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2017-02-24 | Dicke Phase Transition and Collapse of Superradiant Phase in Optomechanical Cavity with Arbitrary Number of Atoms | We in this paper derive the analytical expressions of ground-state energy,
average photon-number, and the atomic population by means of the
spin-coherent-state variational method for arbitrary number of atoms in an
optomechanical cavity. It is found that the existence of mechanical oscil-
lator does not affect the phase boundary between the normal and superradiant
phases. However, the superradiant phase collapses by the resonant damping of
the oscillator when the atom-field coupling increases to a so-called turning
point. As a consequence the system undergoes at this point an additional phase
transition from the superradiant phase to a new normal phase of the atomic
population-inversion state. The region of superradiant phase decreases with the
increase of photon-phonon coupling. It shrinks to zero at a critical value of
the coupling and a direct atomic population transfer appears between two
atom-levels. Moreover we find an unstable nonzero-photon state, which is the
counterpart of the superradiant state. In the absence of oscillator our result
re- duces exactly to that of Dicke model. Particularly the ground-state energy
for N = 1 (i.e. the Rabi model) is in perfect agreement with the numerical
diagonalization in a wide region of coupling constant for both red and blue
detuning. The Dicke phase transition remains for the Rabi model in agreement
with the recent observation. | 1702.07438v1 |
2017-02-28 | Photon-Axion Conversion, Magnetic Field Configuration, and Polarization of Photons | We study the evolution of photon polarization during the photon-axion
conversion process with focusing on the magnetic field configuration
dependence. Most previous studies have been carried out in a conventional model
where a network of magnetic domains is considered and each domain has a
constant magnetic field. We investigate a more general model where a network of
domains is still assumed, but each domain has a helical magnetic field. We find
that the asymptotic behavior does not depend on the configuration of magnetic
fields. Remarkably, we analytically obtain the asymptotic values of the
variance of polarization in the conventional model. When the helicity is small,
we show that there appears the damped oscillating behavior in the early stage
of evolution. Moreover, we see that the constraints on the axion coupling and
the cosmological magnetic fields using polarization observations are affected
by the magnetic field configuration. This is because the different transient
behavior of polarization dynamics is caused by the different magnetic field
configuration. Recently, [C. Wang and D. Lai, J. Cosmol. Astropart. Phys. 06
(2016) 006.] claimed that the photon-axion conversion in helical model behaves
peculiarly. However, our helical model gives much closer predictions to the
conventional discontinuous magnetic field configuration model. | 1702.08843v2 |
2017-05-19 | A Superior but Equally Convenient Alternative to the Steady-State Approximation and Secular Equilibrium | The steady-state approximation (hereafter abbreviated as SSA) consists in
setting $dy/dt=0$, where $y$ denotes the concentration of a short-lived
intermediate subject to first-order decay with a rate constant $k$. The sole
reason for enforcing SSA is to convert the rate equation for $y$ into an
algebraic equation. The conditions under which SSA becomes trustworthy are now
well understood, but a firm grasp of the physical content of the approximation
requires more maturity than few teachers, let alone their students, may be
expected to possess. Furthermore, there is no simple way to gauge the accuracy
of the results derived by imposing SSA. The purpose of this note is to
demonstrate that a better, but equally simple, approximation results if,
instead of setting $dy/dt$ to zero, one substitutes $y(t+\tau)$ for $y+\tau
dy/dt$, where $\tau=1/k$; SSA is a cruder approximation because it neglects the
second term. For systems modelled as damped harmonic oscillators, the "reverse
Taylor approximation" can be extended by retaining one more term in the Taylor
expansion. The utility of the approximation (or its extension) is demonstrated
by examining the following systems: radioactive equilibria, Brownian motion,
dynamic response of linear first- and second-order systems. | 1705.08749v2 |
2017-09-30 | Tuning the diffusion of magnon in Y3Fe5O12 by light excitation | Deliberate control of magnon transportation will lead to an energy-efficient
technology for information transmission and processing. Y3Fe5O12(YIG),
exhibiting extremely large magnon diffusion length due to the low magnetic
damping constant, has been intensively investigated for decades. While most of
the previous works focused on the determination of magnon diffusion length by
various techniques, herein we demonstrated how to tune magnon diffusion by
light excitation. We found that the diffusion length of thermal magnons is
strongly dependent on light wavelength when the magnon is generated by exposing
YIG directly to laser beam. The diffusion length, determined by a nonlocal
geometry at room temperature, is ~30 um for the magnons produced by visible
light (400-650 nm), and ~136-156 um for the laser between 808 nm and 980 nm.
The diffusion distance is much longer than the reported value. In addition to
thermal gradient, we found that light illumination affected the electron
configuration of the Fe3+ ion in YIG. Long wavelength laser triggers a high
spin to low spin state transition of the Fe3+ ions in FeO6 octahedron. This in
turn causes a substantial softening of the magnon thus a dramatic increase in
diffusion distance. The present work paves the way towards an efficient tuning
of magnon transport behavior which is crucially important for magnon
spintronics. | 1710.00222v2 |
2017-10-19 | Global performance metrics for synchronization of heterogeneously rated power systems: The role of machine models and inertia | A recent trend in control of power systems has sought to quantify the
synchronization dynamics in terms of a global performance metric, compute it
under very simplified assumptions, and use it to gain insight on the role of
system parameters, in particular, inertia. In this paper, we wish to extend
this approach to more realistic scenarios, by incorporating the heterogeneity
of machine ratings, more complete machine models, and also to more closely map
it to classical power engineering notions such as Nadir, Rate of Change of
Frequency (RoCoF), and inter-area oscillations.
We consider the system response to a step change in power excitation, and
define the system frequency as a weighted average of generator frequencies
(with weights proportional to each machine's rating); we characterize Nadir and
RoCoF by the $L_\infty$ norm of the system frequency and its derivative,
respectively, and inter-areas oscillations by the $L_2$ norm of the error of
the vector of bus frequencies w.r.t. the system frequency.
For machine models where the dynamic parameters (inertia, damping, etc.) are
proportional to rating, we analytically compute these norms and use them to
show that the role of inertia is more nuanced than in the conventional wisdom.
With the classical swing dynamics, inertia constant plays a secondary role in
performance. It is only when the turbine dynamics are introduced that the
benefits of inertia become more prominent. | 1710.07195v4 |
2019-06-18 | Nonlinear Langevin dynamics via holography | In this work, we consider non-linear corrections to the Langevin effective
theory of a heavy quark moving through a strongly coupled CFT plasma. In
AdS/CFT, this system can be identified with that of a string stretched between
the boundary and the horizon of an asymptotically AdS black-brane solution. We
compute the Feynman-Vernon influence phase for the heavy quark by evaluating
the Nambu-Goto action on a doubled string configuration. This configuration is
the linearised solution of the string motion in the doubled black-brane
geometry which has been proposed as the holographic dual of a thermal
Schwinger-Keldysh contour of the CFT. Our expression for the influence phase
passes non-trivial consistency conditions arising from the underlying unitarity
and thermality of the bath. The local effective theory obeys the recently
proposed non-linear fluctuation dissipation theorem relating the
non-Gaussianity of thermal noise to the thermal jitter in the damping constant.
This furnishes a non-trivial check for the validity of these relations derived
in the weak coupling regime. | 1906.07762v3 |
2019-06-24 | Emergence of localized persistent weakly-evanescent cortical brain wave loops | An inhomogeneous anisotropic physical model of the brain cortex is presented
that predicts the emergence of non--evanescent (weakly damped) wave--like modes
propagating in the thin cortex layers transverse to both the mean neural fiber
direction and to the cortex spatial gradient. Although the amplitude of these
modes stays below the typically observed axon spiking potential, the lifetime
of these modes may significantly exceed the spiking potential inverse decay
constant. Full brain numerical simulations based on parameters extracted from
diffusion and structural MRI confirm the existence and extended duration of
these wave modes. Contrary to the commonly agreed paradigm that the neural
fibers determine the pathways for signal propagation in the brain, the signal
propagation due to the cortex wave modes in the highly folded areas will
exhibit no apparent correlation with the fiber directions. The results are
consistent with numerous recent experimental animal and human brain studies
demonstrating the existence electrostatic field activity in the form of
traveling waves (including studies where neuronal connections were severed) and
with wave loop induced peaks observed in EEG spectra. The localization and
persistence of these cortical wave modes has significant implications in
particular for neuroimaging methods that detect electromagnetic physiological
activity, such as EEG and MEG, and for the understanding of brain activity in
general, including mechanisms of memory. | 1906.09717v1 |
2012-10-30 | Transverse kink oscillations in the presence of twist | Magnetic twist is thought to play an important role in coronal loops. The
effects of magnetic twist on stable magnetohydrodynamic (MHD) waves is poorly
understood because they are seldom studied for relevant cases. The goal of this
work is to study the fingerprints of magnetic twist on stable transverse kink
oscillations. We numerically calculated the eigenmodes of propagating and
standing MHD waves for a model of a loop with magnetic twist. The azimuthal
component of the magnetic field was assumed to be small in comparison to the
longitudinal component. We did not consider resonantly damped modes or kink
instabilities in our analysis. For a nonconstant twist the frequencies of the
MHD wave modes are split, which has important consequences for standing waves.
This is different from the degenerated situation for equilibrium models with
constant twist, which are characterised by an azimuthal component of the
magnetic field that linearly increases with the radial coordinate. In the
presence of twist standing kink solutions are characterised by a change in
polarisation of the transverse displacement along the tube. For weak twist, and
in the thin tube approximation, the frequency of standing modes is unaltered
and the tube oscillates at the kink speed of the corresponding straight tube.
The change in polarisation is linearly proportional to the degree of twist.
This has implications with regard to observations of kink modes, since the
detection of this variation in polarisation can be used as an indirect method
to estimate the twist in oscillating loops. | 1210.8093v1 |
2017-06-21 | Spectral analysis and multigrid preconditioners for two-dimensional space-fractional diffusion equations | Fractional diffusion equations (FDEs) are a mathematical tool used for
describing some special diffusion phenomena arising in many different
applications like porous media and computational finance. In this paper, we
focus on a two-dimensional space-FDE problem discretized by means of a second
order finite difference scheme obtained as combination of the Crank-Nicolson
scheme and the so-called weighted and shifted Gr\"unwald formula.
By fully exploiting the Toeplitz-like structure of the resulting linear
system, we provide a detailed spectral analysis of the coefficient matrix at
each time step, both in the case of constant and variable diffusion
coefficients. Such a spectral analysis has a very crucial role, since it can be
used for designing fast and robust iterative solvers. In particular, we employ
the obtained spectral information to define a Galerkin multigrid method based
on the classical linear interpolation as grid transfer operator and
damped-Jacobi as smoother, and to prove the linear convergence rate of the
corresponding two-grid method. The theoretical analysis suggests that the
proposed grid transfer operator is strong enough for working also with the
V-cycle method and the geometric multigrid. On this basis, we introduce two
computationally favourable variants of the proposed multigrid method and we use
them as preconditioners for Krylov methods. Several numerical results confirm
that the resulting preconditioning strategies still keep a linear convergence
rate. | 1706.06844v1 |
2018-03-07 | Rapidly forming, slowly evolving, spatial patterns from quasi-cycle Mexican Hat coupling | A lattice-indexed family of stochastic processes has quasi-cycle oscillations
if its otherwise-damped oscillations are sustained by noise. Such a family
performs the reaction part of a discrete stochastic reaction-diffusion system
when we insert a local Mexican Hat-type, difference of Gaussians, coupling on a
one-dimensional and on a two-dimensional lattice. Quasi-cycles are a proposed
mechanism for the production of neural oscillations, and Mexican Hat coupling
is ubiquitous in the brain. Thus this combination might provide insight into
the function of neural oscillations in the brain. Importantly, we study this
system only in the transient case, on time intervals before saturation occurs.
In one dimension, for weak coupling, we find that the phases of the coupled
quasi-cycles synchronize (establish a relatively constant relationship, or
phase lock) rapidly at coupling strengths lower than those required to produce
spatial patterns of their amplitudes. In two dimensions the amplitude patterns
form more quickly, but there remain parameter regimes in which phase
synchronization patterns form without being accompanied by clear amplitude
patterns. At higher coupling strengths we find patterns both of phase
synchronization and of amplitude (resembling Turing patterns) corresponding to
the patterns of phase synchronization. Specific properties of these patterns
are controlled by the parameters of the reaction and of the Mexican Hat
coupling. | 1803.02917v2 |
2018-03-23 | Observation of hedgehog skyrmions in sub-100 nm soft magnetic nanodots | Magnetic skyrmions are nanometric spin textures of outstanding potential for
spintronic applications due to unique features governed by their non-trivial
topology. It is well known that skyrmions of definite chirality are stabilized
by the Dzyaloshinskii-Moriya exchange interaction (DMI) in bulk
non-centrosimmetric materials or ultrathin films with strong spin-orbit
coupling in the interface. In this work, we report on the detection of magnetic
hedgehog-skyrmions at room temperature in confined systems with neither DMI nor
perpendicular magnetic anisotropy. We show that soft magnetic (permalloy)
nanodots are able to host non- chiral hedgehog skyrmions that can be further
stabilized by the magnetic field arising from the Magnetic Force Microscopy
probe. Analytical calculations and micromagnetic simulations confirmed the
existence of metastable N\'eel skyrmions in permalloy nanodots even without
external stimuli in a certain size range. Our work implies the existence of a
new degree of freedom to create and manipulate skyrmions in soft nanodots. The
stabilization of skyrmions in soft magnetic materials opens a possibility to
study the skymion magnetization dynamics otherwise limited due to the large
damping constant coming from the high spin-orbit coupling in materials with
high magnetic anisotropy. | 1803.08768v1 |
2018-08-13 | A Nonsmooth Dynamical Systems Perspective on Accelerated Extensions of ADMM | Recently, there has been great interest in connections between
continuous-time dynamical systems and optimization methods, notably in the
context of accelerated methods for smooth and unconstrained problems. In this
paper we extend this perspective to nonsmooth and constrained problems by
obtaining differential inclusions associated to novel accelerated variants of
the alternating direction method of multipliers (ADMM). Through a Lyapunov
analysis, we derive rates of convergence for these dynamical systems in
different settings that illustrate an interesting tradeoff between decaying
versus constant damping strategies. We also obtain modified equations capturing
fine-grained details of these methods, which have improved stability and
preserve the leading order convergence rates. An extension to general nonlinear
equality and inequality constraints in connection with singular perturbation
theory is provided. | 1808.04048v7 |
2018-10-02 | How strongly does diffusion or logistic-type degradation affect existence of global weak solutions in a chemotaxis-Navier--Stokes system? | This paper considers the chemotaxis-Navier--Stokes system with nonlinear
diffusion and logistic-type degradation term \begin{align*} \begin{cases} n_t +
u\cdot\nabla n = \nabla \cdot(D(n)\nabla n) - \nabla\cdot(n \chi(c) \nabla c) +
\kappa n - \mu n^\alpha, & x\in \Omega,\ t>0, \\ c_t + u\cdot\nabla c = \Delta
c - nf(c), & x \in \Omega,\ t>0, \\ u_t + (u\cdot\nabla)u = \Delta u + \nabla P
+ n\nabla\Phi + g, \ \nabla\cdot u = 0, & x \in \Omega,\ t>0, \end{cases}
\end{align*} where $\Omega\subset \mathbb{R}^3$ is a bounded smooth domain; $D
\ge 0$ is a given smooth function such that $D_1 s^{m-1} \le D(s) \le D_2
s^{m-1}$ for all $s\ge 0$ with some $D_2 \ge D_1 > 0$ and some $m > 0$;
$\chi,f$ are given functions satisfying some conditions; $\kappa \in
\mathbb{R},\mu \ge0,\alpha>1$ are constants. This paper shows existence of
global weak solutions to the above system under the condition that
\begin{align*} m >\frac{2}{3},\quad \mu \ge 0 \quad \mbox{and}\quad \alpha >1
\end{align*} hold, or that \begin{align*} m> 0, \quad \mu>0 \quad \mbox{and}
\quad \alpha > \frac{4}{3} \end{align*} hold. This result asserts that `strong'
diffusion effect or `strong' logistic damping derives existence of global weak
solutions even though the other effect is `weak', and can include previous
works. | 1810.01098v2 |
2018-10-05 | Magnetic field direction dependent antiskyrmion motion with microwave electric fields | Magnetic skyrmions are regarded as promising information candidates in future
spintronic devices, which have been investigated theoretically and
experimentally in isotropic system. Recently, the sta- bilization of
antiskyrmions in the presence of anisotropic Dzyaloshinskii-Moriya interaction
and its dynamics driven by current have been investigated. Here, we report the
antiskyrmion motion with the combined action of the in-plane magnetic field and
microwave electric fields. The in-plane mag- netic field breaks the rotation
symmetry of the antiskyrmion, and perpendicular microwave electric field
induces the pumping of magnetic anisotropy, leading to antiskyrmion breathing
mode. With above two effects, the antiskyrmion propagates with a desired
trajectory. Antiskyrmion propagation velocity depends on the frequency,
amplitude of anisotropy pumping, and damping constant as well as strength of
in-plane field, which reaches the maximum value when the frequency of microwave
electric field is in consist with the resonance frequency of antiskyrmion.
