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2016-06-21
|
A stochastic model of supercoiling-dependent transcription
|
We propose a stochastic model for gene transcription coupled to DNA
supercoiling, where we incorporate the experimental observation that
polymerases create supercoiling as they unwind the DNA helix, and that these
enzymes bind more favourably to regions where the genome is unwound. Within
this model, we show that when the transcriptionally induced flux of
supercoiling increases, there is a sharp crossover from a regime where
torsional stresses relax quickly and gene transcription is random, to one where
gene expression is highly correlated and tightly regulated by supercoiling. In
the latter regime, the model displays transcriptional bursts, waves of
supercoiling, and up-regulation of divergent or bidirectional genes. It also
predicts that topological enzymes which relax twist and writhe should provide a
pathway to down-regulate transcription. This article has been published in
Physical Review Letters, May 2016.
|
1606.06555v3
|
2016-06-22
|
Induced magnetization and power loss for a periodically driven system of ferromagnetic nanoparticles with randomly oriented easy axes
|
We study the effect of an elliptically polarized magnetic field on a system
of non-interacting, single-domain ferromagnetic nanoparticles characterized by
a uniform distribution of easy axis directions. Our main goal is to determine
the average magnetization of this system and the power loss in it. In order to
calculate these quantities analytically, we develop a general perturbation
theory for the Landau-Lifshitz-Gilbert (LLG) equation and find its steady-state
solution for small magnetic field amplitudes. On this basis, we derive the
second-order expressions for the average magnetization and power loss,
investigate their dependence on the magnetic field frequency, and analyze the
role of subharmonic resonances resulting from the nonlinear nature of the LLG
equation. For arbitrary amplitudes, the frequency dependence of these
quantities is obtained from the numerical solution of this equation. The impact
of transitions between different regimes of regular and chaotic dynamics of
magnetization, which can be induced in nanoparticles by changing the magnetic
field frequency, is examined in detail.
|
1606.07131v1
|
2016-07-20
|
Performance of Topological Insulator Interconnects
|
The poor performance of copper interconnects at the nanometer scale calls for
new material solutions for continued scaling of integrated circuits. We propose
the use of three dimensional time-reversal-invariant topological insulators
(TIs), which host backscattering-protected surface states, for this purpose.
Using semiclassical methods, we demonstrate that nanoscale TI interconnects
have a resistance 1-3 orders of magnitude lower than copper interconnects and
graphene nanoribbons at the nanometer scale. We use the nonequilibrium Green
function (NEGF) formalism to measure the change in conductance of nanoscale TI
and metal interconnects caused by the presence of impurity disorder. We show
that metal interconnects suffer a resistance increase, relative to the clean
limit, in excess of 500% due to disorder while the TI's surface states increase
less than 35% in the same regime.
|
1607.06131v2
|
2016-07-21
|
Rate-distance tradeoff for codes above graph capacity
|
The capacity of a graph is defined as the rate of exponential growth of
independent sets in the strong powers of the graph. In the strong power an edge
connects two sequences if at each position their letters are equal or adjacent.
We consider a variation of the problem where edges in the power graphs are
removed between sequences which differ in more than a fraction $\delta$ of
coordinates. The proposed generalization can be interpreted as the problem of
determining the highest rate of zero undetected-error communication over a link
with adversarial noise, where only a fraction $\delta$ of symbols can be
perturbed and only some substitutions are allowed.
We derive lower bounds on achievable rates by combining graph homomorphisms
with a graph-theoretic generalization of the Gilbert-Varshamov bound. We then
give an upper bound, based on Delsarte's linear programming approach, which
combines Lov\'asz' theta function with the construction used by McEliece et al.
for bounding the minimum distance of codes in Hamming spaces.
|
1607.06384v1
|
2016-07-25
|
A geometric approach to optimal nonequilibrium control: Minimizing dissipation in nanomagnetic spin systems
|
Optimal control of nanomagnets has become an urgent problem for the field of
spintronics as technological tools approach thermodynamically determined limits
of efficiency. In complex, fluctuating systems, like nanomagnetic bits, finding
optimal protocols is challenging, requiring detailed information about the
dynamical fluctuations of the controlled system. We provide a new, physically
transparent derivation of a metric tensor for which the length of a protocol is
proportional to its dissipation. This perspective simplifies nonequilibrium
optimization problems by recasting them in a geometric language. We then
describe a numerical method, an instance of geometric minimum action methods,
that enables computation of geodesics even when the number of control
parameters is large. We apply these methods to two models of nanomagnetic bits:
a simple Landau-Lifshitz-Gilbert description of a single magnetic spin
controlled by two orthogonal magnetic fields and a two dimensional Ising model
in which the field is spatially controlled. These calculations reveal
nontrivial protocols for bit erasure and reversal, providing important,
experimentally testable predictions for ultra-low power computing.
|
1607.07425v1
|
2016-08-02
|
Affordable echelle spectroscopy of the eccentric HAT-P-2, WASP-14 and XO-3 planetary systems with a sub-meter-class telescope
|
A new off-shelf low-cost echelle spectrograph was installed recently on the
0.6m telescope at the Star\'a Lesn\'a Observatory (Slovakia). In this paper we
describe in details the radial velocity (RV) analysis of the first three
transiting planetary systems, HAT-P-2, WASP-14 and XO-3, observed with this
instrument. Furthermore, we compare our data with the RV data achieved with
echelle spectrographs of other sub-meter-, meter- and two-meter-class
telescopes in terms of their precision. Finally, we investigate the
applicability of our RV data for modeling orbital parameters.
|
1608.00745v1
|
2016-08-06
|
High current, high efficiency graded band gap perovskite solar cells
|
Organic-inorganic halide perovskite materials have emerged as attractive
alternatives to conventional solar cell building blocks. Their high light
absorption coefficients and long diffusion lengths suggest high power
conversion efficiencies (PCE),1-5 and indeed perovskite-based single band gap
and tandem solar cell designs have yielded impressive performances.1-16 One
approach to further enhance solar spectrum utilization is the graded band gap,
but this has not been previously achieved for perovskites. In this study, we
demonstrate graded band gap perovskite solar cells with steady-state conversion
efficiencies averaging 18.4%, with a best of 21.7%, all without reflective
coatings. An analysis of the experimental data yields high fill factors of ~75%
and high short circuit current densities up to 42.1 mA/cm2. These cells, which
are based on a novel architecture of two perovskite layers (MASnI3 and
MAPbI3-xBrx), incorporating GaN, monolayer hexagonal boron nitride, and
graphene aerogel, display the highest efficiency ever reported for perovskite
solar cells.
|
1608.02150v1
|
2016-08-09
|
Existence of weak solutions to an evolutionary model for magnetoelasticity
|
We prove existence of weak solutions to an evolutionary model derived for
magnetoelastic materials. The model is phrased in Eulerian coordinates and
consists in particular of (i) a Navier-Stokes equation that involves magnetic
and elastic terms in the stress tensor obtained by a variational approach, of
(ii) a regularized transport equation for the deformation gradient and of (iii)
the Landau-Lifshitz-Gilbert equation for the dynamics of the magnetization. The
proof is built on a Galerkin method and a fixed-point argument. It is based on
ideas from F.-H. Lin and the third author for systems modeling the flow of
liquid crystals as well as on methods by G. Carbou and P. Fabrie for solutions
of the Landau-Lifshitz equation.
|
1608.02992v1
|
2016-08-16
|
Magnetic Yoking and Tunable Interactions in FePt-Based Hard/Soft Bilayers
|
Assessing and controlling magnetic interactions in magnetic nanostructures
are critical to nanomagnetic and spintronic explorations, such as magnetic
recording media, permanent magnets, magnetic memory and logic devices, etc.