Moreover, we show that the antiskyrmion propagation depends on the direction of
magnetic field, where the antiskyrmion Hall angle can be suppressed or
enhanced. At a critical direction of magnetic field, the Hall angle is zero.
Our results introduce a possible application of antiskyrmion in
antiskyrmion-based spintronic devices with lower energy consumption. | 1810.02464v1 |
2018-10-22 | Polarized Raman spectroscopy study of metallic $(Sr_{1-x}La_{x})_{3}Ir_{2}O_{7}$: a consistent picture of disorder-interrupted unidirectional charge order | We have used rotational anisotropic polarized Raman spectroscopy to study the
symmetries, the temperature and the doping dependence of the charge ordered
state in metallic $(Sr_{1-x}La_{x})_{3}Ir_{2}O_{7}$. Although the Raman probe
size is greater than the charge ordering length, we establish that the charge
ordering breaks the fourfold rotational symmetry of the underlying tetragonal
crystal lattice into twofold, as well as the translational symmetry, and forms
short-range domains with $90^{\circ}$ rotated charge order wave vectors, as
soon as the charge order sets in below $T_{CO} = \sim$ 200K and across the
doping-induced insulator metal transition. We observe that this charge order
mode frequency remains nearly constant over a wide temperature range and up to
the highest doping level. These above features are highly reminiscent of the
ubiquitous unidirectional charge order in underdoped high-$T_C$
copper-oxide-based superconductors (cuprates). We further resolve that the
charge order damping rate diverges when approaching $T_{CO}$ from below and
increases significantly as increasing the La doping level, which resembles the
behaviors for a disorder-interrupted ordered phase and has not been observed
for the charge order in cuprates. | 1810.09087v2 |
2019-01-25 | Gravitational waves from dynamical tides in white dwarf binaries | We study the effect of tidal forcing on gravitational wave signals from
tidally relaxed white dwarf pairs in the LISA, DECIGO and BBO frequency band
($0.1-100\,{\rm mHz}$). We show that for stars not in hydrostatic equilibrium
(in their own rotating frames), tidal forcing will result in energy and angular
momentum exchange between the orbit and the stars, thereby deforming the orbit
and producing gravitational wave power in harmonics not excited in perfectly
circular synchronous binaries. This effect is not present in the usual
orbit-averaged treatment of the equilibrium tide, and is analogous to transit
timing variations in multiplanet systems. It should be present for all LISA
white dwarf pairs since gravitational waves carry away angular momentum faster
than tidal torques can act to synchronize the spins, and when mass transfer
occurs as it does for at least eight LISA verification binaries. With the
strain amplitudes of the excited harmonics depending directly on the density
profiles of the stars, gravitational wave astronomy offers the possibility of
studying the internal structure of white dwarfs, complimenting information
obtained from asteroseismology of pulsating white dwarfs. Since the vast
majority of white-dwarf pairs in this frequency band are expected to be in the
quasi-circular state, we focus here on these binaries, providing general
analytic expressions for the dependence of the induced eccentricity and strain
amplitudes on the stellar apsidal motion constants and their radius and mass
ratios. Tidal dissipation and gravitation wave damping will affect the results
presented here and will be considered elsewhere. | 1901.09045v2 |
2019-01-31 | Angular momentum Josephson effect between two isolated condensates | We demonstrate that the two degenerate energy levels in spin-orbit coupled
trapped Bose gases, coupled by a quenched Zeeman field, can be used for angular
momentum Josephson effect. In a static quenched field, we can realize a
Josephson oscillation with period ranging from millisecond to hundreds of
milliseconds. Moreover, by a driven Zeeman field, we realize a new Josephson
oscillation, in which the population imbalance may have the same expression as
the current in the directed current (dc) Josephson effect. When the dynamics of
condensate can not follow up the modulation frequency, it the self-trapping
regime. This new dynamics is understood from the time dependent evolution of
the constant-energy trajectory in phase space. This model has several salient
advantages as compared with the previous ones. The condensates are isolated
from their excitations by a finite gap, thus can greatly suppress the damping
effect induced by thermal atoms and Bogoliubov excitations. The oscillation
period can be tuned by several order of magnitudes without influencing other
parameters. In experiments, the dynamics can be mapped out from spin and
momentum spaces, thus is not limited by the spatial resolution in imaging. This
system can serve as a promising platform for exploring of matter wave
interferometry. | 1901.11449v2 |
2015-12-02 | Thermodynamics of the heat currents in the longitudinal spin Seebeck and spin Peltier effects | We employ the non-equilibrium thermodynamics of currents and forces to
describe the heat transport caused by a spin current in a Pt/YIG bilayer. By
starting from the constitutive equations of the magnetization currents in both
Pt and YIG, we derive the magnetization potentials and currents. We apply the
theory to the spin Peltier experiments in which a spin current, generated by
the spin Hall effect in Pt, is injected into YIG. We find that efficient
injection is obtained when: i) the thickness of each layer is larger than its
diffusion length: $t_{Pt} > l_{Pt}$ and $t_{YIG} > l_{YIG}$ and ii) the ratio
$(l_{Pt}/\tau_{Pt})/(l_{YIG}/\tau_{YIG})$ is small, where $\tau_i$ is the time
constant of the intrinsic damping ($i=Pt, YIG$). We finally derive the
temperature profile in adiabatic conditions. The scale of the effect is given
by the parameter $\Delta T_{SH}$ which is proportional to the electric current
in Pt. Using known parameters for Pt and YIG we estimate $\Delta T_{SH}/j_e = 4
\cdot 10^{-13}$ K A$^{-1}$m$^2$. This value is of the same order of magnitude
of the spin Peltier experiments. | 1512.00644v2 |
2015-12-11 | Infrared study of lattice dynamics and spin-phonon and electron-phonon interactions in multiferroic TbFe3(BO3)4 and GdFe3(BO3)4 | We present a comparative far-infrared reflection spectroscopy study of
phonons, phase transitions, spin-phonon and electron-phonon interactions in
isostructural multiferroic iron borates of gadolinium and terbium. The behavior
of phonon modes registered in a wide temperature range is consistent with a
weak first-order structural phase transition (Ts = 143 for GdFe3(BO3)4 and 200
K for TbFe3(BO3)4) from high-symmetry high-temperature R32 structure into
low-symmetry low-temperature P3121 one. The temperature dependences of
frequencies, oscillator strengths, and damping constants of some low-frequency
modes reveal an appreciable lattice anharmonicity. Peculiarities in the phonon
mode behavior in both compounds at the temperature of an antiferromagnetic
ordering (TN = 32 K for GdFe3(BO3)4 and 40 K for TbFe3(BO3)4) evidence the
spin-phonon interaction. In the energy range of phonons, GdFe3(BO3)4 has no
electronic levels but TbFe3(BO3)4 possesses several ones. We observe an onset
of new bands in the excitation spectrum of TbFe3(BO3)4, due to a resonance
interaction between a lattice phonon and 4f electronic crystal-field
excitations of Tb3+. This interaction causes delocalization of the CF
excitations, their Davydov splitting, and formation of coupled electron-phonon
modes. | 1512.03527v1 |
2015-12-27 | Electrically Switchable Metadevices via Graphene | Metamaterials bring sub-wavelength resonating structures together to overcome
the limitations of conventional materials. The realization of active
metadevices has been an outstanding challenge that requires electrically
reconfigurable components operating over a broad spectrum with a wide dynamic
range. The existing capability of metamaterials, however, is not sufficient to
realize this goal. Here, by integrating passive metamaterials with active
graphene devices, we demonstrate a new class of electrically controlled active
metadevices working in microwave frequencies. The fabricated active metadevices
enable efficient control of both amplitude (> 50 dB) and phase (> 90{\deg}) of
electromagnetic waves. In this hybrid system, graphene operates as a tunable
Drude metal that controls the radiation of the passive metamaterials.
Furthermore, by integrating individually addressable arrays of metadevices, we
demonstrate a new class of spatially varying digital metasurfaces where the
local dielectric constant can be reconfigured with applied bias voltages.
Additionally, we reconfigure resonance frequency of split ring resonators
without changing its amplitude by damping one of the two coupled metasurfaces
via graphene. Our approach is general enough to implement various metamaterial
systems that could yield new applications ranging from electrically switchable
cloaking devices to adaptive camouflage systems. | 1512.08277v3 |
2016-08-07 | Two-loop RGE of a general renormalizable Yang-Mills theory in a renormalization scheme with an explicit UV cutoff | We perform a systematic one-loop renormalization of a general renormalizable
Yang-Mills theory coupled to scalars and fermions using a regularization scheme
with a smooth momentum cutoff $\Lambda$ (implemented through an exponential
damping factor). We construct the necessary finite counterterms restoring the
BRST invariance of the effective action by analyzing the relevant
Slavnov-Taylor identities. We find the relation between the renormalized
parameters in our scheme and in the conventional $\overline{\rm MS}$ scheme
which allow us to obtain the explicit two-loop renormalization group equations
in our scheme from the known two-loop ones in the $\overline{\rm MS}$ scheme.
We calculate in our scheme the divergences of two-loop vacuum graphs in the
presence of a constant scalar background field which allow us to rederive the
two-loop beta functions for parameters of the scalar potential. We also prove
that consistent application of the proposed regularization leads to
counterterms which, together with the original action, combine to a bare action
expressed in terms of bare parameters. This, together with treating $\Lambda$
as an intrinsic scale of a hypothetical underlying finite theory of all
interactions, offers a possibility of an unconventional solution to the
hierarchy problem if no intermediate scales between the electroweak scale and
the Planck scale exist. | 1608.02270v3 |
2010-09-28 | Accurate interaction energies at DFT level by means of an efficient dispersion correction | This paper presents an approach for obtaining accurate interaction energies
at the DFT level for systems where dispersion interactions are important. This
approach combines Becke and Johnson's [J. Chem. Phys. 127, 154108 (2007)]
method for the evaluation of dispersion energy corrections and a Hirshfeld
method for partitioning of molecular polarizability tensors into atomic
contributions. Due to the availability of atomic polarizability tensors, the
method is extended to incorporate anisotropic contributions, which prove to be
important for complexes of lower symmetry. The method is validated for a set of
eighteen complexes, for which interaction energies were obtained with the
B3LYP, PBE and TPSS functionals combined with the aug-cc-pVTZ basis set and
compared with the values obtained at CCSD(T) level extrapolated to a complete
basis set limit. It is shown that very good quality interaction energies can be
obtained by the proposed method for each of the examined functionals, the
overall performance of the TPSS functional being the best, which with a slope
of 1.00 in the linear regression equation and a constant term of only 0.1
kcal/mol allows to obtain accurate interaction energies without any need of a
damping function for complexes close to their exact equilibrium geometry. | 1009.5631v1 |
2017-03-17 | Quasinormal modes of a scalar field in the Einstein--Gauss--Bonnet-AdS black hole background: Perturbative and non-perturbative branches | It has recently been found that quasinormal modes of asymptotically anti-de
Sitter (AdS) black holes in theories with higher curvature corrections may help
to describe the regime of intermediate 't Hooft coupling in the dual field
theory. Here, we consider quasinormal modes of a scalar field in the background
of spherical Gauss--Bonnet--anti-de Sitter (AdS) black holes. In general, the
eigenvalues of wave equations are found here numerically, but at a fixed
Gauss-Bonnet constant $\alpha = R^2/2$ (where $R$ is the AdS radius), an exact
solution of the scalar field equation has been obtained. Remarkably, the purely
imaginary modes, which are usually appropriate only to some gravitational
perturbations, were found here even for a test scalar field. These purely
imaginary modes of the Einstein--Gauss--Bonnet theory do not have the
Einsteinian limits, because their damping rates grow, when $\alpha$ is
decreasing. Thus, these modes are nonperturbative in $\alpha$. The real
oscillation frequencies of the perturbative branch are linearly related to
their Schwarzschild-AdS limits $Re (\omega_{GB}) = Re (\omega_{SAdS}) (1+ K(D)
(\alpha/R^2))$, where $D$ is the number of spacetime dimensions. Comparison of
the analytical formula with the frequencies found by the shooting method allows
us to test the latter. In addition, we found exact solutions to the master
equations for gravitational perturbations at $\alpha=R^2/2$ and observed that
for the scalar type of gravitational perturbations an eikonal instability
develops. | 1703.06215v3 |
2018-09-20 | Relaxation in a Fuzzy Dark Matter Halo | Dark matter may be composed of light bosons, ${m_b \sim 10^{-22}\,
\mathrm{eV}}$, with a de Broglie wavelength $\lambda \sim 1 \,\mathrm{kpc}$ in
typical galactic potentials. Such `fuzzy' dark matter (FDM) behaves like cold
dark matter (CDM) on much larger scales than the de Broglie wavelength, but may
resolve some of the challenges faced by CDM in explaining the properties of
galaxies on small scales ($\lesssim 10\,\mathrm{kpc}$). Because of its wave
nature, FDM exhibits stochastic density fluctuations on the scale of the de
Broglie wavelength that never damp. The gravitational field from these
fluctuations scatters stars and black holes, causing their orbits to diffuse
through phase space. We show that this relaxation process can be analyzed
quantitatively with the same tools used to analyze classical two-body
relaxation in an $N$-body system, and can be described by treating the FDM
fluctuations as quasiparticles, with effective mass $\sim 10^7 M_\odot
{(1\,\mathrm{kpc}/r)}^2{(10^{-22}\,\mathrm{eV}/m_b)}^3$ in a galaxy with a
constant circular speed of $200\,\mathrm{kms}$. This novel relaxation mechanism
may stall the inspiral of supermassive black holes or globular clusters due to
dynamical friction at radii of a few hundred pc, and can heat and expand the
central regions of galaxies. These processes can be used to constrain the mass
of the light bosons that might comprise FDM. | 1809.07673v2 |
2020-02-25 | The Casimir densities for a sphere in the Milne universe | The influence of a spherical boundary on the vacuum fluctuations of a massive
scalar field is investigated in background of $(D+1)$-dimensional Milne
universe, assuming that the field obeys Robin boundary condition on the sphere.
The normalized mode functions are derived for the regions inside and outside
the sphere and different vacuum states are discussed. For the conformal vacuum,
the Hadamard function is decomposed into boundary-free and sphere-induced
contributions and an integral representation is obtained for the latter in both
the interior and exterior regions. As important local characteristics of the
vacuum state the vacuum expectation values (VEVs) of the field squared and of
the energy-momentum tensor are investigated. It is shown that the vacuum
energy-momentum tensor has an off-diagonal component that corresponds to the
energy flux along the radial direction. Depending on the coefficient in Robin
boundary condition the sphere-induced contribution to the vacuum energy and the
energy flux can be either positive or negative. At late stages of the expansion
and for a massive field the decay of the sphere-induced VEVs, as functions of
time, is damping oscillatory. The geometry under consideration is conformally
related to that for a static spacetime with negative constant curvature space
and the sphere-induced contributions in the corresponding VEVs are compared. | 2003.05372v2 |
2020-03-12 | Skyrmion Dynamics and Transverse Mobility: Skyrmion Hall Angle Reversal on 2D Periodic Substrates with dc and Biharmonic ac Drives | We numerically examine the dynamics of a skyrmion interacting with a
two-dimensional periodic substrate under dc and biharmonic ac drives. We show
that the Magnus force of the skyrmion produces circular orbits that can
resonate with the ac drive and the periodicity of the substrate to create
quantized motion both parallel and perpendicular to the dc drive. The skyrmion
Hall angle exhibits a series of increasing and/or decreasing steps along with
strongly fluctuating regimes. In the phase locked regimes, the skyrmion Hall
angle is constant and the skyrmion motion consists of periodic orbits
encircling an integer number of obstacles per every or every other ac drive
cycle. We also observe phases in which the skyrmion moves at $90^\circ$ with
respect to the driving direction even in the presence of damping, a phenomenon
called absolute transverse mobility that can exhibit reentrance as a function
of dc drive. When the biharmonic ac drives have different amplitudes, in the
two directions we find regimes in which the skyrmion Hall angle shows a sign
reversal from positive to negative, as well as a reentrant pinning effect in
which the skyrmion is mobile at low drives but becomes pinned at higher drives.
These behaviors arise due to the combination of the Magnus force with the
periodic motion of the skyrmions, which produce Shapiro steps, directional
locking, and ratchet effects. | 2003.05972v1 |
2020-03-16 | Dimensional crossovers and Casimir forces for the Bose gas in anisotropic optical lattices | We consider the Bose gas on a $d$-dimensional anisotropic lattice employing
the imperfect (mean-field) gas as a prototype example. We study the dimensional
crossover arising as a result of varying the dispersion relation at finite
temperature $T$. We analyze in particular situations where one of the relevant
effective dimensionalities is located at or below the lower critical dimension,
so that the Bose-Einstein condensate becomes expelled from the system by
anisotropically modifying the lattice parameters controlling the kinetic term
in the Hamiltonian. We clarify the mechanism governing this phenomenon.
Subsequently we study the thermodynamic Casimir effect occurring in this
system. We compute the exact profile of the scaling function for the Casimir
energy. As an effect of strongly anisotropic scale invariance, the Casimir
force below or at the critical temperature $T_c$ may be repulsive even for
periodic boundary conditions. The corresponding Casimir amplitude is universal
only in a restricted sense, and the power law governing the decay of the
Casimir interaction becomes modified. We also demonstrate that, under certain
circumstances, the scaling function is constant for suffciently large values of
the scaling variable, and in consequence is not an analytical function. At $T >
T_c$ the Casimir-like interactions reflect the structure of the correlation
function, and, for certain orientations of the confining walls, show
exponentially damped oscillatory behavior so that the corresponding force is
attractive or repulsive depending on the distance. | 2003.07458v3 |
2020-04-25 | Quasinormal modes of the test fields in the novel 4D Einstein-Gauss-Bonnet-de Sitter gravity | The regularization proposed in [D.~Glavan and C.~Lin, Phys.\ Rev.\ Lett.\
{\bf 124}, 081301 (2020)] led to the black hole solutions which turned out to
be the solutions of the consistent well-defined $4$-dimensional
Einstein-Gauss-Bonnet theory of gravity suggested in [K.~Aoki, M.~Gorji and
S.~Mukohyama, arXiv:2005.03859]. Recently the quasinormal modes of bosonic and
fermionic fields for this theory were studied. Here we calculate quasinormal
frequencies of the test scalar, electromagnetic and Dirac fields for the
spherically symmetric black hole in the novel $4D$ Einstein-Gauss-Bonnet-de
Sitter theory. The values of the quasinormal modes, calculated by the sixth
order WKB method with Pad\'{e} approximants and the time-domain integration,
show that both real oscillation frequency and the damping rate are suppressed
by increasing of the cosmological constant. While the stability of the scalar
and electromagnetic fields follows directly from the positive definiteness of
the effective potential, there is no such positive definiteness for the Dirac
field. Here, with the help of the time domain integration, taking into account
all the modes, we prove stability of the Dirac field in $4D$
Einstein-Gauss-Bonnet-de Sitter theory. | 2004.14172v2 |
2020-08-08 | Axial Gravitational Waves in Bianchi I Universe | In this paper, we have studied the propagation of axial gravitational waves
in Bianchi I universe using the Regge-Wheeler gauge. In this gauge, there are
only two non-zero components of $ h_{\mu\nu} $ in the case of axial waves:
$h_0(t,r)$ and $h_1(t,r)$. The field equations in absence of matter have been
derived both for the unperturbed as well as axially perturbed metric. These
field equations are solved simultaneously by assuming the expansion scalar
$\Theta$ to be proportional to the shear scalar $\sigma$ (so that $a= b^n$,
where $a$, $b$ are the metric coefficients and $n$ is an arbitrary constant),
and the wave equation for the perturbation parameter $h_0(t,r)$ have been
derived. We used the method of separation of variables to solve for this
parameter, and have subsequently determined $h_1(t,r)$. We then discuss a few
special cases in order to interpret the results. We find that the anisotropy of
the background spacetime is responsible for the damping of the gravitational
waves as they propagate through this spacetime. The perturbations depend on the
values of the angular momentum $l$. The field equations in the presence of
matter reveal that the axially perturbed spacetime leads to perturbations only
in the azimuthal velocity of the fluid leaving the matter field undisturbed. | 2008.04780v2 |
2020-08-15 | Stability analysis of the linear discrete teleoperation systems with stochastic sampling and data dropout | This paper addresses the stability conditions of the sampled-data
teleoperation systems consisting continuous time master, slave, operator, and
environment with discrete time controllers over general communication networks.
The output signals of the slave and master robots are quantized with stochastic
sampling periods which are modeled as being from a finite set. By applying an
input delay method, the probabilistic sampling system is converted into a
continuous-time system including stochastic parameters in the system matrices.
The main contribution of this paper is the derivation of the less conservative
stability conditions for linear discrete teleoperation systems taking into
account the challenges such as the stochastic sampling rate, constant time
delay and the possibility of data packet dropout. The numbers of dropouts are
driven by a finite state Markov chain. First, the problem of finding a lower
bound on the maximum sampling period that preserves the stability is
formulated. This problem is constructed as a convex optimization program in
terms of linear matrix inequalities (LMI). Next, Lyapunov Krasovskii based
approaches are applied to propose sufficient conditions for stochastic and
exponential stability of closed-loop sampled-data bilateral teleoperation
system. The proposed criterion notifies the effect of sampling time on the
stability transparency trade-off and imposes bounds on the sampling time,
control gains and the damping of robots. Neglecting this study undermines both
the stability and transparency of teleoperation systems. Numerical simulation
results are used to verify the proposed stability criteria and illustrate the
effectiveness of the sampling architecture. | 2008.06683v1 |
2020-08-25 | The Sandwich Mode for Vertical Shear Instability in Protoplanetary Disks | Turbulence has a profound impact on the evolution of gas and dust in
protoplanetary disks (PPDs), from driving the collisions and the diffusion of
dust grains, to the concentration of pebbles in giant vortices, thus,
facilitating planetesimal formation. The Vertical Shear Instability (VSI) is a
hydrodynamic mechanism, operating in PPDs if the local rate of thermal
relaxation is high enough. Previous studies of the VSI have, however, relied on
the assumption of constant cooling rates, or neglected the finite coupling time
between the gas particles and the dust grains. Here, we present the results of
hydrodynamic simulations of PPDs with the PLUTO code that include a more
realistic thermal relaxation prescription, which enables us to study the VSI in
the optically thick and optically thin parts of the disk under consideration of
the thermal dust-gas coupling. We show the VSI to cause turbulence even in the
optically thick inner regions of PPDs in our two- and three-dimensional
simulations. The collisional decoupling of dust and gas particles in the upper
atmosphere and the correspondingly inefficient thermal relaxation rates lead to
the damping of the VSI turbulence. Long-lived anticyclonic vortices form in our
three-dimensional simulation. These structures emerge from the turbulence in
the VSI-active layer, persist over hundreds of orbits and extend vertically
over the whole extent of the turbulent region. We conclude that the VSI leads
to turbulence and the formation of long-lived dust traps within $\pm$3 pressure
scale heights distance from the disk midplane | 2008.11195v2 |
2021-04-01 | Brownian motion under intermittent harmonic potentials | We study the effects of an intermittent harmonic potential of strength $\mu =
\mu_0 \nu$ -- that switches on and off stochastically at a constant rate
$\gamma$, on an overdamped Brownian particle with damping coefficient $\nu$.
This can be thought of as a realistic model for realisation of stochastic
resetting. We show that this dynamics admits a stationary solution in all
parameter regimes and compute the full time dependent variance for the position
distribution and find the characteristic relaxation time. We find the exact
non-equilibrium stationary state distributions in the limits -- (i)
$\gamma\ll\mu_0 $ which shows a non-trivial distribution, in addition as
$\mu_0\to\infty$, we get back the result for resetting with refractory period;
(ii) $\gamma\gg\mu_0$ where the particle relaxes to a Boltzmann distribution of
an Ornstein-Uhlenbeck process with half the strength of the original potential
and (iii) intermediate $\gamma=2n\mu_0$ for $n=1, 2$. The mean first passage
time (MFPT) to find a target exhibits an optimisation with the switching rate,
however unlike instantaneous resetting the MFPT does not diverge but reaches a
stationary value at large rates. MFPT also shows similar behavior with respect
to the potential strength. Our results can be verified in experiments on
colloids using optical tweezers. | 2104.00609v2 |
2021-05-28 | Designing a Plasma Lens as a Matching Device for the ILC Positron Source | To realise a planned high-luminosity and high-energy $e^+e^-$-collider, as
the ILC, a large amount of positrons have to be produced and the accelerated
particles have to be captured and matched according to the damping ring
acceptances. %There exist several technical possibilities. In this contribution
a new promising alternative method for capturing positrons will be presented,
the application of the plasma lens as an optical matching device. It will be
compared with the current matching device proposed for the ILC, namely the
quarter wave transformer. An advantage of the plasma lens is the different
magnetic field component, which focuses the divergent beam in a more effective
manner. Therefore it will be shown in this paper that the yield requirements
could be achieved more easily. The plasma lens can actually be a promising
alternative for focusing beams as soon as the technical feasibility has been
approved.\\ In the simulation, a tapered active plasma lens has been optimized
using the approximation of a homogeneous electric current density constant in
time. The optimization process led to a plasma lens design that improves on the
ILC's currently proposed optical matching device, namely the quarter wave
transformer, by approximately $50-100\%$. Furthermore the design has been shown
to guarantee a stable captured positron yield within $\pm1.5\%$ for single,
independent parameter deviations of about $\pm10\%$. | 2105.14008v1 |
2007-07-19 | Structure formation in the presence of dark energy perturbations | We study non-linear structure formation in the presence of dark energy. The
influence of dark energy on the growth of large-scale cosmological structures
is exerted both through its background effect on the expansion rate, and
through its perturbations as well. In order to compute the rate of formation of
massive objects we employ the Spherical Collapse formalism, which we generalize
to include fluids with pressure. We show that the resulting non-linear
evolution equations are identical to the ones obtained in the Pseudo-Newtonian
approach to cosmological perturbations, in the regime where an equation of
state serves to describe both the background pressure relative to density, and
the pressure perturbations relative to the density perturbations as well. We
then consider a wide range of constant and time-dependent equations of state
(including phantom models) parametrized in a standard way, and study their
impact on the non-linear growth of structure. The main effect is the formation
of dark energy structure associated with the dark matter halo: non-phantom
equations of state induce the formation of a dark energy halo, damping the
growth of structures; phantom models, on the other hand, generate dark energy
voids, enhancing structure growth. Finally, we employ the Press-Schechter
formalism to compute how dark energy affects the number of massive objects as a
function of redshift. | 0707.2882v3 |
2007-07-25 | Chromoelectric response functions for quark-gluon plasma | We determine the chromoelectric response of quark-gluon plasma (QGP)
systematically within the framework of classical transport equations. The
transport equations are set up in the phase space which includes the SU(3)
group space corresponding to color (which is a dynamical degree of freedom), in
addition to the position - momentum variables. The distribution functions are
defined by projecting the density operators for the quarks and the gluons to
their respective coherent states (defined over the extended phase space). The
full import of the Yang-Mills(YM) dynamics is shown to manifest through the
emergence of an intrinsic nonlinear, nonlocal response, whose behavior we
determine in the long wavelength limit. It also manifests as a tensor response
which is a characteristic of gluons. The response functions are shown to have a
natural interpretation in terms of the renormalizations of the Abelian and the
non-Abelian coupling constants. A detailed analysis of the screening of heavy
quark potential and of the exact role played by the Debye mass screening in the
case of the Cornell potential, is performed. We also discuss the non-Abelian
contribution to Landau damping in QGP. | 0707.3697v3 |
2007-07-26 | Coarse-Grained Finite-Temperature Theory for the Condensate in Optical Lattices | In this work, we derive a coarse-grained finite-temperature theory for a Bose
condensate in a one-dimensional optical lattice, in addition to a confining
harmonic trap potential. We start from a two-particle irreducible (2PI)
effective action on the Schwinger-Keldysh closed-time contour path. In
principle, this action involves all information of equilibrium and
non-equilibrium properties of the condensate and noncondensate atoms. By
assuming an ansatz for the variational function, i.e., the condensate order
parameter in an effective action, we derive a coarse-grained effective action,
which describes the dynamics on the length scale much longer than a lattice
constant. Using the variational principle, coarse-grained equations of motion
for the condensate variables are obtained. These equations include a
dissipative term due to collisions between condensate and noncondensate atoms,
as well as noncondensate mean-field. To illustrate the usefulness of our
formalism, we discuss a Landau instability of the condensate in optical
lattices by using the coarse-grained generalized Gross-Pitaevskii
hydrodynamics. We found that the collisional damping rate due to collisions
between the condensate and noncondensate atoms changes sign when the condensate
velocity exceeds a renormalized sound velocity, leading to a Landau instability
consistent with the Landau criterion. Our results in this work give an insight
into the microscopic origin of the Landau instability. | 0707.3984v1 |
2013-11-13 | Convergent perturbative nuclear effective field theory | We consider the nuclear effective field theory including pions in the
two-nucleon sector in the S waves up to including the next-to-next-to-leading
order (NNLO) terms according to the power counting suggested by the Wilsonian
renormalization group analysis done in a previous paper. We treat only the
leading contact interaction nonperturbatively, and the rest, including the
long-distance part of pion exchange, are treated as perturbations. To define
the long-distance part, it is important to introduce a separation scale, or a
cutoff. We employ a hybrid regularization, in which the loops with only contact
interactions are regularized with Power Divergence Subtraction (PDS), while the
loops with (long-distance part of) pion exchange are regularized with a
Gaussian damping factor (GDF), to simplify the (nonperturbative) leading-order
amplitudes. The scale introduced by PDS is identified with the cutoff of GDF up
to a numerical factor. We emphasize that the introduction of the GDF requires a
careful definition of the coupling constant for the pion exchange. We obtain
the analytic expressions for the phase shifts for the $^1S_0$ and
$^3S_1$-$^3D_1$ channels. By fitting them to the Nijmegen partial wave analysis
data, it is shown that the effective theory expansion with perturbative
long-distance part of pion exchange is converging. | 1311.3063v1 |
2013-11-14 | Electric field gradient wave (EFGW) in iron-based superconductor Ba(0.6)K(0.4)Fe2As2 studied by Mössbauer spectroscopy | The optimally doped 122 iron-based superconductor Ba(0.6)K(0.4)Fe2As2 has
been studied by 57Fe Moessbauer spectroscopy versus temperature ranging from
4.2 K till 300 K with particular attention paid to the superconducting
transition around 38 K. The spectra do not contain magnetic components and they
exhibit quasi-continuous distribution of quadrupole split doublets. A
distribution follows the electric field gradient (EFG) spatial modulation
(wave) - EFGW. The EFGW is accompanied by some charge density wave (CDW) having
about an order of magnitude lesser influence on the spectrum. The EFGW could be
modeled as widely separated narrow sheets with the EFG increasing from small
till maximum value almost linearly and subsequently dropping back to the
original value in a similar fashion - across the sheet. One encounters very
small and almost constant EFG between sheets. The EFGW shape and amplitude as
well as the amplitude of CDW are strongly affected by a superconducting
transition. All modulations are damped significantly at transition (38 K) and
recover at a temperature being about 14 K lower. The maximum quadrupole
splitting at 4.2 K amounts to about 2.1 mm/s, while the dispersion of CDW seen
on the iron nuclei could be estimated far away from the superconducting gap
opening and at low temperature as 0.5 el./a.u.^3. It drops to about 0.3
el./a.u.^3 just below transition to the superconducting state. | 1311.3503v4 |
2016-06-16 | Calculating rotating hydrodynamic and magneto-hydrodynamic waves to understand magnetic effects on dynamical tides | For understanding magnetic effects on dynamical tides, we study the rotating
magneto-hydrodynamic (MHD) flow driven by harmonic forcing. The linear
responses are analytically derived in a periodic box under the local WKB
approximation. Both the kinetic and Ohmic dissipations at the resonant
frequencies are calculated and the various parameters are investigated.
Although magnetic pressure may be negligible compared to thermal pressure,
magnetic field can be important for the first-order perturbation, e.g.
dynamical tides. It is found that magnetic field splits the resonant frequency,
namely the rotating hydrodynamic flow has only one resonant frequency but the
rotating MHD flow has two, one positive and the other negative. In the weak
field regime the dissipations are asymmetric around the two resonant
frequencies and this asymmetry is more striking with a weaker magnetic field.