Here we demonstrate an extremely sensitive magnetic yoking effect and tunable
interactions in FePt based hard/soft bilayers mediated by the soft layer. Below
the exchange length, a thin soft layer strongly exchange couples to the
perpendicular moments of the hard layer; above the exchange length, just a few
nanometers thicker, the soft layer moments turn in-plane and act to yoke the
dipolar fields from the adjacent hard layer perpendicular domains. The
evolution from exchange to dipolar-dominated interactions is experimentally
captured by first-order reversal curves, the delta-M method, and polarized
neutron reflectometry, and confirmed by micromagnetic simulations. These
findings demonstrate an effective yoking approach to design and control
magnetic interactions in wide varieties of magnetic nanostructures and devices.
|
1608.04630v1
|
2016-08-17
|
Current-induced instability of domain walls in cylindrical nanowires
|
We study the current-driven domain wall (DW) motion in cylindrical nanowires
using micromagnetic simulations by implementing the Landau-Lifshitz-Gilbert
equation with nonlocal spin-transfer torque in a finite difference
micromagnetic package. We find that in the presence of DW Gaussian wave packets
(spin waves) will be generated when the charge current is applied to the system
suddenly. And this effect is excluded when using the local spin-transfer
torque. The existence of spin waves emission indicates that transverse domain
walls can not move arbitrarily fast in cylindrical nanowires although they are
free from the Walker limit. We establish an upper-velocity limit for the DW
motion by analyzing the stability of Gaussian wave packets using the local
spin-transfer torque. Micromagnetic simulations show that the stable region
obtained by using nonlocal spin-transfer torque is smaller than that by using
its local counterpart. This limitation is essential for multiple domain walls
since the instability of Gaussian wave packets will break the structure of
multiple domain walls.
|
1608.04876v2
|
2016-08-22
|
Disorder Induced Phase Transitions of Type-II Weyl Semimetal
|
Weyl semimetals are a newly discovered class of materials that host
relativistic massless Weyl fermions as their low-energy bulk excitations. Among
this new class of materials, there exist two general types of semimetals that
are of particular interest: type-I Weyl semimetals, that have broken inversion
or time-reversal symmetry symmetry, and type-II Weyl semimetals, that
additionally breaks Lorentz invariance. In this work, we use Born approximation
to analytically demonstrate that the type-I Weyl semimetals may undergo a
quantum phase transition to type-II Weyl semimetals in the presence of the
finite charge and magnetic disorder when non-zero tilt exist. The phase
transition occurs when the disorder renormalizes the topological mass, thereby
reducing the Fermi velocity near the Weyl cone below the tilt of the cone. We
also confirm the presence of the disorder induced phase transition in Weyl
semimetals using exact diagonalization of a three-dimensional tight-binding
model to calculate the resultant phase diagram of the type-I Weyl semimetal.
|
1608.06311v1
|
2016-08-25
|
Convergence of a mass-lumped finite element method for the Landau-Lifshitz equation
|
The dynamics of the magnetic distribution in a ferromagnetic material is
governed by the Landau-Lifshitz equation, which is a nonlinear geometric
dispersive equation with a nonconvex constraint that requires the magnetization
to remain of unit length throughout the domain. In this article, we present a
mass-lumped finite element method for the Landau-Lifshitz equation. This method
preserves the nonconvex constraint at each node of the finite element mesh, and
is energy nonincreasing. We show that the numerical solution of our method for
the Landau-Lifshitz equation converges to a weak solution of the
Landau-Lifshitz-Gilbert equation using a simple proof technique that cancels
out the product of weakly convergent sequences. Numerical tests for both
explicit and implicit versions of the method on a unit square with periodic
boundary conditions are provided for structured and unstructured meshes.
|
1608.07312v3
|
2016-08-30
|
LiRa: A New Likelihood-Based Similarity Score for Collaborative Filtering
|
Recommender system data presents unique challenges to the data mining,
machine learning, and algorithms communities. The high missing data rate, in
combination with the large scale and high dimensionality that is typical of
recommender systems data, requires new tools and methods for efficient data
analysis. Here, we address the challenge of evaluating similarity between two
users in a recommender system, where for each user only a small set of ratings
is available. We present a new similarity score, that we call LiRa, based on a
statistical model of user similarity, for large-scale, discrete valued data
with many missing values. We show that this score, based on a ratio of
likelihoods, is more effective at identifying similar users than traditional
similarity scores in user-based collaborative filtering, such as the Pearson
correlation coefficient. We argue that our approach has significant potential
to improve both accuracy and scalability in collaborative filtering.
|
1608.08646v2
|
2016-09-09
|
An Empirical Study of Cycle Toggling Based Laplacian Solvers
|
We study the performance of linear solvers for graph Laplacians based on the
combinatorial cycle adjustment methodology proposed by
[Kelner-Orecchia-Sidford-Zhu STOC-13]. The approach finds a dual flow solution
to this linear system through a sequence of flow adjustments along cycles. We
study both data structure oriented and recursive methods for handling these
adjustments.
The primary difficulty faced by this approach, updating and querying long
cycles, motivated us to study an important special case: instances where all
cycles are formed by fundamental cycles on a length $n$ path. Our methods
demonstrate significant speedups over previous implementations, and are
competitive with standard numerical routines.
|
1609.02957v1
|
2016-09-21
|
Harmonic space analysis of pulsar timing array redshift maps
|
In this paper, we propose a new framework for treating the angular
information in the pulsar timing array response to a gravitational wave
background based on standard cosmic microwave background techniques. We
calculate the angular power spectrum of the all-sky gravitational redshift
pattern induced at the earth for both a single bright source of gravitational
radiation and a statistically isotropic, unpolarized Gaussian random
gravitational wave background. The angular power spectrum is the harmonic
transform of the Hellings & Downs curve. We use the power spectrum to examine
the expected variance in the Hellings & Downs curve in both cases. Finally, we
discuss the extent to which pulsar timing arrays are sensitive to the angular
power spectrum and find that the power spectrum sensitivity is dominated by the
quadrupole anisotropy of the gravitational redshift map.
|
1609.06758v2
|
2016-09-22
|
Ultrafast generation of skyrmionic defects with vortex beams: printing laser profiles on magnets
|
Controlling electric and magnetic properties of matter by laser beams is
actively explored in the broad region of condensed matter physics, including
spintronics and magneto-optics. Here we theoretically propose an application of
optical and electron vortex beams carrying intrinsic orbital angular momentum
to chiral ferro- and antiferro- magnets. We analyze the time evolution of spins
in chiral magnets under irradiation of vortex beams, by using the stochastic
Landau-Lifshitz-Gilbert equation. We show that beam-driven nonuniform
temperature lead to a class of ring-shaped magnetic defects, what we call
skyrmion multiplex, as well as conventional skyrmions. We discuss the proper
beam parameters and the optimal way of applying the beams for the creation of
these topological defects. Our findings provide an ultrafast scheme of
generating topological magnetic defects in a way applicable to both metallic
and insulating chiral (anti-) ferromagnets.
|
1609.06816v3
|
2016-10-02
|
Syntactic Structures and Code Parameters
|
We assign binary and ternary error-correcting codes to the data of syntactic
structures of world languages and we study the distribution of code points in
the space of code parameters. We show that, while most codes populate the lower
region approximating a superposition of Thomae functions, there is a
substantial presence of codes above the Gilbert-Varshamov bound and even above
the asymptotic bound and the Plotkin bound. We investigate the dynamics induced
on the space of code parameters by spin glass models of language change, and
show that, in the presence of entailment relations between syntactic parameters
the dynamics can sometimes improve the code. For large sets of languages and
syntactic data, one can gain information on the spin glass dynamics from the
induced dynamics in the space of code parameters.
|
1610.00311v1
|
2016-10-03
|
Linear dynamics of classical spin as Möbius transformation
|
Although the overwhelming majority of natural processes occurs far from the
equilibrium, general theoretical approaches to non-equilibrium phase
transitions remain scarce. Recent breakthroughs introducing description of open
dissipative systems in terms of non-Hermitian quantum mechanics allowed to
identify a class of non-equilibrium phase transitions associated with the loss
of combined parity (reflection) and time-reversal symmetries. Here we report
that time evolution of a single classical spin (e.g. monodomain ferromagnet)
governed by the Landau-Lifshitz-Gilbert-Slonczewski equation in absence of
higher-order anisotropy terms is described by a M\"{o}bius transformation in
complex stereographic coordinates. We identify the \textit{parity-time}
symmetry-breaking phase transition occurring in spin-transfer torque-driven
linear spin systems as a transition between hyperbolic and loxodromic classes
of M\"{o}bius transformations, with the critical point of the transition
corresponding to the parabolic transformation. This establishes the
understanding of non-equilibrium phase transitions as topological transitions
in configuration space.
|
1610.00762v1
|
2016-10-11
|
Self-Consistent Field Theory studies of the thermodynamics and quantum spin dynamics of magnetic Skyrmions
|
A self-consistent field theory is introduced and used to investigate the
thermodynamics and spin dynamics of an $S = 1$ quantum spin system with a
magnetic Skyrmion. The temperature dependence of the Skyrmion profile as well
as the phase diagram are calculated. It is shown that the Skyrmion carries a
phase transition to the ferromagnetic phase of first order with increasing
temperature, while the magnetization of the surrounding ferromagnet undergoes a
phase transition of second order when changing to the paramagnetic phase.