It is also found that both the kinetic and Ohmic dissipations at the resonant
frequencies are inversely proportional to the Ekman number and the square of
wavenumber. The dissipation at the resonant frequency on small scales is almost
equal to the dissipation at the non-resonant frequencies, namely the resonance
takes its effect on the dissipation at intermediate length scales. Moreover,
the waves with phase propagation perpendicular to magnetic field are much more
damped. It is also interesting to find that the frequency-averaged dissipation
is constant. This result suggests that in compact objects magnetic effects on
tidal dissipation should be considered. | 1606.06232v1 |
2016-12-01 | Echoes from the Abyss: Tentative evidence for Planck-scale structure at black hole horizons | In classical General Relativity (GR), an observer falling into an
astrophysical black hole is not expected to experience anything dramatic as she
crosses the event horizon. However, tentative resolutions to problems in
quantum gravity, such as the cosmological constant problem, or the black hole
information paradox, invoke significant departures from classicality in the
vicinity of the horizon. It was recently pointed out that such near-horizon
structures can lead to late-time echoes in the black hole merger gravitational
wave signals that are otherwise indistinguishable from GR. We search for
observational signatures of these echoes in the gravitational wave data
released by advanced Laser Interferometer Gravitational-Wave Observatory
(LIGO), following the three black hole merger events GW150914, GW151226, and
LVT151012. In particular, we look for repeating damped echoes with time-delays
of $8 M \log M$ (+spin corrections, in Planck units), corresponding to
Planck-scale departures from GR near their respective horizons. Accounting for
the "look elsewhere" effect due to uncertainty in the echo template, we find
tentative evidence for Planck-scale structure near black hole horizons at false
detection probability of $1\%$ (corresponding to $2.5\sigma$ significance
level). Future observations from interferometric detectors at higher
sensitivity, along with more physical echo templates, will be able to confirm
(or rule out) this finding, providing possible empirical evidence for
alternatives to classical black holes, such as in ${\it firewall}$ or ${\it
fuzzball}$ paradigms. | 1612.00266v2 |
2016-12-21 | Cosmological singularity resolution from quantum gravity: the emergent-bouncing universe | Alternative scenarios to the Big Bang singularity have been subject of
intense research for several decades by now. Most popular in this sense have
been frameworks were such singularity is replaced by a bounce around some
minimal cosmological volume or by some early quantum phase. This latter
scenario was devised a long time ago and referred as an "emergent universe" (in
the sense that our universe emerged from a constant volume quantum phase). We
show here that within an improved framework of canonical quantum gravity (the
so called Quantum Reduced Loop Gravity) the Friedmann equations for cosmology
are modified in such a way to replace the big bang singularity with a short
bounce {preceded by a metastable quantum phase in which the volume of the
universe oscillates between a series of local maxima and minima}. We call this
hybrid scenario an "emergent-bouncing universe" since after a pure oscillating
quantum phase the classical Friedmann spacetime emerges. Perspective
developments and possible tests of this scenario are discussed in the end. | 1612.07116v2 |
2017-07-07 | Interplay between the edge-state magnetism and long-range Coulomb interaction in zigzag graphene nanoribbons: quantum Monte Carlo study | We perform projective quantum Monte Carlo simulations of zigzag graphene
nanoribbons within a realistic model with long-range Coulomb interactions.
Increasing the relative strength of nonlocal interactions with respect to the
on-site repulsion does not generate a phase transition but has a number of
nontrivial effects. At the single-particle level we observe a marked
enhancement of the Fermi velocity at the Dirac points. At the two-particle
level, spin- and charge-density-wave fluctuations compete. As a consequence,
the edge magnetic moment is reduced but the edge dispersion relation increases
in the sense that the single-particle gap at momentum $q=\pi/|{\pmb a}_1|$
grows. We attribute this to nonlocal charge fluctuations which assist the spin
fluctuations to generate the aforementioned gap. In contrast, the net result of
the interaction-induced renormalization of different energy scales is a
constant spin-wave velocity of the edge modes. However, since the particle-hole
continuum is shifted to higher energies---due to the renormalization of the
Fermi velocity---Landau damping is reduced. As a result, a roughly linear
spin-wave-like mode at the edge spreads out through a larger part of the
Brillouin zone. | 1707.02065v2 |
2018-05-08 | Fitting a function to time-dependent ensemble averaged data | Time-dependent ensemble averages, i.e., trajectory-based averages of some
observable, are of importance in many fields of science. A crucial objective
when interpreting such data is to fit these averages (for instance, squared
displacements) with a function and extract parameters (such as diffusion
constants). A commonly overlooked challenge in such function fitting procedures
is that fluctuations around mean values, by construction, exhibit temporal
correlations. We show that the only available general purpose function fitting
methods, correlated chi-square method and the weighted least squares method
(which neglects correlation), fail at either robust parameter estimation or
accurate error estimation. We remedy this by deriving a new closed-form error
estimation formula for weighted least square fitting. The new formula uses the
full covariance matrix, i.e., rigorously includes temporal correlations, but is
free of the robustness issues, inherent to the correlated chi-square method. We
demonstrate its accuracy in four examples of importance in many fields:
Brownian motion, damped harmonic oscillation, fractional Brownian motion and
continuous time random walks. We also successfully apply our method, weighted
least squares including correlation in error estimation (WLS-ICE), to particle
tracking data. The WLS-ICE method is applicable to arbitrary fit functions, and
we provide a publically available WLS-ICE software. | 1805.03057v1 |
2018-05-22 | Time dilation in the oscillating decay laws of moving two-mass unstable quantum states | The decay of a moving system is studied in case the system is initially
prepared in a two-mass unstable quantum state. The survival probability
$\mathcal{P}_p(t)$ is evaluated over short and long times in the reference
frame where the unstable system moves with constant linear momentum $p$. The
mass distribution densities of the two mass states are tailored as power laws
with powers $\alpha_1$ and $\alpha_2$ near the non-vanishing lower bounds
$\mu_{0,1}$ and $\mu_{0,2}$ of the mass spectra, respectively. If the powers
$\alpha_1$ and $\alpha_2$ differ, the long-time survival probability
$\mathcal{P}_p(t)$ exhibits a dominant inverse-power-law decay and is
approximately related to the survival probability at rest $\mathcal{P}_0(t)$ by
a time dilation. The corresponding scaling factor $\chi_{p,k}$ reads
$\sqrt{1+p^2/\mu_{0,k}^2}$, the power $\alpha_k$ being the lower of the powers
$\alpha_1$ and $\alpha_2$. If the two powers coincide and the lower bounds
$\mu_{0,1}$ and $\mu_{0,2}$ differ, the scaling relation is lost and damped
oscillations of the survival probability $\mathcal{P}_p(t)$ appear over long
times. By changing reference frame, the period $T_0$ of the oscillations at
rest transforms in the longer period $T_p$ according to a factor which is the
weighted mean of the scaling factors of each mass, with non-normalized weights
$\mu_{0,1}$ and $\mu_{0,2}$. | 1805.08335v1 |
2018-05-23 | The classical D-type expansion of spherical H II regions | Recent numerical and analytic work has highlighted some shortcomings in our
understanding of the dynamics of H II region expansion, especially at late
times, when the H II region approaches pressure equilibrium with the ambient
medium. Here we reconsider the idealized case of a constant radiation source in
a uniform and spherically symmetric ambient medium, with an isothermal equation
of state. A thick-shell solution is developed which captures the stalling of
the ionization front and the decay of the leading shock to a weak compression
wave as it escapes to large radii. An acoustic approximation is introduced to
capture the late-time damped oscillations of the H II region about the
stagnation radius. Putting these together, a matched asymptotic equation is
derived for the radius of the ionization front which accounts for both the
inertia of the expanding shell and the finite temperature of the ambient
medium. The solution to this equation is shown to agree very well with the
numerical solution at all times, and is superior to all previously published
solutions. The matched asymptotic solution can also accurately model the
variation of H II region radius for a time-varying radiation source. | 1805.09273v1 |
2018-05-24 | Quantum surface diffusion in Bohmian Mechanics | Surface diffusion of small adsorbates is analyzed in terms of the so-called
intermediate scattering function and dynamic structure factor, observables in
experiments using the well-known quasielastic Helium atom scattering and Helium
spin echo techniques. The linear theory used is an extension of the neutron
scattering due to van Hove and considers the time evolution of the position of
the adsorbates in the surface. This approach allows us to use a stochastic
trajectory description following the classical, quantum and Bohmian frameworks.
Three regimes of motion are clearly identified in the diffusion process:
ballistic, Brownian and intermediate which are well characterized, for the
first two regimes, through the mean square displacements and Einstein relation
for the diffusion constant. The Langevin formalism is used by considering Ohmic
friction, moderate surface temperatures and small coverages. In the Bohmian
framework, the starting point is the so-called Schr\"odinger-Langevin equation
which is a nonlinear, logarithmic differential equation. By assuming a Gaussian
function for the probability density, the corresponding quantum stochastic
trajectories are given by a dressing scheme consisting of a classical
stochastic trajectory of the center of the Gaussian wave packet, issued from
solving the Langevin equation (particle property), plus the time evolution of
its width governed by the damped Pinney differential equation (wave property).
The velocity autocorrelation function is the same as the classical one when the
initial spread rate is assumed to be zero. If not, in the diffusion regime, the
Brownian-Bohmian motion shows a weak anomalous diffusion. | 1805.09536v5 |
2019-03-27 | Field-free spin-orbit-torque switching of perpendicular magnetization aided by uniaxial shape anisotropy | It has been demonstrated that the switching of perpendicular magnetization
can be achieved with spin orbit torque (SOT) at an ultrafast speed and low
energy consumption. However, to make the switching deterministic, an
undesirable magnetic field or unconventional device geometry is required to
break the structure inverse symmetry. Here we propose a novel scheme for
SOT-induced field-free deterministic switching of perpendicular magnetization.
The proposed scheme can be implemented in a simple magnetic tunnel junction
(MTJ) /heavy-metal system, without the need of complicated device structure.
The perpendicular-anisotropy MTJ is patterned into elliptical shape and
misaligned with the axis of the heavy metal, so that the uniaxial shape
anisotropy aids the magnetization switching. Furthermore, unlike the
conventional switching scheme where the switched final magnetization state is
dependent on the direction of the applied current, in our scheme the bipolar
switching is implemented by choosing different current paths, which offers a
new freedom for developing novel spintronics memories or logic devices. Through
the macrospin simulation, we show that a wide operation window of the applied
current pulse can be obtained in the proposed scheme. The precise control of
pulse amplitude or pulse duration is not required. The influences of key
parameters such as damping constant and field-like torque strength are
discussed as well. | 1903.11487v1 |
2019-12-18 | Tidal evolution of circumbinary systems with arbitrary eccentricities: applications for Kepler systems | We present an extended version of the Constant Time Lag analytical approach
for the tidal evolution of circumbinary planets introduced in our previous
work. The model is self-consistent, in the sense that all tidal interactions
between pairs are computed, regardless of their size. We derive analytical
expressions for the variational equations governing the spin and orbital
evolution, which are expressed as high-order elliptical expansions in the
semimajor axis ratio but retain closed form in terms of the binary and
planetary eccentricities. These are found to reproduce the results of the
numerical simulations with arbitrary eccentricities very well, as well as
reducing to our previous results in the low-eccentric case. Our model is then
applied to the well-characterised Kepler circumbinary systems by analysing the
tidal timescales and unveiling the tidal flow around each different system. In
all cases we find that the spins reach stationary values much faster than the
characteristic timescale of the orbital evolution, indicating that all Kepler
circumbinary planets are expected to be in a sub-synchronous state. On the
other hand, all systems are located in a tidal flow leading to outward
migration; thus the proximity of the planets to the orbital instability limit
may have been even greater in the past. Additionally, Kepler systems may have
suffered a significant tidally induced eccentricity damping, which may be
related to their proximity to the capture eccentricity. To help understand the
predictions of our model, we also offer a simple geometrical interpretation of
our results. | 1912.08728v1 |
2020-01-01 | Nonequilibrium Landau-Zener Tunneling in Exciton-Polariton Condensates | For a coherent quantum mechanical two-level system driven with a linearly
time-dependent detuning, the Landau-Zener model has served over decades as a
textbook model of quantum dynamics. A particularly intriguing question is
whether that framework can be extended to capture an intrinsical nonequilibrium
nature for a quantum system with coherent and dissipative dynamics occurring on
an equal footing. In this work, we are motivated to investigate the
Landau-Zenner problem of polariton condensates in a periodic potential under
nonresonant pumping, considering driven-dissipative Gross-Pitaevskii equations
coupled to the rate equation of a reservoir. Using a two-mode approach, we find
fluctuation of the reservoir can be considered as a constant and the relative
phase plays a very important role. The evolution of the dissipative
Landau-Zener model we obtain presents its adiabatic process very different from
the closed system because the fluctuation of the reservoir has a peak and leads
to the damping of the condensates. We substitute the fluctuation of the
reservoir to Hamiltonian and get an effective two-level model. The motion of
Hamiltonian in phase space is also discussed and is directly corresponding to
the pumping rate. The instability of the band structure can also be studied by
the curvatures in phase space and there may be two loops in the middle of the
Brillouin zone when the pumping rate is far beyond the threshold. | 2001.00151v1 |
2020-01-14 | A bound on the 12C/13C ratio in near-pristine gas with ESPRESSO | Using science verification observations obtained with ESPRESSO at the Very
Large Telescope (VLT) in 4UT mode, we report the first bound on the carbon
isotope ratio 12C/13C of a quiescent, near-pristine damped Ly-alpha (DLA)
system at z=2.34. We recover a limit log10(12C/13C) > +0.37 (2 sigma). We use
the abundance pattern of this DLA, combined with a stochastic chemical
enrichment model, to infer the properties of the enriching stars, finding the
total gas mass of this system to be log10(M_gas/M_sun)=6.3+1.4-0.9 and the
total stellar mass to be log10(M_*/M_sun)=4.8+/-1.3. The current observations
disfavour enrichment by metal-poor Asymptotic Giant Branch (AGB) stars with
masses <2.4 Msun, limiting the epoch at which this DLA formed most of its
enriching stars. Our modelling suggests that this DLA formed very few stars
until >1 Gyr after the cosmic reionization of hydrogen and, despite its very
low metallicity (~1/1000 of solar), this DLA appears to have formed most of its
stars in the past few hundred Myr. Combining the inferred star formation
history with evidence that some of the most metal-poor DLAs display an elevated
[C/O] ratio at redshift z<3, we suggest that very metal-poor DLAs may have been
affected by reionization quenching. Finally, given the simplicity and
quiescence of the absorption features associated with the DLA studied here, we
use these ESPRESSO data to place a bound on the possible variability of the
fine-structure constant, Delta alpha/alpha=(-1.2 +/- 1.1)x10^-5. | 2001.04983v1 |
2020-01-14 | Limits on Mode Coherence in Pulsating DA White Dwarfs Due to a Non-static Convection Zone | The standard theory of pulsations deals with the frequencies and growth rates
of infinitesimal perturbations in a stellar model. Modes which are calculated
to be linearly driven should increase their amplitudes exponentially with time;
the fact that nearly constant amplitudes are usually observed is evidence that
nonlinear mechanisms inhibit the growth of finite amplitude pulsations. Models
predict that the mass of convection zones in pulsating hydrogen-atmosphere
(DAV) white dwarfs is very sensitive to temperature (i.e., $M_{\rm CZ} \propto
T_{\rm eff}^{-90}$), leading to the possibility that even low-amplitude
pulsators may experience significant nonlinear effects. In particular, the
outer turning point of finite-amplitude g-mode pulsations can vary with the
local surface temperature, producing a reflected wave that is out of phase with
what is required for a standing wave. This can lead to a lack of coherence of
the mode and a reduction in its global amplitude. In this paper we show that:
(1) whether a mode is calculated to propagate to the base of the convection
zone is an accurate predictor of its width in the Fourier spectrum, (2) the
phase shifts produced by reflection from the outer turning point are large
enough to produce significant damping, and (3) amplitudes and periods are
predicted to increase from the blue edge to the middle of the instability
strip, and subsequently decrease as the red edge is approached. This amplitude
decrease is in agreement with the observational data while the period decrease
has not yet been systematically studied. | 2001.05048v1 |
2020-01-30 | An auto-parameter denoising method for nuclear magnetic resonance spectroscopy based on low-rank Hankel matrix | Nuclear Magnetic Resonance (NMR) spectroscopy, which is modeled as the sum of
damped exponential signals, has become an indispensable tool in various
scenarios, such as the structure and function determination, chemical analysis,
and disease diagnosis. NMR spectroscopy signals, however, are usually corrupted
by Gaussian noise in practice, raising difficulties in sequential analysis and
quantification of the signals. The low-rank Hankel property plays an important