Furthermore, the electric field driven annihilation process of the Skyrmion is
described quantum mechanical by solving the time dependent Schr\"odinger
equation. The results are compared with the trajectories of the semi-classical
description of the spin expectation values using a differential equation
similar to the classical Landau-Lifshitz-Gilbert equation.
|
1610.03191v2
|
2016-10-12
|
Variational approximation of functionals defined on 1-dimensional connected sets: the planar case
|
In this paper we consider variational problems involving 1-dimensional
connected sets in the Euclidean plane, such as the classical Steiner tree
problem and the irrigation (Gilbert-Steiner) problem. We relate them to optimal
partition problems and provide a variational approximation through
Modica-Mortola type energies proving a $\Gamma$-convergence result. We also
introduce a suitable convex relaxation and develop the corresponding numerical
implementations. The proposed methods are quite general and the results we
obtain can be extended to $n$-dimensional Euclidean space or to more general
manifold ambients, as shown in the companion paper [11].
|
1610.03839v5
|
2016-10-24
|
Field-free, spin-current control of magnetization in non-collinear chiral antiferromagnets
|
Non-collinear chiral antiferromagnets like Mn3Sn and Mn3Ge are known to show
gigantic anomalous Hall response depending on the orientation of their inverse
chiral magnetic order of Mn atoms in Kagome layers. Here we study the stability
of such magnetic order in the absence of external magnetic fields on the basis
of stochastic Landau-Lifshitz-Gilbert equation for a simplified two-dimensional
model of these materials. We find that even without external magnetic fields,
the ordered state is, once formed, highly stable against thermal fluctuations.
Moreover, we show that if Mn spins are well confined inside each Kagome layers,
by injecting spin-current using spin-filtering effect of ferromagnetic metals,
we can control the in-plane magnetic structure in a field free way.
|
1610.07615v2
|
2016-11-04
|
Struwe-like solutions for the Stochastic Harmonic Map Flow
|
We give a new result on the well-posedness of the two-dimensional Stochastic
Harmonic Map flow, whose study is motivated by the Landau-Lifshitz-Gilbert
model for thermal fluctuations in micromagnetics. We construct strong solutions
that belong locally to the spaces $C([s,t);H^1)\cap L^2([s,t);H^2)$, $0\leq
s<t\leq T$. It that sense, these maps are a counterpart of the so-called
"Struwe solutions" of the deterministic model. We also give a natural criterion
of uniqueness that extends A.\ Freire's Theorem to the stochastic case. Both
results are obtained under the condition that the noise term has a trace-class
covariance in space.
|
1611.01565v2
|
2016-11-07
|
Compactness results for static and dynamic chiral skyrmions near the conformal limit
|
We examine lower order perturbations of the harmonic map prob- lem from
$\mathbb{R}^2$ to $\mathbb{S}^2$ including chiral interaction in form of a
helicity term that prefers modulation, and a potential term that enables decay
to a uniform background state. Energy functionals of this type arise in the
context of magnetic systems without inversion symmetry. In the almost conformal
regime, where these perturbations are weighted with a small parameter, we
examine the existence of relative minimizers in a non-trivial homotopy class,
so-called chiral skyrmions, strong compactness of almost minimizers, and their
asymptotic limit. Finally we examine dynamic stability and compactness of
almost minimizers in the context of the Landau-Lifshitz-Gilbert equation
including spin-transfer torques arising from the interaction with an external
current.
|
1611.01984v1
|
2016-11-08
|
A variational method for spectral functions
|
The Generalized Eigenvalue Problem (GEVP) has been used extensively in the
past in order to reliably extract energy levels from time-dependent Euclidean
correlators calculated in Lattice QCD. We propose a formulation of the GEVP in
frequency space. Our approach consists of applying the model-independent
Backus-Gilbert method to a set of Euclidean two-point functions with common
quantum numbers. A GEVP analysis in frequency space is then applied to a matrix
of estimators that allows us, among other things, to obtain particular linear
combinations of the initial set of operators that optimally overlap to
different local regions in frequency. We apply this method to lattice data from
NRQCD. This approach can be interesting both for vacuum physics as well as for
finite-temperature problems.
|
1611.02499v1
|
2016-11-14
|
Hidden fluctuations close to a quantum bicritical point
|
In this paper we describe physical properties arising in the vicinity of two
coupled quantum phase transitions. We consider a phenomenological model based
on two scalar order parameter fields locally coupled biquadratically and having
a common quantum critical point as a function of a quantum tuning parameter
such as pressure or magnetic field. A self-consistent treatment suggests that
the uniform static susceptibilities of the two order parameter fields may have
the same qualitative form at low temperature even where the forms differ
sharply in the absence of the biquadratic coupling. The possible limitations of
the self-consistent analysis leading to this prediction are considered.
|
1611.04621v2
|
2016-11-22
|
New bounds of permutation codes under Hamming metric and Kendall's $τ$-metric
|
Permutation codes are widely studied objects due to their numerous
applications in various areas, such as power line communications, block
ciphers, and the rank modulation scheme for flash memories. Several kinds of
metrics are considered for permutation codes according to their specific
applications. This paper concerns some improvements on the bounds of
permutation codes under Hamming metric and Kendall's $\tau$-metric
respectively, using mainly a graph coloring approach. Specifically, under
Hamming metric, we improve the Gilbert-Varshamov bound asymptotically by a
factor $n$, when the minimum Hamming distance $d$ is fixed and the code length
$n$ goes to infinity. Under Kendall's $\tau$-metric, we narrow the gap between
the known lower bounds and upper bounds. Besides, we also obtain some sporadic
results under Kendall's $\tau$-metric for small parameters.
|
1611.07188v1
|
2016-12-01
|
Encoding orbital angular momentum of light in magnets
|
Breaking the diffraction limit and focusing laser beams to subwavelength
scale are becoming possible with the help of recent developments in plasmonics.
Such subwavelength focusing bridges different length scales of laser beams and
matter. Here we consider optical vortex, or laser beam carrying orbital angular
momentum (OAM) and discuss potential subwavelength magnetic phenomena induced
by such laser. On the basis of numerical calculations using
Landau-Lifshitz-Gilbert equation, we propose two OAM-dependent phenomena
induced by optical vortices, generation of radially anisotropic spin waves and
generation of topological defects in chiral magnets. The former could lead to
the transient topological Hall effect through the laser-induced scalar spin
chirality, and the latter reduces the timescale of generating skyrmionic
defects by several orders compared to other known means.
|
1612.00176v2
|
2016-12-26
|
Credibility and Dynamics of Collective Attention
|
Today, social media provide the means by which billions of people experience
news and events happening around the world. However, the absence of traditional
journalistic gatekeeping allows information to flow unencumbered through these
platforms, often raising questions of veracity and credibility of the reported
information. Here we ask: How do the dynamics of collective attention directed
toward an event reported on social media vary with its perceived credibility?