role in the denoising issue, but selecting an appropriate parameter still
remains a problem. In this work, we explore the effect of the regularization
parameter of a convex optimization denoising method based on low-rank Hankel
matrices for exponential signals corrupted by Gaussian noise. An accurate
estimate on the spectral norm of weighted Hankel matrices is provided as a
guidance to set the regularization parameter. The bound can be efficiently
calculated since it only depends on the standard deviation of the noise and a
constant. Aided by the bound, one can easily obtain an auto-setting
regularization parameter to produce promising denoised results. Our experiments
on synthetic and realistic NMR spectroscopy data demonstrate a superior
denoising performance of our proposed approach in comparison with the typical
Cadzow and the state-of-the-art QR decomposition methods, especially in the low
signal-to-noise ratio regime. | 2001.11815v2 |
2020-02-26 | Velocity-coherent Filaments in NGC 1333: Evidence for Accretion Flow? | Recent observations of global velocity gradients across and along molecular
filaments have been interpreted as signs of gas accreting onto and along these
filaments, potentially feeding star-forming cores and proto-clusters. The
behavior of velocity gradients in filaments, however, has not been studied in
detail, particularly on small scales (< 0.1 pc). In this paper, we present
MUFASA, an efficient, robust, and automatic method to fit ammonia lines with
multiple velocity components, generalizable to other molecular species. We also
present CRISPy, a Python package to identify filament spines in 3D images
(e.g., position-position-velocity cubes), along with a complementary technique
to sort fitted velocity components into velocity-coherent filaments. In NGC
1333, we find a wealth of velocity gradient structures on a beam-resolved scale
of ~0.05 pc. Interestingly, these local velocity gradients are not randomly
oriented with respect to filament spines and their perpendicular, i.e., radial,
component decreases in magnitude towards the spine for many filaments. Together
with remarkably constant velocity gradients on larger scales along many
filaments, these results suggest a scenario in which gas falling onto filaments
is progressively damped and redirected to flow along these filaments. | 2002.11736v1 |
2020-06-08 | Detection and parameter estimation of binary neutron star merger remnants | Detection and parameter estimation of binary neutron star merger remnants can
shed light on the physics of hot matter at supranuclear densities. Here we
develop a fast, simple model that can generate gravitational waveforms, and
show it can be used for both detection and parameter estimation of post-merger
remnants. The model consists of three exponentially-damped sinusoids with a
linear frequency-drift term. The median fitting factors between the model
waveforms and numerical-relativity simulations exceed 0.90. We detect remnants
at a post-merger signal-to-noise ratio of $\ge 7$ using a Bayes-factor
detection statistic with a threshold of 3000. We can constrain the primary
post-merger frequency to $\pm_{1.2}^{1.4}\%$ at post-merger signal-to-noise
ratios of 15 with an increase in precision to $\pm_{0.2}^{0.3}\%$ for
post-merger signal-to-noise ratios of 50. The tidal coupling constant can be
constrained to $\pm^{9}_{12}\%$ at post-merger signal-to-noise ratios of 15,
and $\pm 5\%$ at post-merger signal-to-noise ratios of 50 using a hierarchical
inference model. | 2006.04396v1 |
2020-07-29 | Dynamics of antiferromagnetic skyrmion in absence and presence of pinning defect | A theoretical study on the dynamics of an antiferromagnetic (AFM) skyrmion is
indispensable for revealing the underlying physics and understanding the
numerical and experimental observations. In this work, we present a reliable
theoretical treatment of the spin current induced motion of an AFM skyrmion in
the absence and presence of pinning defect. For an ideal AFM system free of
defect, the skyrmion motion velocity as a function of the intrinsic parameters
can be derived, based on the concept that the skyrmion profile agrees well with
the 360 domain wall formula, leading to an explicit description of the skyrmion
dynamics. However, for an AFM lattice containing a defect, the skyrmion can be
pinned and the depinning field as a function of damping constant and pinning
strength can be described by the Thiele approach. It is revealed that the
depinning behavior can be remarkably influenced by the time dependent
oscillation of the skyrmion trajectory. The present theory provides a
comprehensive scenario for manipulating the dynamics of AFM skyrmion,
informative for future spintronic applications based on antiferromagnets. | 2007.14562v1 |
2020-09-29 | The one-dimensional stochastic Keller--Segel model with time-homogeneous spatial Wiener processes | Chemotaxis is a fundamental mechanism of cells and organisms, which is
responsible for attracting microbes to food, embryonic cells into developing
tissues, or immune cells to infection sites. Mathematically chemotaxis is
described by the Patlak--Keller--Segel model. This macroscopic system of
equations is derived from the microscopic model when limiting behaviour is
studied. However, on taking the limit and passing from the microscopic
equations to the macroscopic equations, fluctuations are neglected. Perturbing
the system by a Gaussian random field restitutes the inherent randomness of the
system. This gives us the motivation to study the classical
Patlak--Keller--Segel system perturbed by random processes.
We study a stochastic version of the classical Patlak--Keller--Segel system
under homogeneous Neumann boundary conditions on an interval
$\mathcal{O}=[0,1]$. In particular, let $\mathcal{W}_1$, $\mathcal{W}_2$ be two
time-homogeneous spatial Wiener processes over a filtered probability space
$\mathfrak{A}$. Let $u$ and $v$ denote the cell density and concentration of
the chemical signal. We investigate the coupled system \begin{align*} & d {u} -
( r_u\Delta u- \chi {\rm div }( u\nabla v) )\, dt =u\circ d\mathcal{W}_1, \\ &
d{v} -(r_v \Delta v -\alpha v)\, dt = \beta u \, dt+ v\circ d\mathcal{W}_2,
\end{align*} with initial conditions $(u(0),v(0))=(u_0,v_0)$. The positive
terms $r_u$ and $r_v$ are the diffusivity of the cells and chemoattractant,
respectively, the positive value $\chi$ is the chemotactic sensitivity,
$\alpha\ge0$ is the so-called damping constant. The noise is interpreted in the
Stratonovich sense. Given $T>0$, we will prove the existence of a martingale
solution on $[0,T]$. | 2009.13789v1 |
2020-10-15 | Delayed bifurcation in elastic snap-through instabilities | We study elastic snap-through induced by a control parameter that evolves
dynamically. In particular, we study an elastic arch subject to an
end-shortening that evolves linearly with time, i.e. at a constant rate. For
large end-shortening the arch is bistable but, below a critical end-shortening,
the arch becomes monostable. We study when and how the arch transitions between
states and show that the end-shortening at which the fast 'snap' happens
depends on the rate at which the end-shortening is reduced. This lag in
snap-through is a consequence of delayed bifurcation and occurs even in the
perfectly elastic case when viscous (and viscoelastic) effects are negligible.
We present the results of numerical simulations to determine the magnitude of
this lag as the loading rate and the importance of external viscous damping
vary. We also present an asymptotic analysis of the geometrically-nonlinear
problem that reduces the salient dynamics to that of an ordinary differential
equation; the form of this reduced equation is generic for snap-through
instabilities in which the relevant control parameter is ramped linearly in
time. Moreover, this asymptotic reduction allows us to derive analytical
results for the observed lag in snap-through that are in good agreement with
the numerical results of our simulations. Finally, we discuss scaling laws for
the lag that should be expected in other examples of delayed bifurcation in
elastic instabilities. | 2010.07850v1 |
2020-10-29 | Connecting cosmological accretion to strong Lyman-alpha absorbers | We present an analytical model for the cosmological accretion of gas onto
dark matter halos, based on a similarity solution applicable to spherical
systems. Performing simplified radiative transfer, we compute how the accreting
gas turns increasingly neutral as it self-shields from the ionising background,
and obtain the column density, $N_{\rm HI}$, as a function of impact parameter.
The resulting column-density distribution function (CDDF) is in excellent
agreement with observations. The analytical expression elucidates (1) why halos
over a large range in mass contribute about equally to the CDDF as well as (2)
why the CDDF evolves so little with redshift in the range $z=2\rightarrow 5$.
We show that the model also predicts reasonable DLA line-widths ($v_{90}$),
bias and molecular fractions. Integrating over the CDDF yields the mass density
in neutral gas, $\Omega_{\rm HI}$, which agrees well with observations.
$\Omega_{\rm HI}(z)$ is nearly constant even though the accretion rate onto
halos evolves. We show that this occurs because the fraction of time that the
inflowing gas is neutral depends on the dynamical time of the halo, which is
inversely proportional to the accretion rate. Encapsulating results from
cosmological simulations, the simple model shows that most Lyman-limit system
and damped Lyman-alpha absorbers are associated with the cosmological accretion
of gas onto halos. | 2010.15857v1 |
2020-11-25 | Early modified gravity in light of the $H_0$ tension and LSS data | We present a model of Early Modified Gravity (EMG) consisting in a scalar
field $\sigma$ with a non-minimal coupling to the Ricci curvature of the type
$M^2_{\rm pl}+\xi \sigma^2$ plus a cosmological constant and a small effective
mass and demonstrate its ability to alleviate the $H_0$ tension while providing
a good fit to Cosmic Microwave Background (CMB) anisotropies and Baryon
Acoustic Oscillations (BAO) data. In this model the scalar field, frozen deep
in the radiation era, grows around the redshift of matter-radiation equality
because of the coupling to non-relativistic matter. The small effective mass,
which we consider here as induced by a quartic potential, then damps the scalar
field into coherent oscillations around its minimum at $\sigma=0$, leading to a
weaker gravitational strength at early times and naturally recovering the
consistency with laboratory and Solar System tests of gravity. We analyze the
capability of EMG with positive $\xi$ to fit current cosmological observations
and compare our results to the case without an effective mass and to the
popular early dark energy models with $\xi=0$. We show that EMG with a quartic
coupling of the order of $\lambda\sim\mathcal{O}({\rm eV}^4/M_{\rm pl}^4)$ can
substantially alleviate the $H_0$ tension also when the full shape of the
matter power spectrum is included in the fit in addition to CMB and Supernovae
(SN) data. | 2011.12934v2 |
2021-01-16 | Excitation and evolution of coronal oscillations in self-consistent 3D radiative MHD simulations of the solar atmosphere | Solar coronal loops are commonly subject to oscillations. Observations of
coronal oscillations are used to infer physical properties of the coronal
plasma using coronal seismology. Excitation and evolution of oscillations in
coronal loops is typically studied using highly idealised models of magnetic
flux-tubes. In order to improve our understanding of coronal oscillations, it
is necessary to consider the effect of realistic magnetic field topology and
evolution. We study excitation and evolution of coronal oscillations in
three-dimensional self-consistent simulations of solar atmosphere spanning from
convection zone to solar corona using radiation-MHD code Bifrost. We use
forward-modelled EUV emission and three-dimensional tracing of magnetic field
to analyse oscillatory behaviour of individual magnetic loops. We further
analyse the evolution of individual plasma velocity components along the loops
using wavelet power spectra to capture changes in the oscillation periods.
Various types of oscillations commonly observed in the corona are present in
the simulation. We detect standing oscillations in both transverse and
longitudinal velocity components, including higher order oscillation harmonics.
We also show that self-consistent simulations reproduce existence of two
distinct regimes of transverse coronal oscillations: rapidly decaying
oscillations triggered by impulsive events and sustained small-scale
oscillations showing no observable damping. No harmonic drivers are detected at
the footpoints of oscillating loops. We show that coronal loop oscillations are
abundant in self-consistent 3D MHD simulations of the solar atmosphere. The
dynamic evolution and variability of individual magnetic loops suggest we need
to reevaluate our models of monolithic and static coronal loops with constant
lengths in favour of more realistic models. | 2101.06430v1 |
2021-02-17 | Linear Nearest Neighbor Flocks with All Distinct Agents | This paper analyzes the global dynamics of 1-dimensional agent arrays with
nearest neighbor linear couplings. The equations of motion are second order
linear ODEs with constant coeffcients. The novel part of this research is that
the couplings are different for each distinct agent. We allow the forces to
depend on the positions and velocity (damping terms) but the magnitudes of both
the position and velocity couplings are different for each agent. We, also, do
not assume that the forces are "Newtonian" (i.e. the force due to A on B equals
the minus the force of B on A) as this assumption does not apply to certain
situations, such as traffic modeling. For example, driver A reacting to driver
B does not imply the opposite reaction in driver B. There are no known
analytical means to solve these systems, even though they are linear, and so
relatively little is known about them. This paper is a generalization of
previous work that computed the global dynamics of 1-dimensional sequences of
identical agents [3] assuming periodic boundary conditions. In this paper, we
push that method further, similar to [2], and use an extended periodic boundary
condition to to gain quantitative insights to the systems under consideration.
We find that we can approximate the global dynamics of such a system by
carefully analyzing the low-frequency behavior of the system with (generalized)
periodic boundary conditions. | 2102.09020v1 |
2021-03-18 | The APOGEE Data Release 16 Spectral Line List | The updated H-band spectral line list (from \lambda 15,000 - 17,000\AA)
adopted by the Apache Point Observatory Galactic Evolution Experiment (APOGEE)
for the SDSS IV Data Release 16 (DR16) is presented here. The APOGEE line list
is a combination of atomic and molecular lines with data from laboratory,
theoretical, and astrophysical sources. Oscillator strengths and damping
constants are adjusted using high signal-to-noise, high-resolution spectra of
the Sun and alpha Boo (Arcturus) as "standard stars". Updates to the DR16 line
list, when compared to the previous DR14 version, are the inclusion of
molecular H_2O and FeH lines, as well as a much larger (by a factor of ~4)
atomic line list, which includes significantly more transitions with hyperfine
splitting. More recent references and line lists for the crucial molecules CO
and OH were used, as well as for C_2 and SiH. In contrast to DR14, DR16
contains measurable lines from the heavy neutron-capture elements cerium (as Ce
II), neodymium (as Nd II), and ytterbium (as Yb II), as well as one line from
rubidium (as Rb I), that may be detectable in a small fraction of APOGEE red
giants. | 2103.10112v1 |
2021-03-18 | Soft mode theory of ferroelectric phase transitions in the low-temperature phase | Historically, the soft mode theory of ferroelectric phase transitions has
been developed for the high-temperature (paraelectric) phase, where the phonon
mode softens upon decreasing the temperature. In the low-temperature
ferroelectric phase, a similar phonon softening occurs, also leading to a
bosonic condensation of the frozen-in mode at the transition, but in this case
the phonon softening occurs upon increasing the temperature. Here we present a
soft mode theory of ferroelectric and displacive phase transitions by
describing what happens in the low-temperature phase in terms of phonon
softening and instability. A new derivation of the generalized
Lyddane-Sachs-Teller (LST) relation for materials with strong anharmonic phonon
damping is also presented which leads to the expression
$\varepsilon_{0}/\varepsilon_{\infty}=|\omega_{LO}|^{2}/|\omega_{TO}|^{2}$. The
theory provides a microscopic expression for $T_c$ as a function of physical
parameters, including the mode specific Gr\"uneisen parameter. The theory also
shows that $\omega_{TO} \sim (T_{c}-T)^{1/2}$, and again specifies the
prefactors in terms of Gr\"uneisen parameter and fundamental physical
constants. Using the generalized LST relation, the softening of the TO mode
leads to the divergence of $\epsilon_0$ and to a polarization catastrophe at
$T_c$. A quantitative microscopic form of the Curie-Weiss law is derived with
prefactors that depend on microscopic physical parameters. | 2103.10262v1 |
2021-03-23 | High-order implicit time integration scheme based on Padé expansions | A single-step high-order implicit time integration scheme for the solution of
transient and wave propagation problems is presented. It is constructed from
the Pad\'e expansions of the matrix exponential solution of a system of
first-order ordinary differential equations formulated in the state-space. A
computationally efficient scheme is developed exploiting the techniques of
polynomial factorization and partial fractions of rational functions, and by
decoupling the solution for the displacement and velocity vectors. An important
feature of the novel algorithm is that no direct inversion of the mass matrix
is required. From the diagonal Pad\'e expansion of order $M$ a time-stepping
scheme of order $2M$ is developed. Here, each elevation of the accuracy by two
orders results in an additional system of real or complex sparse equations to
be solved. These systems are comparable in complexity to the standard Newmark
method, i.e., the effective system matrix is a linear combination of the static
stiffness, damping, and mass matrices. It is shown that the second-order scheme
is equivalent to Newmark's constant average acceleration method, often also
referred to as trapezoidal rule. The proposed time integrator has been
implemented in MATLAB using the built-in direct linear equation solvers. In
this article, numerical examples featuring nearly one million degrees of
freedom are presented. High-accuracy and efficiency in comparison with common
second-order time integration schemes are observed. The MATLAB-implementation
is available from the authors upon request or from the GitHub repository (to be
added). | 2103.12282v1 |
2021-07-13 | Tuning the Optical Properties of an MoSe$_2$ Monolayer Using Nanoscale Plasmonic Antennas | Nanoplasmonic systems combined with optically-active two-dimensional
materials provide intriguing opportunities to explore and control light-matter
interactions at extreme sub-wavelength lengthscales approaching the exciton
Bohr radius. Here, we present room- and cryogenic-temperature investigations of
light-matter interactions between an MoSe$_2$ monolayer and individual
lithographically defined gold dipole nanoantennas having sub-10 nm feed gaps.