By examining the first large-scale, systematically tracked credibility database
of public Twitter messages (47M messages corresponding to 1,138 real-world
events over a span of three months), we established a relationship between the
temporal dynamics of events reported on social media and their associated level
of credibility judgments. Representing collective attention by the aggregate
temporal signatures of an event reportage, we found that the amount of
continued attention focused on an event provides information about its
associated levels of perceived credibility. Events exhibiting sustained,
intermittent bursts of attention were found to be associated with lower levels
of perceived credibility. In other words, as more people showed interest during
moments of transient collective attention, the associated uncertainty
surrounding these events also increased.
|
1612.08440v1
|
2017-01-07
|
Modulation calorimetry in diamond anvil cells I: heat flow models
|
Numerical simulations of heat transport in diamond anvil cells reveal a
possibility for absolute measurements of specific heat via high-frequency
modulation calorimetry. Such experiments could reveal and help characterize
temperature-driven phase transitions at high-pressure, such as melting, the
glass transition, magnetic and electric orderings, or superconducting
transitions. Specifically, we show that calorimetric information of a sample
cannot be directly extracted from measurements at frequencies slower than the
timescale of conduction to the diamond anvils (10s to 100s of kHz) since the
experiment is far from adiabatic. At higher frequencies, laser-heating
experiments allow relative calorimetric measurements, where changes in specific
heat of the sample are discriminated from changes in other material properties
by scanning the heating frequency from $\sim 1$ MHz to 1 GHz. But laser-heating
generates large temperature gradients in metal samples, preventing absolute
heat capacities to be inferred. High-frequency Joule heating, on the other
hand, allows accurate, absolute specific heat measurements if it can be
performed at high-enough frequency: assuming a thin layer of KBr insulation,
the specific heat of a 5 $\mu$m-thick metal sample heated at 100 kHz, 1 MHz, or
10 MHz frequency would be measured with 30%, 8% or 2% accuracy, respectively.
|
1701.01872v1
|
2017-01-08
|
Topological crystalline superconductors with linearly and projectively represented $C_{n}$ symmetry
|
We study superconductors with $n$-fold rotational invariance both in the
presence and in the absence of spin-orbit interactions. More specifically, we
classify the non-interacting Hamiltonians by defining a series of $Z$-numbers
for the Bogoliubov-de Gennes (BdG) symmetry classes of the Altland-Zimbauer
classification of random matrices in $1$D, $2$D, and $3$D in the presence of
discrete rotational invariance. Our analysis emphasizes the important role
played by the angular momentum of the Cooper pairs in the system: for pairings
of nonzero angular momentum, the rotation symmetry may be represented
projectively, and a projective representation of rotation symmetry may have
anomalous properties, including the anti-commutation with the time-reversal
symmetry. In 1D and 3D, we show how an $n$-fold axis enhances the topological
classification and give additional topological numbers; in 2D, we establish a
relation between the Chern number (in class D and CI) and the eigenvalues of
rotation symmetry at high-symmetry points. For each nontrivial class in 3D, we
write down a minimal effective theory for the surface Majorana states.
|
1701.01944v1
|
2017-01-09
|
Temperature dependence of shear viscosity of $SU(3)$--gluodynamics within lattice simulation
|
In this paper we study the shear viscosity temperature dependence of
$SU(3)$--gluodynamics within lattice simulation. To do so, we measure the
correlation functions of energy-momentum tensor in the range of temperatures
$T/T_c\in [0.9, 1.5]$. To extract the values of shear viscosity we used two
approaches. The first one is to fit the lattice data with some physically
motivated ansatz for the spectral function with unknown parameters and then
determine shear viscosity. The second approach is to apply the Backus-Gilbert
method which allows to extract shear viscosity from the lattice data
nonparametrically. The results obtained within both approaches agree with each
other. Our results allow us to conclude that within the temperature range
$T/T_c \in [0.9, 1.5]$ SU(3)--gluodynamics reveals the properties of a strongly
interacting system, which cannot be described perturbatively, and has the ratio
$\eta/s$ close to the value ${1}/{4\pi}$ in $N = 4$ Supersymmetric Yang-Mills
theory.
|
1701.02266v2
|
2017-01-25
|
Statistical Decoding
|
The security of code-based cryptography relies primarily on the hardness of
generic decoding with linear codes. The best generic decoding algorithms are
all improvements of an old algorithm due to Prange: they are known under the
name of information set decoding techniques (ISD). A while ago a generic
decoding algorithm which does not belong to this family was proposed:
statistical decoding. It is a randomized algorithm that requires the
computation of a large set of parity-check equations of moderate weight. We
solve here several open problems related to this decoding algorithm.
We give in particular the asymptotic complexity of this algorithm, give a
rather efficient way of computing the parity-check equations needed for it
inspired by ISD techniques and give a lower bound on its complexity showing
that when it comes to decoding on the Gilbert-Varshamov bound it can never be
better than Prange's algorithm.
|
1701.07416v2
|
2017-02-03
|
Enhanced Spin Conductance of a Thin-Film Insulating Antiferromagnet
|
We investigate spin transport by thermally excited spin waves in an
antiferromagnetic insulator. Starting from a stochastic Landau-Lifshitz-Gilbert
phenomenology, we obtain the out-of-equilibrium spin-wave properties. In linear
response to spin biasing and a temperature gradient, we compute the spin
transport through a normal metal$|$antiferromagnet$|$normal metal
heterostructure. We show that the spin conductance diverges as one approaches
the spin-flop transition; this enhancement of the conductance should be readily
observable by sweeping the magnetic field across the spin-flop transition. The
results from such experiments may, on the one hand, enhance our understanding
of spin transport near a phase transition, and on the other be useful for
applications that require a large degree of tunability of spin currents. In
contrast, the spin Seebeck coefficient does not diverge at the spin-flop
transition. Furthermore, the spin Seebeck coefficient is finite even at zero
magnetic field, provided that the normal metal contacts break the symmetry
between the antiferromagnetic sublattices.
|
1702.00975v2
|
2017-02-07
|
Giant-spin nonlinear response theory of magnetic nanoparticle hyperthermia: a field dependence study
|
Understanding high-field amplitude electromagnetic heat loss phenomena is of
great importance, in particular in the biomedical field, since the
heat-delivery treatment plans might rely on analytical models that are only
valid at low field amplitudes. Here, we develop a nonlinear response model
valid for single- domain nanoparticles of larger particle sizes and higher
field amplitudes in comparison to linear response theory. A nonlinear
magnetization expression and a generalized heat loss power equation are
obtained and compared with the exact solution of the stochastic
Landau-Lifshitz-Gilbert equation assuming the giant-spin hypothesis. The model
is valid within the hyperthermia therapeutic window and predicts a shift of
optimum particle size and distinct heat loss field amplitude exponents.
Experimental hyperthermia data with distinct ferrite-based nanoparticles, as
well as third harmonic magnetization data supports the nonlinear model, which
also has implications for magnetic particle imaging and magnetic thermometry.
|
1702.02022v1
|
2017-02-17
|
Spin-orbit torque in MgO/CoFeB/Ta/CoFeB/MgO symmetric structure with interlayer antiferromagnetic coupling
|
Spin current generated by spin Hall effect in the heavy metal would diffuse
up and down to adjacent ferromagnetic layers and exert torque on their
magnetization, called spin-orbit torque. Antiferromagnetically coupled
trilayers, namely the so-called synthetic antiferromagnets (SAF), are usually
employed to serve as the pinned layer of spintronic devices based on spin
valves and magnetic tunnel junctions to reduce the stray field and/or increase
the pinning field. Here we investigate the spin-orbit torque in
MgO/CoFeB/Ta/CoFeB/MgO perpendicularly magnetized multilayer with interlayer
antiferromagnetic coupling. It is found that the magnetization of two CoFeB
layers can be switched between two antiparallel states simultaneously. This
observation is replicated by the theoretical calculations by solving
Stoner-Wohlfarth model and Landau-Lifshitz-Gilbert equation. Our findings
combine spin-orbit torque and interlayer coupling, which might advance the
magnetic memories with low stray field and low power consumption.
|
1702.05331v1
|
2017-02-19
|
Multiscale simulations of topological transformations in magnetic Skyrmions
|
Magnetic Skyrmions belong to the most interesting spin structures for the
development of future information technology as they have been predicted to be
topologically protected. To quantify their stability, we use an innovative
multiscale approach to simulating spin dynamics based on the
Landau-Lifshitz-Gilbert equation. The multiscale approach overcomes the
micromagnetic limitations that have hindered realistic studies using
conventional techniques. We first demonstrate how the stability of a Skyrmion
is influenced by the refinement of the computational mesh and reveal that
conventionally employed traditional micromagnetic simulations are inadequate
for this task. Furthermore, we determine the stability quantitatively using our
multiscale approach. As a key operation for devices, the process of
annihilating a Skyrmion by exciting it with a spin polarized current pulse is
analyzed, showing that Skyrmions can be reliably deleted by designing the pulse
shape.