By progressively tuning the nanoantenna size, their dipolar resonance is tuned
relative to the A-exciton transition in a proximal MoSe$_2$ monolayer achieving
a total tuning of $\sim 130\;\mathrm{meV}$. Differential reflectance
measurements performed on $> 100$ structures reveal an apparent avoided
crossing between exciton and dipolar mode and an exciton-plasmon coupling
constant of $g= 55\;\mathrm{meV}$, representing $g/(\hbar\omega_X)\geq3\%$ of
the transition energy. This places our hybrid system in the
intermediate-coupling regime where spectra exhibit a characteristic Fano-like
shape, indicative of the interplay between pronounced light-matter coupling and
significant damping. We also demonstrate active control of the optical response
by varying the polarization of the excitation light to programmably suppress
coupling to the dipole mode. We further study the emerging optical signatures
of the monolayer localized at dipole nanoantennas at $10\;\mathrm{K}$. Our
findings represent a key step towards realizing non-linear photonic devices
based on 2D materials with potential for low-energy and ultrafast performance. | 2107.06410v2 |
2021-09-10 | Electrical spectroscopy of the spin-wave dispersion and bistability in gallium-doped yttrium iron garnet | Yttrium iron garnet (YIG) is a magnetic insulator with record-low damping,
allowing spin-wave transport over macroscopic distances. Doping YIG with
gallium ions greatly reduces the demagnetizing field and introduces a
perpendicular magnetic anisotropy, which leads to an isotropic spin-wave
dispersion that facilitates spin-wave optics and spin-wave steering. Here, we
characterize the dispersion of a gallium-doped YIG (Ga:YIG) thin film using
electrical spectroscopy. We determine the magnetic anisotropy parameters from
the ferromagnetic resonance frequency and use propagating spin wave
spectroscopy in the Damon-Eshbach configuration to detect the small spin-wave
magnetic fields of this ultrathin weak magnet over a wide range of wavevectors,
enabling the extraction of the exchange constant $\alpha=1.3(2)\times10^{-12}$
J/m. The frequencies of the spin waves shift with increasing drive power, which
eventually leads to the foldover of the spin-wave modes. Our results shed light
on isotropic spin-wave transport in Ga:YIG and highlight the potential of
electrical spectroscopy to map out the dispersion and bistability of
propagating spin waves in magnets with a low saturation magnetization. | 2109.05045v1 |
2021-09-17 | Adaptive Steering Control for Steer-by-Wire Systems | Steer-by-Wire (SBW) systems are being adapted widely in semi-autonomous and
fully autonomous vehicles. The main control challenge in a SBW system is to
follow the steering commands in the face of parametric uncertainties, external
disturbances and actuator delay; crucially, perturbations in inertial
parameters and damping forces give rise to state-dependent uncertainties, which
cannot be bounded a priori by a constant. However, the state-of-the-art control
methods of SBW system rely on a priori bounded uncertainties, and thus, become
inapplicable when state-dependent dynamics become unknown. In addition,
ensuring tracking accuracy under actuator delay is always a challenging task.
This work proposes two control frameworks to overcome these challenges.
Firstly, an adaptive controller is proposed to tackle the state-dependent
uncertainties and external disturbances in a typical SBW system without any a
priori knowledge of their structures and of their bounds. The stability of the
closed-loop system is studied analytically via uniformly ultimately bounded
notion and the effectiveness of the proposed solution is verified via
simulations against the state-of-the-art solution. While this proposed
controller handles the uncertainties and external perturbations, it does not
consider the actuator delay which sometimes result in decreased accuracy.
Therefore, a new adaptive-robust control framework is devised to tackle the
same control problem of an SBW system under the influence of time-varying input
delay. In comparison to the existing strategies, the proposed framework removes
the conservative assumption of a priori bounded uncertainty and, in addition,
the Razumikhin theorem based stability analysis allows the proposed scheme to
deal with arbitrary variation in input delay. The effectiveness of the both
controllers is proved using comparative simulation studies. | 2109.08380v1 |
2021-11-04 | Momentum-space decoherence of distinguishable and identical particles in the Caldeira-Leggett formalism | In this work, momentum-space decoherence using minimum and
nonminimum-uncertainty-product (stretched) Gaussian wave packets in the
framework of Caldeira-Leggett formalism and under the presence of a linear
potential is studied. As a dimensionless measure of decoherence, purity, a
quantity appearing in the definition of the {\it linear entropy}, is studied
taking into account the role of the stretching parameter. Special emphasis is
on the open dynamics of the well-known cat states and bosons and fermions
compared to distinguishable particles. For the cat state, while the stretching
parameter speeds up the decoherence, the external linear potential strength
does not affect the decoherence time; only the interference pattern is shifted.
Furthermore, the interference pattern is not observed for
minimum-uncertainty-product-Gaussian wave packets in the momentum space.
Concerning bosons and fermions, the question we have addressed is how the
symmetry of the wave functions of indistinguishable particles is manifested in
the decoherence process, which is understood here as the loss of being
indistinguishable due to the gradual emergence of classical statistics with
time. We have observed that the initial bunching and anti-bunching character of
bosons and fermions, respectively, in the momentum space are not preserved as a
function of the environmental parameters, temperature and damping constant.
However, fermionic distributions are slightly broader than the distinguishable
ones and these similar to the bosonic distributions. This general behavior
could be interpreted as a residual reminder of the symmetry of the wave
functions in the momentum space for this open dynamics. | 2111.03127v1 |
2022-01-20 | Oxygen-enhanced extremely metal-poor DLAs: A signpost of the first stars? | We present precise abundance determinations of two near-pristine damped
Ly$\alpha$ systems (DLAs) to assess the nature of the [O/Fe] ratio at [Fe/H] <
-3 (i.e. <1/1000 of the solar metallicity). Prior observations indicate that
the [O/Fe] ratio is consistent with a constant value, [O/Fe] ~ +0.4, when -3 <
[Fe/H] < -2, but this ratio may increase when [Fe/H] < -3. In this paper, we
test this picture by reporting new, high-precision [O/Fe] abundances in two of
the most metal-poor DLAs currently known. We derive values of [O/Fe] = +0.50
+/- 0.10 and [O/Fe] = +0.62 +/- 0.05 for these two z ~ 3 near-pristine gas
clouds. These results strengthen the idea that the [O/Fe] abundances of the
most metal-poor DLAs are elevated compared to DLAs with [Fe/H] > -3. We compare
the observed abundance pattern of the latter system to the nucleosynthetic
yields of Population III supernovae (SNe), and find that the enrichment can be
described by a (19-25) M$_{\odot}$ Population III SN that underwent a
(0.9-2.4)$\times 10^{51}$ erg explosion. These high-precision measurements
showcase the behaviour of [O/Fe] in the most metal-poor environments. Future
high-precision measurements in new systems will contribute to a firm detection
of the relationship between [O/Fe] and [Fe/H]. These data will reveal whether
we are witnessing a chemical signature of enrichment from Population III stars
and allow us to rule out contamination from Population II stars. | 2201.08394v1 |
2022-02-18 | Massive neutrino self-interactions with a light mediator in cosmology | Nonstandard self-interactions can alter the evolution of cosmological
neutrinos, mainly by damping free streaming, which should leave traces in
cosmological observables. Although overall effects are opposite to those
produced by neutrino mass and a larger $N_{\rm eff}$, they cannot be totally
canceled by these last. We harness cosmological data that includes Cosmic
Microwave Background from Plank 2018, BAO measurements, local $H_0$,
Ly-$\alpha$ and SNIa, to constrain massive neutrino self-interactions with a
very light scalar mediator. We find that the effective coupling constant, at
the 95\% C.L., should be $g_{\rm eff}< 1.94 \times 10^{-7}$ for only Planck
2018 data and $1.97\times10^{-7}$ when Planck + BAO are considered. This bound
relaxes to $2.27\times 10^{-7}$ ($2.3\times 10^{-7}$) for $H_0$
($H_0$+SNe+Ly-$\alpha$) data. Using the Planck + BAO dataset, the $H_0$ tension
lowers from 4.3$\sigma$ (for $\Lambda$CDM) to 3.2$\sigma$. The Akaike
Information Criterion penalizes the self-interacting model due to its larger
parameter space for Plank or Planck + BAO data, but favors the interacting
model when we use local $H_0$ measurements. A somewhat larger value for $H_0$
is preferred when we include the whole data pool, which comes accompanied with
a larger value of $N_{\rm eff}$ and a more constricted bound on $\Sigma m_\nu$. | 2202.09310v2 |
2022-02-16 | Egg-speriments: Stretch, crack, and spin | Eggs are key ingredients in our kitchens because of their nutritional values
and functional properties such as foaming, emulsifying and gelling, offering a
wide variety of culinary achievements. They also constitute ideal objects to
illustrate a myriad of scientific concepts. In this article, we focus on
several experiments (egg-speriments) that involve the singular properties of
the liquids contained inside the eggshell, especially the egg white. We first
characterize the rheology of an egg white in a rotational rheometer for
constant and oscillatory shear stresses revealing its shear-thinning behavior
and visco-elastic properties. Then, we measure the tendency of the fluid to
generate very long filaments when stretched that we relate to the shear modulus
of the material. Second, we explore the anisotropic crack pattern that forms on
a thin film of egg white after it is spread on a surface and let dried. The
anisotropy results from the long protein chains present in the egg white which
are straightened during film deposition. Finally, we consider the "spin test"
that permits to distinguish between raw and hard-boiled eggs. To do so, we
measure the residual rotation of a spinning raw egg after a short stop which
reflects the continuation of the internal flow. These observations are
interpreted in terms of viscous damping of the internal flow consistently with
the measurements deduced from rheology. | 2202.10243v1 |
2022-03-15 | Thermodynamic engine powered by anisotropic fluctuations | The purpose of this work is to present the concept of an autonomous
Stirling-like engine powered by anisotropy of thermodynamic fluctuations.
Specifically, simultaneous contact of a thermodynamic system with two heat
baths along coupled degrees of freedom generates torque and circulatory
currents -- an arrangement referred to as a Brownian gyrator. The embodiment
that constitutes the engine includes an inertial wheel to sustain rotary motion
and average out the generated fluctuating torque, ultimately delivering power
to an external load. We detail an electrical model for such an engine that
consists of two resistors in different temperatures and three reactive elements
in the form of variable capacitors. The resistors generate Johnson-Nyquist
current fluctuations that power the engine, while the capacitors generate
driving forces via a coupling of their dielectric material with the inertial
wheel. A proof-of-concept is established via stability analysis to ensure the
existence of a stable periodic orbit generating sustained power output. We
conclude by drawing a connection to the dynamics of a damped pendulum with
constant torque and to those of a macroscopic Stirling engine. The sought
insights aim at nano-engines and biological processes that are similarly
powered by anisotropy in temperature and chemical potentials. | 2203.07573v2 |
2022-03-27 | Giant bulk spin-orbit torque and efficient electrical switching in single ferrimagnetic FeTb layers with strong perpendicular magnetic anisotropy | Efficient manipulation of antiferromagnetically coupled materials that are
integration-friendly and have strong perpendicular magnetic anisotropy (PMA) is
of great interest for low-power, fast, dense magnetic storage and computing.
Here, we report a distinct, giant bulk damping-like spin-orbit torque in
strong-PMA ferrimagnetic Fe100-xTbx single layers that are integration-friendly
(composition-uniform, amorphous, sputter-deposited). For sufficiently-thick
layers, this bulk torque is constant in the efficiency per unit layer
thickness, {\xi}_DL^j/t, with a record-high value of 0.036nm-1, and the
dampinglike torque efficiency {\xi}_DL^j achieves very large values for thick
layers, up to 300% for 90 nm layers. This giant bulk torque by itself switches
tens of nm thick Fe100-xTbx layers that have very strong PMA and high
coercivity at current densities as low as a few MA/cm2. Surprisingly, for a
given layer thickness, {\xi}_DL^j shows strong composition dependence and
becomes negative for composition where the total angular momentum is oriented
parallel to the magnetization rather than antiparallel. Our findings of giant
bulk spin torque efficiency and intriguing torque-compensation correlation will
stimulate study of such unique spin-orbit phenomena in a variety of
ferrimagnetic hosts. This work paves a promising avenue for developing
ultralow-power, fast, dense ferrimagnetic storage and computing devices. | 2203.14193v1 |
2022-04-11 | Diffusion of elastic waves in a continuum solid with a random array of pinned dislocations | The propagation of incoherent elastic energy in a three-dimensional solid due
to the scattering by many, randomly placed and oriented, pinned dislocation
segments, is considered in a continuum mechanics framework. The scattering
mechanism is that of an elastic string of length L that re-radiates as a
response to an incoming wave. The scatterers are thus not static but have their
own dynamics. A Bethe-Salpeter (BS) equation is established, and a
Ward-Takahashi Identity (WTI) is demonstrated. The BS equation is written as a
spectral problem that, using the WTI, is solved in the diffusive limit. To
leading order a diffusion behavior indeed results, and an explicit formula for
the diffusion coeffcient is obtained. It can be evaluated in an Independent
Scattering Approximation (ISA) in the absence of intrinsic damping. It depends
not only on the bare longitudinal and transverse wave velocities but also on
the renormalized velocities, as well as attenuation coeffcients, of the
coherent waves. The influence of the length scale given by L, and of the
resonant behavior for frequencies near the resonance frequency of the strings,
can be explicitly identified. A Kubo representation for the diffusion constant
can be identified. Previous generic results, obtained with an energy transfer
formalism, are recovered when the number of dislocations per unit volume is
small. This includes the equipartition of diffusive energy density which,
however, does not hold in general. The formalism bears a number of similarities
with the behavior of electromagnetic waves in a medium with a random
distribution of dielectric scatterers; the elastic interaction, however, is
momentum dependent. | 2204.05140v1 |
2022-04-17 | Dynamics of co-orbital exoplanets in a first order resonance chain with tidal dissipation | Co-orbital planets (in a $1:1$ mean motion resonance) can be formed within a
Laplace resonance chain. Here, we develop a secular model to study the dynamics
of the resonance chain $p:p:p+1$, where the co-orbital pair is in a first-order
mean motion resonance with the outermost third planet. Our model takes into
account tidal dissipation through the use of a Hamiltonian version of the
constant time-lag model, which extends the Hamiltonian formalism of the
point-mass case. We show the existence of several families of equilibria, and
how these equilibria extend to the complete system. In one family, which we
call the main branch, a secular resonance between the libration frequency of
the co-orbitals and the precession frequency of the pericentres has unexpected
dynamical consequences when tidal dissipation is added. We report the existence
of two distinct mechanisms that make co-orbital planets much more stable within
the $p:p:p+1$ resonance chain rather than outside it. The first one is due to
negative real parts of the eigenvalues of the linearised system with tides, in
the region of the secular resonance mentioned above. The second one comes from
non-linear contributions of the vector field and it is due to eccentricity
damping. These two stabilising mechanisms increase the chances of a
still-to-come detection of exoplanets in the co-orbital configuration. | 2204.08074v1 |
2022-04-26 | Quintom fields from chiral K-essence cosmology | In this paper, we present an analysis of a chiral cosmological scenario from
the perspective of K-essence formalism. In this setup, several scalar fields
interact within the kinetic and potential sectors. However, we only consider a
flat Friedmann--Robertson--Lama\^{\i}tre--Walker universe coupled minimally to
two quintom fields: one quintessence and one phantom. We examine a classical
cosmological framework, where analytical solutions are obtained. Indeed, we
present an explanation of the ``big-bang'' singularity by means of a
``big-bounce''. Moreover, having a barotropic fluid description and for a
particular set of parameters, the phantom line is in fact crossed.
Additionally, for the quantum counterpart, the Wheeler--DeWitt equation is
analytically solved for various instances, where the factor-ordering problem
has been taken into account (measured by the factor Q). Hence, this approach
allows us to compute the probability density of the previous two classical
subcases. It turns out that its behavior is in effect damped as the scale
factor and the scalar fields evolve. It also tends towards the phantom sector
when the factor ordering constant $\rm Q\ll 0$. | 2204.12083v2 |
2022-07-01 | Particle acceleration and radiation reaction in a strongly magnetized rotating dipole | Abridged. Neutron stars are surrounded by ultra-relativistic particles
efficiently accelerated by ultra strong electromagnetic fields. However so far,
no numerical simulations were able to handle such extreme regimes of very high
Lorentz factors and magnetic field strengths. It is the purpose of this paper
to study particle acceleration and radiation reaction damping in a rotating
magnetic dipole with realistic field strengths typical of millisecond and young
pulsars as well as of magnetars. To this end, we implemented an exact
analytical particle pusher including radiation reaction in the reduced
Landau-Lifshitz approximation where the electromagnetic field is assumed
constant in time and uniform in space during one time step integration. The
position update is performed using a velocity Verlet method. We extensively
tested our algorithm against time independent background electromagnetic fields
like the electric drift in cross electric and magnetic fields and the magnetic
drift and mirror motion in a dipole. Eventually, we apply it to realistic
neutron star environments. We investigated particle acceleration and the impact
of radiation reaction for electrons, protons and iron nuclei plunged around
millisecond pulsars, young pulsars and magnetars, comparing it to situations
without radiation reaction. We found that the maximum Lorentz factor depends on
the particle species but only weakly on the neutron star type. Electrons reach
energies up to $\gamma_e \approx 10^8-10^9$ whereas protons energies up to
$\gamma_p \approx 10^5-10^6$ and iron up to $\gamma \approx 10^4-10^5$. While
protons and irons are not affected by radiation reaction, electrons are
drastically decelerated, reducing their maximum Lorentz factor by 2 orders of
magnitude. We also found that the radiation reaction limit trajectories fairly
agree with the reduced Landau-Lifshitz approximation in almost all cases. | 2207.00624v1 |
2022-07-04 | Selectivity of Protein Interactions Stimulated by Terahertz Signals | It has been established that Terahertz (THz) band signals can interact with
biomolecules through resonant modes. Specifically, of interest here, protein
activation. Our research goal is to show how directing the mechanical signaling
inside protein molecules using THz signals can control changes in their
structure and activate associated biochemical and biomechanical events. To
establish that, we formulate a selectivity metric that quantifies the system
performance and captures the capability of the nanoantenna to induce a
conformational change in the desired protein molecule/population. The metric
provides a score between -1 and 1 that indicates the degree of control we have
over the system to achieve targeted protein interactions. To develop the
selectivity measure, we first use the Langevin stochastic equation driven by an
external force to model the protein behavior. We then determine the probability
of protein folding by computing the steady-state energy of the driven protein
and then generalize our model to account for protein populations. Our numerical
analysis results indicate that a maximum selectivity score is attained when
only the targeted population experiences a folding behavior due to the
impinging THz signal. From the achieved selectivity values, we conclude that
the system response not only depends on the resonant frequency but also on the
system controlling parameters namely, the nanoantenna force, the damping
constant, and the abundance of each protein population. The presented work
sheds light on the potential associated with the electromagnetic-based control
of protein networks, which could lead to a plethora of applications in the
medical field ranging from bio-sensing to targeted therapy. | 2207.01572v1 |
2022-07-10 | Revealing the drag instability in one-fluid nonideal MHD simulations of a 1D isothermal C-shock | C-type shocks are believed to be ubiquitous in turbulent molecular clouds
thanks to ambipolar diffusion. We investigate whether the drag instability in
1D isothermal C-shocks, inferred from the local linear theory of Gu & Chen, can
appear in non-ideal magnetohydrodynamic simulations. Two C-shock models (with
narrow and broad steady-state shock widths) are considered to represent the
typical environment of star-forming clouds. The ionization-recombination
equilibrium is adopted for the one-fluid approach. In the 1D simulation, the
inflow gas is continuously perturbed by a sinusoidal density fluctuation with a
constant frequency. The perturbations clearly grow after entering the C-shock
region until they start being damped at the transition to the postshock region.
We show that the profiles of a predominant Fourier mode extracted locally from
the simulated growing perturbation match those of the growing mode derived from
the linear analysis. Moreover, the local growth rate and wave frequency derived
from the predominant mode generally agree with those from the linear theory.
Therefore, we confirm the presence of the drag instability in simulated 1D
isothermal C-shocks. We also explore the nonlinear behavior of the instability
by imposing larger-amplitude perturbations to the simulation. We find that the
drag instability is subject to wave steepening, leading to saturated
perturbation growth. Issues concerning local analysis, nonlinear effects,
one-fluid approach, and astrophysical applications are discussed. | 2207.04355v2 |
2022-08-10 | Theoretical model of a new type tunneling transistor | A tunneling transistor without heterojunction as a theoretical design, or
more precisely controlled electron current transmission by barrier potential,
is under consideration.
The electrons from the conduction band of the source tunnel through the
forbidden gap $E_g$ of the channel to the conduction band of the drain. The
tunneling current $J$ calculations made at helium temperature for the example
InAs-InAs-InAs, Au-GaSe-Au and Al-AlN-Al structures show that for a constant
source-drain voltage, $V_C$, of several mV, changes in the gate voltage, $V_G$,
applied to the channel within the voltage range of 0 - $E_g/$2e change $J$ by
even 10 orders of magnitude. Unlike the existing solutions such as tunnel
field-effect-transistor (TFET), the proposed device uses the change of $V_G$
(gate voltage), i.e. the change of the electrostatic potential in the channel,
to modify the imaginary wave vector $k_z$ of tunnel current electrons.
Consequently, the gate voltage controls the damping force of the electrons wave
functions and thus the magnitude of the tunneling current, $J$. The effect of
increasing temperature, T, on $J(V_G)$ relation was also tested. It was found
that only in structures with a wide forbidden channel gap this effect is
insignificant (at least up to T=300 K). | 2208.05188v3 |
2022-08-11 | Statistical distribution of HI 21cm intervening absorbers as potential cosmic acceleration probes | Damped Lyman-$\alpha$ Absorber (DLA), or HI 21cm Absorber (H21A), is an
important probe to model-independently measure the acceleration of
spectroscopic velocity ($v_\mathrm{S}$) via the Sandage-Loeb (SL) effect.
Confined by the shortage of DLAs and Background Radio Sources (BRSs) with
adequate information, the detectable amount of DLAs is ambiguous in the bulk of
previous work. After differing the acceleration of scale factor ($\ddot{a}$)
from the first order time derivative of spectroscopic velocity
($\dot{v}_\mathrm{S}$), we make a statistical investigation of the amount of
potential DLAs in the most of this paper. Using Kernel Density Estimation (KDE)
to depict general redshift distributions of BRSs, observed DLAs and a DLA
detection rate with different limitations (1.4GHz flux, HI column density and
spin temperature), we provide fitted multi-Gaussian expressions of the three
components and their 1$\sigma$ regions by bootstrap, with a proportional
constant of H21As in detected DLAs, leading to the measurable number
predictions of H21As for FAST, ASKAP and SKA1-Mid in HI absorption blind
survey. In our most optimistic condition ($F_\mathrm{1.4GHz}$>10mJy,
$N_\mathrm{HI}>2\times10^{20}\mathrm{cm^{-2}}$ and $T_\mathrm{S}$>500K), the
FAST, AKSAP and SKA1-Mid would probe about 80, 500 and 600 H21As respectively. | 2208.05639v3 |
2022-10-03 | Quintom fields from chiral anisotropic cosmology | In this paper we present an analysis of a chiral anisotropic cosmological
scenario from the perspective of quintom fields. In this setup quintessence and
phantom fields interact in a non-standard (chiral) way within an anisotropic
Bianchi type I background. We present our examination from two fronts:
classical and quantum approaches. In the classical program we find analytical
solutions given by a particular choice of the emerged relevant parameters.
Remarkably, we present an explanation of the ''big-bang'' singularity by means
of a ''big-bounce''. Moreover, isotropization is in fact reached as the time
evolves. On the quantum counterpart the Wheeler-DeWitt equation is analytically
solved for various instances given by the same parameter space from the
classical study, and we also include the factor ordering $\rm Q$. Having
solutions in this scheme we compute the probability density, which is in effect
damped as the volume function and the scalar fields evolve; and it also tends
towards a flat FLRW framework when the factor ordering constant $\rm Q \ll 0$.
This result might indicate that for a fixed set of parameters, the anisotropies
quantum-mechanically vanish for very small values of the parameter $\rm Q$.
Finally, classical and quantum solutions reduce to their flat FLRW counterparts
when the anisotropies vanish. | 2210.01186v2 |
2022-10-06 | Effects of a Pre-inflationary de Sitter Bounce on the Primordial Gravitational Waves in $f(R)$ Gravity Theories | In this work we examine the effects of a pre-inflationary de Sitter bounce on
the energy spectrum of the primordial gravitational waves. Specifically we
assume that the Universe is described by several evolution patches, starting
with a de Sitter pre-inflationary bounce which is followed by an quasi-de
Sitter slow-roll inflationary era, followed by a constant equation of state
parameter abnormal reheating era, which is followed by the radiation and matter
domination eras and the late-time acceleration eras. The bounce and the
inflationary era can be realized by vacuum $f(R)$ gravity and the abnormal
reheating and the late-time acceleration eras by the synergy of $f(R)$ gravity
and the prefect matter fluids present. Using well-known reconstruction
techniques we find which $f(R)$ gravity can realize each evolution patch,
except from the matter and radiation domination eras which are realized by the
corresponding matter fluids. Accordingly, we calculate the damping factor of
the primordial de Sitter bounce, and as we show, the signal can be detected by
only one gravitational wave future experiment, in contrast to the case in which
the bounce is absent. We discuss in detail the consequences of our results and
the future perspectives. | 2210.02861v1 |
2022-10-11 | Switching Dynamics of Shallow Arches | This paper presents an analytical method to predict the delayed switching
dynamics of nonlinear shallow arches while switching from one state to another
state for different loading cases. We study an elastic arch subject to static
loading and time-dependent loading separately. In particular, we consider a
time-dependent loading that evolves linearly with time at a constant rate. In
both cases, we observed that the switching does not occur abruptly when the
load exceeds the static switching load, rather the time scale of the dynamics
drastically slows down; hence there is a delay in switching. For
time-independent loading, this delay increases as the applied load approach the
static switching load. Whereas for a time-dependent loading, the delay is
proportional to the rate of the applied load. Other than the loading
parameters, the delay switching time also depends on the local curvature of the
force-displacement function at the static switching point and the damping
coefficient of the arch material. The delay switching occurs due to the
flatness of the energy curve at static switching load. Therefore, we linearize
the arch near the static switching point and get a reduced nonlinear ordinary
differential equation to study the switching dynamics of the arch. This reduced
equation allows us to derive analytical expressions for the delay switching
time of the. We further compare the derived analytical results with the
numerical solutions and observed a good agreement between them. Finally, the
derived analytical formulae can be used to design arches for self-offloading
dynamic footwear for diabetics. | 2210.05734v2 |
2022-10-17 | Pion dynamics in a soft-wall AdS-QCD model | Pseudo-Goldstone modes appear in many physical systems and display robust
universal features. First, their mass $m$ obeys the so-called
Gell-Mann-Oakes-Renner (GMOR) relation $f^2\,m^2=H\,\bar{\sigma}$, with $f$ the
Goldstone stiffness, $H$ the explicit breaking scale and $\bar{\sigma}$ the
spontaneous condensate. More recently, it has been shown that their damping
$\Omega$ is constrained to follow the relation $\Omega=m^2 D_\varphi$, where
$D_\varphi$ is the Goldstone diffusivity in the purely spontaneous phase. Pions
are the most paradigmatic example of pseudo-Goldstone modes and they are
related to chiral symmetry breaking in QCD. In this work, we consider a
bottom-up soft-wall AdS-QCD model with broken ${\rm{SU}}(2)_L \times
{\rm{SU}}(2)_R$ symmetry and we study the nature of the associated
pseudo-Goldstone modes -- the pions. In particular, we perform a detailed
investigation of their dispersion relation in presence of dissipation, of the
role of the explicit breaking induced by the quark masses and of the dynamics
near the critical point. Taking advantage of the microscopic information
provided by the holographic model, we give quantitative predictions for all the
coefficients appearing in the effective description. In particular, we estimate
the finite temperature behavior of the kinetic parameter $\mathfrak{r^2}$
defined as the ration between the Goldstone diffusivity $D_\varphi$ and the
pion attenuation constant $D_A$. Interestingly, we observe important deviations
from the value $\mathfrak{r^2}=3/4$ computed in chiral perturbation theory in
the limit of zero temperature. | 2210.09088v1 |
2022-10-23 | Robust Adaptive Prescribed-Time Control for Parameter-Varying Nonlinear Systems | It is an interesting open problem to achieve adaptive prescribed-time control
for strict-feedback systems with unknown and fast or even abrupt time-varying
parameters. In this paper we present a solution with the aid of several design
and analysis innovations. First, by using a spatiotemporal transformation, we
convert the original system operational over finite time interval into one
operational over infinite time interval, allowing for Lyapunov asymptotic
design and recasting prescribed-time stabilization on finite time domain into
asymptotic stabilization on infinite time domain. Second, to deal with
time-varying parameters with unknown variation boundaries, we use congelation
of variables method and establish three separate adaptive laws for parameter
estimation (two for the unknown parameters in the feedback path and one for the
unknown parameter in the input path), in doing so we utilize two tuning
functions to eliminate over-parametrization. Third, to achieve asymptotic
convergence for the transformed system, we make use of nonlinear damping design
and non-regressor-based design to cope with time-varying perturbations, and
finally, we derive the prescribed-time control scheme from the asymptotic
controller via inverse temporal-scale transformation. The boundedness of all
closed-loop signals and control input is proved rigorously through Lyapunov
analysis, squeeze theorem, and two novel lemmas built upon the method of
variation of constants. Numerical simulation verifies the effectiveness of the
proposed method. | 2210.12706v1 |
2022-11-22 | Possible enhancement of the superconducting $T_c$ due to sharp Kohn-like soft phonon anomalies | Phonon softening is a ubiquitous phenomenon in condensed matter systems which
is often associated with charge density wave (CDW) instabilities and
anharmonicity. The interplay between phonon softening, CDW and
superconductivity is a topic of intense debate. In this work, the effects of
anomalous soft phonon instabilities on superconductivity are studied based on a
recently developed theoretical framework that accounts for phonon damping and
softening within the Migdal-Eliashberg theory. Model calculations show that the
phonon softening in the form of a sharp dip in the phonon dispersion relation,
either acoustic or optical (including the case of Kohn-type anomalies typically
associated with CDW), can cause a manifold increase of the electron-phonon
coupling constant $\lambda$. This, under certain conditions, which are
consistent with the concept of optimal frequency introduced by Bergmann and
Rainer, can produce a large increase of the superconducting transition
temperature $T_c$. In summary, our results suggest the possibility of reaching
high-temperature superconductivity by exploiting soft phonon anomalies
restricted in momentum space. | 2211.12015v3 |
2022-11-22 | Understanding Sparse Feature Updates in Deep Networks using Iterative Linearisation | Larger and deeper networks generalise well despite their increased capacity
to overfit. Understanding why this happens is theoretically and practically
important. One recent approach looks at the infinitely wide limits of such
networks and their corresponding kernels. However, these theoretical tools
cannot fully explain finite networks as the empirical kernel changes
significantly during gradient-descent-based training in contrast to infinite
networks. In this work, we derive an iterative linearised training method as a
novel empirical tool to further investigate this distinction, allowing us to
control for sparse (i.e. infrequent) feature updates and quantify the frequency
of feature learning needed to achieve comparable performance. We justify
iterative linearisation as an interpolation between a finite analog of the
infinite width regime, which does not learn features, and standard gradient
descent training, which does. Informally, we also show that it is analogous to
a damped version of the Gauss-Newton algorithm -- a second-order method. We
show that in a variety of cases, iterative linearised training surprisingly
performs on par with standard training, noting in particular how much less
frequent feature learning is required to achieve comparable performance. We
also show that feature learning is essential for good performance. Since such
feature learning inevitably causes changes in the NTK kernel, we provide direct
negative evidence for the NTK theory, which states the NTK kernel remains
constant during training. | 2211.12345v4 |
2023-01-23 | (Non)-penalized Multilevel methods for non-uniformly log-concave distributions | We study and develop multilevel methods for the numerical approximation of a
log-concave probability $\pi$ on $\mathbb{R}^d$, based on (over-damped)
Langevin diffusion. In the continuity of \cite{art:egeapanloup2021multilevel}
concentrated on the uniformly log-concave setting, we here study the procedure
in the absence of the uniformity assumption. More precisely, we first adapt an
idea of \cite{art:DalalyanRiouKaragulyan} by adding a penalization term to the
potential to recover the uniformly convex setting. Such approach leads to an
\textit{$\varepsilon$-complexity} of the order $\varepsilon^{-5} \pi(|.|^2)^{3}
d$ (up to logarithmic terms). Then, in the spirit of \cite{art:gadat2020cost},
we propose to explore the robustness of the method in a weakly convex
parametric setting where the lowest eigenvalue of the Hessian of the potential
$U$ is controlled by the function $U(x)^{-r}$ for $r \in (0,1)$. In this
intermediary framework between the strongly convex setting ($r=0$) and the
``Laplace case'' ($r=1$), we show that with the help of the control of
exponential moments of the Euler scheme, we can adapt some fundamental
properties for the efficiency of the method. In the ``best'' setting where $U$
is ${\mathcal{C}}^3$ and $U(x)^{-r}$ control the largest eigenvalue of the
Hessian, we obtain an $\varepsilon$-complexity of the order
$c_{\rho,\delta}\varepsilon^{-2-\rho} d^{1+\frac{\rho}{2}+(4-\rho+\delta) r}$
for any $\rho>0$ (but with a constant $c_{\rho,\delta}$ which increases when
$\rho$ and $\delta$ go to $0$). | 2301.09471v1 |
2023-02-02 | The Power of Preconditioning in Overparameterized Low-Rank Matrix Sensing | We propose $\textsf{ScaledGD($\lambda$)}$, a preconditioned gradient descent
method to tackle the low-rank matrix sensing problem when the true rank is
unknown, and when the matrix is possibly ill-conditioned. Using
overparametrized factor representations, $\textsf{ScaledGD($\lambda$)}$ starts
from a small random initialization, and proceeds by gradient descent with a
specific form of damped preconditioning to combat bad curvatures induced by
overparameterization and ill-conditioning. At the expense of light
computational overhead incurred by preconditioners,
$\textsf{ScaledGD($\lambda$)}$ is remarkably robust to ill-conditioning
compared to vanilla gradient descent ($\textsf{GD}$) even with
overprameterization. Specifically, we show that, under the Gaussian design,
$\textsf{ScaledGD($\lambda$)}$ converges to the true low-rank matrix at a
constant linear rate after a small number of iterations that scales only
logarithmically with respect to the condition number and the problem dimension.