|
1702.05767v1
|
2017-02-21
|
Spin texture motion in antiferromagnetic and ferromagnetic nanowires
|
We propose a Hamiltonian dynamics formalism for the current and magnetic
field driven dynamics of ferromagnetic and antiferromagnetic domain walls in
one dimensional systems. To demonstrate the power of this formalism, we derive
Hamilton equations of motion via Poisson brackets based on the
Landau-Lifshitz-Gilbert phenomenology, and add dissipative dynamics via the
evolution of the energy. We use this approach to study current induced domain
wall motion and compute the drift velocity. For the antiferromagnetic case, we
show that a nonzero magnetic moment is induced in the domain wall, which
indicates that an additional application of a magnetic field would influence
the antiferromagnetic domain-wall dynamics. We consider both cases of the
magnetic field being parallel and transverse to the N{\'e}el field. Based on
this formalism, we predict an orientation switch mechanism for
antiferromagnetic domain walls which can be tested with the recently discovered
N{\'e}el spin orbit torques.
|
1702.06274v1
|
2017-02-23
|
Fulde-Ferrell States in Inverse Proximity Coupled Magnetically-Doped Topological Heterostructures
|
We study the superconducting properties of the thin film BCS superconductor
proximity coupled to a magnetically doped topological insulator(TI). Using the
mean field theory, we show that Fulde-Ferrell(FF) pairing can be induced in the
conventional superconductor by having inverse proximity effect(IPE). This
occurs when the IPE of the TI to the superconductor is large enough that the
normal band of the superconductor possesses a proximity induced spin-orbit
coupling and magnetization. We find that the energetics of the different
pairings are dependent on the coupling strength between the TI and the BCS
superconductor and the thickness of the superconductor film. As the thickness
of the superconductor film is increased, we find a crossover from the FF
pairing to the BCS pairing phase. This is a consequence of the increased number
of the superconducting bands, which favor the BCS pairing, implying that the FF
phase can only be observed in the thin-film limit. In addition, we also propose
transport experiments that show distinct signatures of the FF phase.
|
1702.07383v1
|
2017-02-27
|
Quantum critical scaling and fluctuations in Kondo lattice materials
|
We propose a new phenomenological framework for three classes of Kondo
lattice materials that incorporates the interplay between the fluctuations
associated with the antiferromagnetic quantum critical point and those produced
by the hybridization quantum critical point that marks the end of local moment
behavior. We show that these fluctuations give rise to two distinct regions of
quantum critical scaling: hybridization fluctuations are responsible for the
logarithmic scaling in the density of states of the heavy electron Kondo liquid
that emerges below the coherence temperature T*; while the unconventional power
law scaling in the resistivity that emerges at lower temperatures below T_QC
may reflect the combined effects of hybridization and antiferromagnetic quantum
critical fluctuations. Our framework is supported by experimental measurements
on CeCoIn5, CeRhIn5 and other heavy electron materials.
|
1702.08132v2
|
2017-03-09
|
On linear complementary-dual multinegacirculant codes
|
Linear codes with complementary-duals (LCD) are linear codes that intersect
with their dual trivially. Multinegacirculant codes of index $2$ that are LCD
are characterized algebraically and some good codes are found in this family.
Exact enumeration is performed for indices 2 and 3, and for all indices $t$ for
a special case of the co-index by using their concatenated structure.
Asymptotic existence results are derived for the special class of such codes
that are one-generator and have co-index a power of two by means of Dickson
polynomials. This shows that there are infinite families of LCD
multinegacirculant codes with relative distance satisfying a modified
Varshamov-Gilbert bound.
|
1703.03115v1
|
2017-03-16
|
Electronic transport in a two-dimensional superlattice engineered via self-assembled nanostructures
|
Nanoscience offers a unique opportunity to design modern materials from the
bottom up, via low-cost, solution processed assembly of nanoscale building
blocks. These systems promise electronic band structure engineering using not
only the nanoscale structural modulation, but also the mesoscale spatial
patterning, although experimental realization of the latter has been
challenging. Here we design and fabricate a new type of artificial solid by
stacking graphene on a self-assembled, nearly periodic array of nanospheres,
and experimentally observe superlattice miniband effects. We find conductance
dips at commensurate fillings of charge carriers per superlattice unit cell,
which are key features of minibands that are induced by the quasi-periodic
deformation of the graphene lattice. These dips become stronger when the
lattice strain is larger. Using a tight-binding model, we simulate the effect
of lattice deformation as a parameter affecting the inter-atomic hopping
integral, and confirm the superlattice transport behavior. This 2D
material-nanoparticle heterostructure enables facile band structure engineering
via self-assembly, promising for large area electronics and optoelectronics
applications.
|
1703.05689v3
|
2017-03-27
|
Dipolar ferromagnetism in three-dimensional superlattices of nanoparticles
|
A series of atomistic finite temperature simulations on a model of an FCC
lattice of maghemite nanoparticles using the stochastic Landau-Lifshitz-Gilbert
(sLLG) equation are presented. The model exhibits a ferromagnetic transition
that is in good agreement with theoretical expectations. The simulations also
reveal an orientational disorder in the orientational order parameter for $T <
0.5 T_c$ due to pinning of the surface domain walls of the nanoparticles by
surface vacancies. The extent of the competition between surface pinning and
dipolar interactions provides support for the conjecture that recent
measurements on systems of FCC superlattices of iron-oxide nanoparticles
provide evidence for dipolar ferromagnetism is discussed.
|
1703.09290v1
|
2017-03-06
|
Ultralow Energy Analog Straintronics Using Multiferroic Composites
|
Electric field-induced magnetization switching in multiferroics holds
profound promise for ultra-low-energy computing in beyond Moore's law era.
Bistable nanomagnets in the multiferroics are usually deemed to be suitable for
storing a binary bit of information and switching between the two stable states
allows us to process digital information. However, it requires to process
continuous analog signals too for seamless integration of nanomagnetic devices
in our future information processing systems. Here, we show that it is possible
to harness the analog nature in the magnetostrictive nanomagnets, contrary to
writing a digital bit of information. By solving stochastic
Landau-Lifshitz-Gilbert equation of magnetization dynamics at room-temperature,
we demonstrate such possibility and show that there exists a transistor-like
high-gain region in the input-output characteristics of the magnetostrictive
nanomagnets in strain-mediated multiferroic composites. This can be the basis
of ultra-low-energy analog and mixed signal precessing in our future
information processing systems and it eliminates the requirement of using
charge-based transistors.
|
1704.02337v1
|
2017-04-12
|
The gradient condition and the contribution of the dynamical part of Green-Kubo formula to the diffusion coefficient
|
In the diffusive hydrodynamic limit for a symmetric interacting particle
system (such as the exclusion process, the zero range process, the stochastic
Ginzburg-Landau model, the energy exchange model), a possibly non-linear
diffusion equation is derived as the hydrodynamic equation. The bulk diffusion
coefficient of the limiting equation is given by Green-Kubo formula and it can
be characterized by a variational formula. In the case the system satisfies the
gradient condition, the variational problem is explicitly solved and the
diffusion coefficient is given from the Green-Kubo formula through a static
average only. In other words, the contribution of the dynamical part of
Green-Kubo formula is 0. In this paper, we consider the converse, namely if the
contribution of the dynamical part of Green-Kubo formula is 0, does it imply
the system satisfies the gradient condition or not. We show that if the
equilibrium measure {\mu} is product and {L^2} space of its single site
marginal is separable, then the converse also holds. As an application of the
result, we consider a class of stochastic models for energy transport studied
by Gaspard and Gilbert in [1, 2], where the exact problem is discussed for this
specific model.
|
1704.03745v2
|
2017-04-12
|
The Origin of Sequential Chromospheric Brightenings
|
Sequential chromospheric brightenings (SCBs) are often observed in the
immediate vicinity of erupting flares and are associated with coronal mass
ejections. Since their initial discovery in 2005, there have been several
subsequent investigations of SCBs. These studies have used differing detection
and analysis techniques, making it difficult to compare results between
studies. This work employs the automated detection algorithm of Kirk et al.
(Solar Phys. 283, 97, 2013) to extract the physical characteristics of SCBs in
11 flares of varying size and intensity. We demonstrate that the magnetic
substructure within the SCB appears to have a significantly smaller area than
the corresponding H-alpha emission. We conclude that SCBs originate in the
lower corona around 0.1 R_sun above the photosphere, propagate away from the
flare center at speeds of 35 - 85 km/s, and have peak photosphere magnetic
intensities of 148 +/- 2.9 G. In light of these measurements, we infer SCBs to
be distinctive chromospheric signatures of erupting coronal mass ejections.
|
1704.03828v1
|
2017-04-12
|
Relationships Between Sequential Chromospheric Brightening and the Corona
|
The chromosphere is a complex region that acts as an intermediary between the
magnetic flux emergence in the photosphere and the magnetic features seen in
the corona. Large eruptions in the chromosphere of flares and filaments are
often accompanied by ejections of coronal mass off the sun. Several studies
have observed fast-moving progressive trains of compact bright points (called
Sequential Chromospheric Brightenings or SCBs) streaming away from
chromospheric flares that also produce a coronal mass ejection (CME). In this
work, we review studies of SCBs and search for commonalties between them. We
place these findings into a larger context with contemporary chromospheric and
coronal observations. SCBs are fleeting indicators of the solar atmospheric
environment as it existed before their associated eruption. Since they appear
at the very outset of a flare eruption, SCBs are good early indication of a CME
measured in the chromosphere.
|
1704.03835v1
|
2017-05-16
|
Constructive mathematics
|
This text is reproduced with the kind permission of Fran\c{c}ois Ap\'ery.
It was originally edited by Fran\c{c}ois Gu\'enard and Gilbert Leli\`evre for
the book "Penser les math\'ematiques".
It is the modified and abridged version of a text that appeared previously as
S\'eminaire de philosophie et math\'ematiques de l'\'Ecole normale sup\'erieure
(s\'eance du 26 avril 1976), Paris, IREM Paris-Nord, 1980, 15 pp.,
http://www.numdam.org/item?id=SPHM_1976___1_A1_0, as well as in Langage et
pens\'ee math\'ematiques: actes du colloque international (Luxembourg, 9-11
juin 1976), Luxembourg, Centre universitaire de Luxembourg, 1976, pp. 391--410.
In its introduction, Ap\'ery writes: "In default of convincing, this text can
set the record straight: we show that the constructive conception does not
mutilate, on the contrary, it enriches classical mathematics."
|
1705.05581v2
|
2017-05-17
|
Magnetic-Visual Sensor Fusion based Medical SLAM for Endoscopic Capsule Robot
|
A reliable, real-time simultaneous localization and mapping (SLAM) method is
crucial for the navigation of actively controlled capsule endoscopy robots.
These robots are an emerging, minimally invasive diagnostic and therapeutic
technology for use in the gastrointestinal (GI) tract. In this study, we
propose a dense, non-rigidly deformable, and real-time map fusion approach for
actively controlled endoscopic capsule robot applications. The method combines
magnetic and vision based localization, and makes use of frame-to-model fusion
and model-to-model loop closure. The performance of the method is demonstrated
using an ex-vivo porcine stomach model. Across four trajectories of varying
speed and complexity, and across three cameras, the root mean square
localization errors range from 0.42 to 1.92 cm, and the root mean square
surface reconstruction errors range from 1.23 to 2.39 cm.
|
1705.06196v2
|
2017-05-21
|
First-Order Reversal Curves of the Magnetostructural Phase Transition in FeTe
|
We apply the first-order reversal curve (FORC) method, borrowed from studies
of ferromagnetic materials, to the magneto-structural phase transition of FeTe.
FORC measurements reveal two features in the hysteretic phase transition, even
in samples where traditional temperature measurements display only a single
transition. For Fe1.13Te, the influence of magnetic field suggests that the
main feature is primarily structural while a smaller, slightly
higher-temperature transition is magnetic in origin. By contrast Fe1.03Te has a
single transition which shows a uniform response to magnetic field, indicating
a stronger coupling of the magnetic and structural phase transitions. We also
introduce uniaxial stress, which spreads the distribution width without
changing the underlying energy barrier of the transformation. The work shows
how FORC can help disentangle the roles of the magnetic and structural phase
transitions in FeTe.
|
1705.07380v1
|
2017-05-24
|
Towards Understanding the Invertibility of Convolutional Neural Networks
|
Several recent works have empirically observed that Convolutional Neural Nets
(CNNs) are (approximately) invertible. To understand this approximate
invertibility phenomenon and how to leverage it more effectively, we focus on a
theoretical explanation and develop a mathematical model of sparse signal
recovery that is consistent with CNNs with random weights. We give an exact
connection to a particular model of model-based compressive sensing (and its
recovery algorithms) and random-weight CNNs. We show empirically that several
learned networks are consistent with our mathematical analysis and then
demonstrate that with such a simple theoretical framework, we can obtain
reasonable re- construction results on real images. We also discuss gaps
between our model assumptions and the CNN trained for classification in
practical scenarios.
|
1705.08664v1
|
2017-03-06
|
Ultra-low-energy Electric field-induced Magnetization Switching in Multiferroic Heterostructures
|
Electric field-induced magnetization switching in multiferroics is intriguing
for both fundamental studies and potential technological applications. Here, we
review the recent developments on electric field-induced magnetization
switching in multiferroic heterostructures. Particularly, we study the dynamics
of magnetization switching between the two stable states in a shape-anisotropic
single-domain nanomagnet using stochastic Landau-Lifshitz-Gilbert (LLG)
equation in the presence of thermal fluctuations. For magnetostrictive
nanomagnets in strain-coupled multiferroic composites, such study of
magnetization dynamics, contrary to steady-state scenario, revealed intriguing
new phenomena on binary switching mechanism. While the traditional method of
binary switching requires to tilt the potential profile to the desired state of
switching, we show that no such tilting is necessary to switch successfully
since the magnetization's excursion out of magnet's plane can generate a
built-in asymmetry during switching. We also study the switching dynamics in
multiferroic heterostructures having magnetoelectric coupling at the interface
and magnetic exchange coupling that can facilitate to maintain the direction of
switching with the polarity of the applied electric field. We calculate the
performance metrics like switching delay and energy dissipation during
switching while simulating LLG dynamics. The performance metrics turn out to be
very encouraging for potential technological applications.
|
1706.03039v1
|
2017-06-11
|
Local List Recovery of High-rate Tensor Codes and Applications
|
In this work, we give the first construction of high-rate locally
list-recoverable codes. List-recovery has been an extremely useful building
block in coding theory, and our motivation is to use these codes as such a
building block. In particular, our construction gives the first
capacity-achieving locally list-decodable codes (over constant-sized alphabet);
the first capacity achieving globally list-decodable codes with nearly linear
time list decoding algorithm (once more, over constant-sized alphabet); and a
randomized construction of binary codes on the Gilbert-Varshamov bound that can
be uniquely decoded in near-linear-time, with higher rate than was previously
known.
Our techniques are actually quite simple, and are inspired by an approach of
Gopalan, Guruswami, and Raghavendra (Siam Journal on Computing, 2011) for
list-decoding tensor codes. We show that tensor powers of (globally)
list-recoverable codes are "approximately" locally list-recoverable, and that
the "approximately" modifier may be removed by pre-encoding the message with a
suitable locally decodable code. Instantiating this with known constructions of
high-rate globally list-recoverable codes and high-rate locally decodable codes
finishes the construction.
|
1706.03383v1
|
2017-05-31
|
Local Differential Privacy for Physical Sensor Data and Sparse Recovery
|
In this work we explore the utility of locally differentially private thermal
sensor data. We design a locally differentially private recovery algorithm for
the 1-dimensional, discrete heat source location problem and analyse its
performance in terms of the Earth Mover Distance error. Our work indicates that
it is possible to produce locally private sensor measurements that both keep
the exact locations of the heat sources private and permit recovery of the
"general geographic vicinity" of the sources. We also discuss the relationship
between the property of an inverse problem being ill-conditioned and the amount
of noise needed to maintain privacy.