This significantly improves over the convergence rate of vanilla $\textsf{GD}$
which suffers from a polynomial dependency on the condition number. Our work
provides evidence on the power of preconditioning in accelerating the
convergence without hurting generalization in overparameterized learning. | 2302.01186v3 |
2023-03-28 | Nonlocal Nonholonomic Source Seeking Despite Local Extrema | In this paper, we investigate the problem of source seeking with a unicycle
in the presence of local extrema. Our study is motivated by the fact that most
of the existing source seeking methods follow the gradient direction of the
signal function and thus only lead to local convergence into a neighborhood of
the nearest local extremum. So far, only a few studies present ideas on how to
overcome local extrema in order to reach a global extremum. None of them apply
to second-order (force- and torque-actuated) nonholonomic vehicles. We consider
what is possibly the simplest conceivable algorithm for such vehicles, which
employs a constant torque and a translational/surge force in proportion to an
approximately differentiated measured signal. We show that the algorithm steers
the unicycle through local extrema towards a global extremum. In contrast to
the previous extremum-seeking studies, in our analysis we do not approximate
the gradient of the objective function but of the objective function's local
spatial average. Such a spatially averaged objective function is expected to
have fewer critical points than the original objective function. Under suitable
assumptions on the averaged objective function and on sufficiently strong
translational damping, we show that the control law achieves practical uniform
asymptotic stability and robustness to sufficiently weak measurement noise and
disturbances to the force and torque inputs. | 2303.16027v1 |
2023-04-18 | A blue depression in the optical spectra of M dwarfs | A blue depression is found in the spectra of M dwarfs from 4000 to 4500A.
This depression shows an increase toward lower temperatures though is
particularly sensitive to gravity and metallicity. It is the single most
sensitive feature in the optical spectra of M dwarfs. The depression appears as
centered on the neutral calcium resonance line at 4227A and leads to nearby
features being weaker by about two orders of magnitude than predicted. We
consider a variety of possible causes for the depression including temperature,
gravity, metallicity, dust, damping constants, and atmospheric stratification.
We also consider relevant molecular opacities which might be the cause
identifying AlH, SiH, and NaH in the spectral region. However, none of these
solutions are satisfactory. In the absence of a more accurate determination of
the broadening of the calcium line perturbed by molecular hydrogen, we find a
promising empirical fit using a modified Lorentzian line profile for the
calcium resonance line. Such fits provide a simplistic line-broadening
description for this calcium resonance line and potentially other un-modelled
resonance lines in cool high-pressure atmospheres. Thus we claim the most
plausible cause of the blue depression in the optical spectra of M dwarfs is a
lack of appropriate treatment of line broadening for atomic calcium. The broad
wings of the calcium resonance line develop at temperatures below about 4000K
and are analogous to the neutral sodium and potassium features which dominate
the red optical spectra of L dwarfs. | 2304.09219v2 |
2023-04-25 | Flow-induced oscillations of pitching swept wings: Stability boundary, vortex dynamics and force partitioning | We experimentally study the aeroelastic instability boundaries and
three-dimensional vortex dynamics of pitching swept wings, with the sweep angle
ranging from 0 to 25 degrees. The structural dynamics of the wings are
simulated using a cyber-physical control system. With a constant flow speed, a
prescribed high inertia and a small structural damping, we show that the system
undergoes a subcritical Hopf bifurcation to large-amplitude limit-cycle
oscillations (LCOs) for all the sweep angles. The onset of LCOs depends largely
on the static characteristics of the wing. The saddle-node point is found to
change non-monotonically with the sweep angle, which we attribute to the
non-monotonic power transfer between the ambient fluid and the elastic mount.
An optimal sweep angle is observed to enhance the power extraction performance
and thus promote LCOs and destabilize the aeroelastic system. The frequency
response of the system reveals a structural-hydrodynamic oscillation mode for
wings with relatively high sweep angles. Force, moment, and three-dimensional
flow structures measured using multi-layer stereoscopic particle image
velocimetry are analyzed to explain the differences in power extraction for
different swept wings. Finally, we employ a physics-based Force and Moment
Partitioning Method (FMPM) to quantitatively correlate the three-dimensional
vortex dynamics with the resultant unsteady aerodynamic moment. | 2304.12544v2 |
2023-07-04 | Exponential stability of Euler-Bernoulli beam under boundary controls in rotation and angular velocity | This paper addresses the analysis of a boundary feedback system involving a
non-homogeneous Euler-Bernoulli beam governed by the equation
$m(x)u_{tt}+\mu(x)u_{t}$$+\left(r(x)u_{xx}\right)_{xx}=0$, subject to the
initial $u(x,0)=u_0(x)$, $u_t(x,0)=v_0(x)$ and boundary conditions $u(0,t)=0$,
$\left (-r(x)u_{xx}(x,t)\right )_{x=0}=-k^{-}_r u_{x}(0,t)-k^{-}_a
u_{xt}(0,t)$, $u(\ell,t)=0$, $\left (-r(x)u_{xx}(x,t)\right )_{x=\ell}=-k^{+}_r
u_{x}(\ell,t)-k^{+}_a u_{xt}(\ell,t)$, with boundary control at both ends
resulting from the rotation and angular velocity. The approach proposed in this
study relies on the utilization of regular weak solutions, energy identity, and
a physically motivated Lyapunov function. By imposing natural assumptions
concerning physical parameters and other inputs, which ensure the existence of
a regular weak solution, we successfully derive a uniform exponential decay
estimate for the system's energy. The decay rate constant featured in this
estimate is solely dependent on the physical and geometric properties of the
beam. These properties encompass crucial parameters such as the viscous
external damping coefficient $\mu(x)$, as well as the boundary springs
$k^{-}_r,k^+_r $ and dampers $k^{-}_a,k^+_a$. To illustrate the practical
effectiveness of our theoretical findings, numerical examples are provided.
These examples serve to demonstrate the applicability and relevance of our
derived results in real-world scenarios. | 2307.01518v1 |
2023-07-13 | Exciton-polaritons in CsPbBr$_3$ crystals revealed by optical reflectivity in high magnetic fields and two-photon spectroscopy | Cesium lead bromide (CsPbBr$_3$) is a representative material of the emerging
class of lead halide perovskite semiconductors that possess remarkable
optoelectronic properties. Its optical properties in the vicinity of the band
gap energy are greatly contributed by excitons, which form exciton-polaritons
due to strong light-matter interactions. We examine exciton-polaritons in
solution-grown CsPbBr$_3$ crystals by means of circularly-polarized reflection
spectroscopy measured in high magnetic fields up to 60 T. The excited 2P
exciton state is measured by two-photon absorption. Comprehensive modeling and
analysis provides detailed quantitative information about the exciton-polariton
parameters: exciton binding energy of 32.5 meV, oscillator strength
characterized by longitudinal-tranverse splitting of 5.3 meV, damping of 6.7
meV, reduced exciton mass of $0.18 m_0$, exciton diamagnetic shift of 1.6
$\mu$eV/T$^2$, and exciton Land\'e factor $g_X=+2.35$. We show that the exciton
states can be well described within a hydrogen-like model with an effective
dielectric constant of 8.7. From the measured exciton longitudinal-transverse
splitting we evaluate the Kane energy of $E_p=15$ eV, which is in reasonable
agreement with values of $11.8-12.5$ eV derived from the carrier effective
masses. | 2307.07035v1 |
2023-07-19 | Impact of bulk viscosity on the post-merger gravitational-wave signal from merging neutron stars | In the violent post-merger of binary neutron-star mergers strong oscillations
are present that impact the emitted gravitational-wave (GW) signal. The
frequencies, temperatures and densities involved in these oscillations allow
for violations of the chemical equilibrium promoted by weak-interactions, thus
leading to a nonzero bulk viscosity that can impact dynamics and GW signals. We
present the first simulations of binary neutron-star mergers employing the
self-consistent and second-order formulation of the equations of relativistic
hydrodynamics for dissipative fluids proposed by M\"uller, Israel and Stewart.
With the spirit of obtaining a first assessment of the impact of bulk viscosity
on the structure and radiative efficiency of the merger remnant we adopt a
simplified approach for the viscosity, which we assume to be constant within
the stars, but which we vary in strength for different binaries, thus exploring
the possible behaviours and obtaining strict upper limits. In this way, we find
that large bulk viscosities are very effective at damping the
collision-and-bounce oscillations that characterize the dynamics of the stellar
cores right after the merger. As a result, the $m=2$ deformations and the
gravitational-radiation efficiency of the remnant are considerably reduced,
with qualitative and quantitative changes in the post-merger spectrum that can
be large in the case of the most extreme configurations. Overall, our crude but
self-consistent results indicate that bulk viscosity reduces the energy
radiated in GWs by $\lesssim 1\%$ in the (realistic) scenario of small
viscosity, and by $\lesssim 15\%$ in the (unrealistic) scenario of large
viscosity. | 2307.10464v1 |
2023-07-21 | Non-ideal magnetohydrodynamics on a moving mesh I: Ohmic and ambipolar diffusion | Especially in cold and high-density regions, the assumptions of ideal
magnetohydrodynamics (MHD) can break down, making first order non-ideal terms
such as Ohmic and ambipolar diffusion as well as the Hall effect important. In
this study we present a new numerical scheme for the first two resistive terms,
which we implement in the moving-mesh code AREPO using the single-fluid
approximation combined with a new gradient estimation technique based on a
least-squares fit per interface. Through various test calculations including
the diffusion of a magnetic peak, the structure of a magnetic C-shock, and the
damping of an Alfv\'en wave, we show that we can achieve an accuracy comparable
to the state-of-the-art code ATHENA++. We apply the scheme to the linear growth
of the magnetorotational instability and find good agreement with the
analytical growth rates. By simulating the collapse of a magnetised cloud with
constant magnetic diffusion, we show that the new scheme is stable even for
large density contrasts. Thanks to the Lagrangian nature of the moving mesh
method the new scheme is thus well suited for intended future applications
where a high resolution in the dense cores of collapsing protostellar clouds
needs to be achieved. In a forthcoming work we will extend the scheme to the
Hall effect. | 2307.11814v1 |
2023-09-14 | The cost of solving linear differential equations on a quantum computer: fast-forwarding to explicit resource counts | How well can quantum computers simulate classical dynamical systems? There is
increasing effort in developing quantum algorithms to efficiently simulate
dynamics beyond Hamiltonian simulation, but so far exact resource estimates are
not known. In this work, we provide two significant contributions. First, we
give the first non-asymptotic computation of the cost of encoding the solution
to general linear ordinary differential equations into quantum states -- either
the solution at a final time, or an encoding of the whole history within a time
interval. Second, we show that the stability properties of a large class of
classical dynamics allow their fast-forwarding, making their quantum simulation
much more time-efficient. From this point of view, quantum Hamiltonian dynamics
is a boundary case that does not allow this form of stability-induced
fast-forwarding. In particular, we find that the history state can always be
output with complexity $O(T^{1/2})$ for any stable linear system. We present a
range of asymptotic improvements over state-of-the-art in various regimes. We
illustrate our results with a family of dynamics including linearized
collisional plasma problems, coupled, damped, forced harmonic oscillators and
dissipative nonlinear problems. In this case the scaling is quadratically
improved, and leads to significant reductions in the query counts after
inclusion of all relevant constant prefactors. | 2309.07881v2 |
2023-09-25 | Domain wall dynamics driven by a transient laser-induced magnetisation | One of the fundamental effects of the laser-matter interaction is the
appearance of an induced transient magnetisation. While the underlying
phenomena differ in their microscopic origin and cover a diverse array of
materials, here we address a fundamental question about the possibility to
drive domain-wall dynamics on the femtosecond timescale of the exchange
interactions solely by longitudinal changes of the magnetic moments. We verify
the viability of this hypothesis in the case of a generic ferromagnetic system
described in the framework of the high-temperature micromagnetic model based on
the Landau-Lifshitz-Bloch equation. The effect is investigated in a 1D model at
constant temperature as well as in a full micromagnetic framework considering
realistic laser-induced heating. Our results demonstrate that domain-wall
deformation in a femtosecond timeframe leads to the displacement of the wall on
a larger timescale up to nanoseconds accompanied by a release of excess energy
in the form of spin waves. The domain wall deformation leads to the appearance
of a magnetisation gradient across the wall which promotes the motion towards
the region consisting of spins with decreased magnetisation length. The total
displacement is enhanced at larger temperatures and smaller damping due to an
increase of the longitudinal relaxation time which ensures the longer presence
of the induced magnetisation gradient. We also demonstrate an enhanced domain
wall motion in the presence of the Dzyaloshinskii-Moriya interaction attributed
to augmented magnonic torques. Our results are important towards the
understanding of ultrafast magnetism phenomena on the sub-picosecond timescale. | 2309.14287v1 |
2023-10-03 | Controlled Quasi-Latitudinal Solutions for ultra-fast Spin-Torque Precessional Magnetization Switching | The aim of the paper is to present a novel class of time-dependent controls
to realize ultra-fast magnetization switching in nanomagnets driven by
spin-torques produced by spin-polarized electric currents. Magnetization
dynamics in such systems is governed by the Landau-Lifshitz-Slonczewski
equation which describes the precessional motion of (dimensionless)
magnetization vector on the unit-sphere. The relevant case of nanoparticles
with uniaxial anisotropy having in-plane easy and intermediate axes and
out-of-plane hard axis is considered. By exploiting the characteristic
smallness of damping and spin-torque intensity, the aforementioned controls are
constructed via suitable perturbative tools in a way to realise approximate
\emph{latitudinal solutions} (i.e. motions on a sphere in which the
out-of-plane magnetization component stays constant) with the effect to fast
``switch'' the system from one stationary state to another. The possibility to
keep a (``small'') bounded value of the out-of-plane coordinate throughout this
process of ``transfer'', turns out to be advantageous in the applications as it
sensibly reduces the post-switching relaxation oscillations that may cause the
failure of switching in real samples. Further relevant quantitative results on
the behaviour of the solutions during the pre- and post-switching stages
(termed ``expulsion'' and ``attraction'', respectively), are given as a
byproduct. A selection of validating numerical experiments is presented
alongside the corresponding theoretical results. | 2310.02070v1 |
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