|
1706.05916v4
|
2017-06-22
|
High-performance nanoscale topological energy transduction
|
The realization of high-performance, small-footprint, on-chip inductors
remains a challenge in radio-frequency and power microelectronics, where they
perform vital energy transduction in filters and power converters. Modern
planar inductors consist of metallic spirals that consume significant chip
area, resulting in low inductance densities. We present a novel method for
magnetic energy transduction that utilizes ferromagnetic islands (FIs) on the
surface of a 3D time-reversal-invariant topological insulator (TI) to produce
paradigmatically different inductors. Depending on the chemical potential, the
FIs induce either an anomalous or quantum anomalous Hall effect in the
topological surface states. These Hall effects direct current around the FIs,
concentrating magnetic flux and producing a highly inductive device. Using a
novel self-consistent simulation that couples AC non-equilibrium Green
functions to fully electrodynamic solutions of Maxwell's equations, we
demonstrate excellent inductance densities up to terahertz frequencies, thus
harnessing the unique properties of topological materials for practical device
applications.
|
1706.07381v1
|
2017-06-28
|
Extension and Applications of a Variational Approach with Deformed Derivatives
|
We have recently presented an extension of the standard variational calculus
to include the presence of deformed derivatives in the Lagrangian of a system
of particles and in the Lagrangian density of field-theoretic models. Classical
Euler-Lagrange equations and the Hamiltonian formalism have been re-assessed in
this approach. Whenever applied to a number of physical systems, the resulting
dynamical equations come out to be the correct ones found in the literature,
specially with mass-dependent and with non-linear equations for classical and
quantum-mechanical systems. In the present contribution, we extend the
variational approach with the intervalar form of deformed derivatives to study
higher-order dissipative systems, with application to concrete situations, such
as an accelerated point charge - this is the problem of the
Abraham-Lorentz-Dirac force - to stochastic dynamics like the Langevin, the
advection-convection-reaction and Fokker-Planck equations, Korteweg-de Vries
equation, Landau-Lifshits-Gilbert equation and the Caldirola-Kanai Hamiltonian.
By considering these different applications, we show that the formulation
investigated in this paper may be a simple and promising path for dealing with
dissipative, non-linear and stochastic systems through the variational
approach.
|
1706.09504v1
|
2017-07-06
|
Relaxation time and critical slowing down of a spin-torque oscillator
|
The relaxation phenomena of spin-torque oscillators consisting of
nanostructured ferromagnets are interesting research targets in magnetism. A
theoretical study on the relaxation time of a spin-torque oscillator from one
self-oscillation state to another is investigated. By solving the
Landau-Lifshitz-Gilbert equation both analytically and numerically, it is shown
that the oscillator relaxes to the self-oscillation state exponentially within
a few nanoseconds, except when magnetization is close to a critical point. The
relaxation rate, which is an inverse of relaxation time, is proportional to the
current. On the other hand, a critical slowing down appears near the critical
point, where relaxation is inversely proportional to time, and the relaxation
time becomes on the order of hundreds of nanoseconds. These conclusions are
primarily obtained for a spin-torque oscillator consisting of a perpendicularly
magnetized free layer and an in-plane magnetized pinned layer, and are further
developed for application to arbitrary types of spin-torque oscillators.
|
1707.01960v1
|
2017-07-07
|
Pandeia: A Multi-mission Exposure Time Calculator for JWST and WFIRST
|
Pandeia is the exposure time calculator (ETC) system developed for the James
Webb Space Telescope (JWST) that will be used for creating JWST proposals. It
includes a simulation-hybrid Python engine that calculates the two-dimensional
pixel-by-pixel signal and noise properties of the JWST instruments. This allows
for appropriate handling of realistic point spread functions, MULTIACCUM
detector readouts, correlated detector readnoise, and multiple photometric and
spectral extraction strategies. Pandeia includes support for all the JWST
observing modes, including imaging, slitted/slitless spectroscopy, integral
field spectroscopy, and coronagraphy. Its highly modular, data-driven design
makes it easily adaptable to other observatories. An implementation for use
with WFIRST is also available.
|
1707.02202v2
|
2017-07-10
|
Convex optimization over classes of multiparticle entanglement
|
A well-known strategy to characterize multiparticle entanglement utilizes the
notion of stochastic local operations and classical communication (SLOCC), but
characterizing the resulting entanglement classes is difficult. Given a
multiparticle quantum state, we first show that Gilbert's algorithm can be
adapted to prove separability or membership in a certain entanglement class. We
then present two algorithms for convex optimization over SLOCC classes. The
first algorithm uses a simple gradient approach, while the other one employs
the accelerated projected-gradient method. For demonstration, the algorithms
are applied to the likelihood-ratio test using experimental data on bound
entanglement of a noisy four-photon Smolin state [Phys. Rev. Lett. 105, 130501
(2010)].
|
1707.02958v3
|
2017-07-13
|
Collective charge excitations and the metal-insulator transition in the square lattice Hubbard-Coulomb model
|
In this article, we discuss the non-trivial collective charge excitations
(plasmons) of the extended square-lattice Hubbard model. Using a fully
non-perturbative approach, we employ the hybrid Monte Carlo algorithm to
simulate the system at half-filling. A modified Backus-Gilbert method is
introduced to obtain the spectral functions via numerical analytic
continuation. We directly compute the single-particle density of states which
demonstrates the formation of Hubbard bands in the strongly-correlated phase.
The momentum-resolved charge susceptibility is also computed on the basis of
the Euclidean charge density-density correlator. In agreement with previous
EDMFT studies, we find that at large strength of the electron-electron
interaction, the plasmon dispersion develops two branches.
|
1707.04212v2
|
2017-07-25
|
Partially Ionized Plasmas in Astrophysics
|
Partially ionized plasmas are found across the Universe in many different
astrophysical environments. They constitute an essential ingredient of the
solar atmosphere, molecular clouds, planetary ionospheres and protoplanetary
disks, among other environments, and display a richness of physical effects
which are not present in fully ionized plasmas. This review provides an
overview of the physics of partially ionized plasmas, including recent advances
in different astrophysical areas in which partial ionization plays a
fundamental role. We outline outstanding observational and theoretical
questions and discuss possible directions for future progress.
|
1707.07975v2
|
2017-07-14
|
Spatially variant PSF modeling in confocal macroscopy
|
Point spread function (PSF) plays an essential role in image reconstruction.
In the context of confocal microscopy, optical performance degrades towards the
edge of the field of view as astigmatism, coma and vignetting. Thus, one should
expect the related artifacts to be even stronger in macroscopy, where the field
of view is much larger. The field aberrations in macroscopy fluorescence
imaging system was observed to be symmetrical and to increase with the distance
from the center of the field of view. In this paper we propose an experiment
and an optimization method for assessing the center of the field of view. The
obtained results constitute a step towards reducing the number of parameters in
macroscopy PSF model.
|
1707.09858v1
|
2017-08-09
|
Bubble Magnetometry of Nanoparticle Heterogeneity and Interaction
|
Bubbles have a rich history as transducers in particle-physics experiments.
In a solid-state analogue, we use bubble domains in nanomagnetic films to
measure magnetic nanoparticles. This technique can determine the magnetic
orientation of a single nanoparticle in a fraction of a second, and generate a
full hysteresis loop in a few seconds, which is much faster than any other
reported technique. We achieve this unprecedented speed by tuning the
nanomagnetic properties of the films, including the Dzyaloshinskii-Moriya
interaction, in the first application of topological protection from the
skyrmion state to a nanoparticle sensor. We demonstrate the technique on
iron/nickel nanorods and iron oxide nanoparticles, which delineate a wide range
of properties and applications. Bubble magnetometry enables the first
measurement with high throughput for statistical analysis of the magnetic
hysteresis of dispersed nanoparticles, and the first direct measurement of a
transition from superparamagnetic behavior as single nanoparticles to
collective behavior in nanoscale agglomerates. These results demonstrate a
breakthrough capability for measuring the heterogeneity and interaction of
magnetic nanoparticles.
|
1708.02706v2
|
2017-08-18
|
Antiferromagnetic nano-oscillator in external magnetic fields
|
We describe the dynamics of an antiferromagnetic nano-oscillator in an
external magnetic field of any given time distribution. The oscillator is
powered by a spin current originating from spin-orbit effects in a neighboring
heavy metal layer, and is capable of emitting a THz signal in the presence of
an additional easy-plane anisotropy. We derive an analytical formula describing
the interaction between such a system and an external field, which can affect
the output signal character. Interactions with magnetic pulses of different
shapes, with a sinusoidal magnetic field and with a sequence of rapidly
changing magnetic fields are discussed. We also perform numerical simulations
based on the Landau-Lifshitz-Gilbert equation with spin-transfer torque effects
to verify the obtained results and find a very good quantitative agreement
between analytical and numerical predictions.
|
1708.05590v2
|
2017-09-07
|
A sharp bound for winning within a proportion of the maximum of a sequence
|
This note considers a variation of the full-information secretary problem
where the random variables to be observed are independent and identically
distributed. Consider $X_1,\dots,X_n$ to be an independent sequence of random
variables, let $M_n:=\max\{X_1,\dots,X_n\}$, and the objective is to select the
maximum of the sequence. What is the maximum probability of "stopping at the
maximum"? That is, what is the stopping time $\tau$ adapted to $X_1,...,X_n$
that maximizes $P(X_{\tau}=M_n)$? This problem was examined by Gilbert and
Mosteller \cite{GilMost} when in addition the common distribution is
continuous. The optimal win probability in this case is denoted by
$v_{n,max}^*$. What if it is desired to "stop within a proportion of the
maximum"? That is, for $0<\alpha<1$, what is the stopping rule $\tau$ that
maximizes $P(X_{\tau} \geq \alpha M_n)$? In this note both problems are treated
as games, it is proven that for any continuous random variable $X$, if $\tau^*$
is the optimal stopping rule then $P(X_{\tau^*} \geq \alpha M_n)\geq
v_{n,max}^*$, and this lower bound is sharp. Some examples and another
interesting result are presented.
|
1709.02416v1
|
2017-09-18
|
Endo-VMFuseNet: Deep Visual-Magnetic Sensor Fusion Approach for Uncalibrated, Unsynchronized and Asymmetric Endoscopic Capsule Robot Localization Data
|
In the last decade, researchers and medical device companies have made major
advances towards transforming passive capsule endoscopes into active medical
robots. One of the major challenges is to endow capsule robots with accurate
perception of the environment inside the human body, which will provide
necessary information and enable improved medical procedures. We extend the
success of deep learning approaches from various research fields to the problem
of uncalibrated, asynchronous, and asymmetric sensor fusion for endoscopic
capsule robots. The results performed on real pig stomach datasets show that
our method achieves sub-millimeter precision for both translational and
rotational movements and contains various advantages over traditional sensor
fusion techniques.
|
1709.06041v2
|
2017-09-22
|
On self-dual negacirculant codes of index two and four
|
In this paper, we study a special kind of factorization of $x^n+1$ over
$\mathbb{F}_q, $ with $q$ a prime power $\equiv 3~({\rm mod}~4)$ when $n=2p,$
with $p\equiv 3~({\rm mod}~4)$ and $p$ is a prime. Given such a $q$ infinitely
many such $p$'s exist that admit $q$ as a primitive root by the Artin
conjecture in arithmetic progressions. This number theory conjecture is known
to hold under GRH. We study the double (resp. four)-negacirculant codes over
finite fields $\mathbb{F}_q, $ of co-index such $n$'s, including the exact
enumeration of the self-dual subclass, and a modified Varshamov-Gilbert bound
on the relative distance of the codes it contains.
|
1709.07546v2
|
2017-03-06
|
Ultra-Low-Energy Straintronics Using Multiferroic Composites
|
The primary impediment to continued improvement of charge-based electronics
is the excessive energy dissipation incurred in switching a bit of information.
With suitable choice of materials, devices made of multiferroic composites,
i.e., strain-coupled piezoelectric-magnetostrictive heterostructures, dissipate
miniscule amount of energy of ~1 attojoule at room-temperature, while switching
in sub-nanosecond delay. Apart from devising memory bits, such devices can be
also utilized for building logic, so that they can be deemed suitable for
computing purposes as well. Here, we first review the current state of the art
for building nanoelectronics using multiferroic composites. On a recent
development, it is shown that these multiferroic straintronic devices can be
also utilized for analog signal processing, with suitable choice of materials.
By solving stochastic Landau-Lifshitz-Gilbert equation of magnetization
dynamics at room-temperature, it is shown that we can achieve a voltage gain,
i.e., these straintronic devices can act as voltage amplifiers.
|
1710.00612v1
|
2017-10-12
|
Modeling the aging kinetics of zirconia ceramics
|
Yttria-stabilized tetragonal zirconia polycrystals (3Y-TZP) with different
microstructures were elaborated. The isothermal tetragonal to monoclinic
transformation was investigated at 134 {\deg}C in steam by X-ray diffraction,
Atomic Force Microscopy (AFM) and optical interferometry. The aging kinetics
were analyzed in terms of nucleation and growth, using the Mehl-Avrami-Johnson
(MAJ) formalism. Numerical simulation of the aging of zirconia surfaces was
also conducted, and the results were used to better fit the aging kinetics. The
simulation shows that the exponent of the MAJ laws is controlled not only by
the nucleation and growth mechanisms, but also - and mainly - by their
respective kinetic parameters. Measurements of nucleation and growth rates at
the surface, at the beginning of aging, and the use of numerical simulation
allow the accurate prediction of aging kinetics.
|
1710.04454v1
|
2017-10-13
|
Microstructural investigation of the aging behavior of $(3Y-TZP)-Al_2O_3$ composites
|
The low temperature autoclave aging behavior of zirconia toughened alumina
composites processed by a classical powder mixing processing route was analyzed
using atomic force microscopy (AFM), scanning electron microscopy and X-Ray
diffraction. The transformation was evaluated in terms of nucleation and
growth, assessed by XRD. The time-temperature equivalency of the transformation
was used to measure an apparent activation energy of the nucleation stage of
the transformation of 78kJ/mol. The microstructural features influencing the
transformation were identified, and the influence of the alumina matrix on the
transformation was investigated. Transformation progression grain by grain was
observed by AFM. Transformation does not only occur in zirconia agglomerates
but also in isolated zirconia grains. The matrix could partially inhibit the
transformation. This behavior could be rationalized considering the
constraining effect of the alumina matrix, shape strain accommodation arguments
and microstructural homogeneity effects.
|
1710.04923v1
|
2017-10-16
|
Tunable Low Density Palladium Nanowire Foams
|
Nanostructured palladium foams offer exciting potential for applications in
diverse fields such as catalyst, fuel cell, and particularly hydrogen storage
technologies. We have fabricated palladium nanowire foams using a cross-linking
and freeze-drying technique. These foams have a tunable density down to 0.1% of
the bulk, and a surface area to volume ratio of up to 1,540,000:1. They exhibit
highly attractive characteristics for hydrogen storage, in terms of loading
capacity, rate of absorption and heat of absorption. The hydrogen
absorption/desorption process is hysteretic in nature, accompanied by
substantial lattice expansion/contraction as the foam converts between Pd and
PdHx.
|
1710.05906v1
|
2017-10-19
|
An estimate for the thermal photon rate from lattice QCD
|
We estimate the production rate of photons by the quark-gluon plasma in
lattice QCD. We propose a new correlation function which provides better
control over the systematic uncertainty in estimating the photon production
rate at photon momenta in the range {\pi}T/2 to 2{\pi}T. The relevant Euclidean
vector current correlation functions are computed with $N_{\mathrm f}$ = 2
Wilson clover fermions in the chirally-symmetric phase. In order to estimate
the photon rate, an ill-posed problem for the vector-channel spectral function
must be regularized. We use both a direct model for the spectral function and a
model-independent estimate from the Backus-Gilbert method to give an estimate
for the photon rate.
|
1710.07050v1
|
2017-10-26
|
Spin dynamics in MgO based magnetic tunnel junctions with dynamical exchange coupling
|
We study the spin dynamics in Fe|MgO|Fe tunnel junction with the dynamical
exchange coupling by coupled Landau-Lifshitz-Gilbert equations. The effects of
spin pumping on the spin dynamics are investigated in detail. It is observed
that the spin pumping can stabilize a quasi-antiparallel state rather than a
quasi-parallel one. More interestingly, our work suggests that the spin pumping
torque can efficiently modulate the magnetization, similar to the
thermal-bias-driven and electricbias-driven spin torques.
|
1710.09666v2
|
2017-10-27
|
Bulk and edge spin transport in topological magnon insulators
|
We investigate the spin transport properties of a topological magnon
insulator, a magnetic insulator characterized by topologically nontrivial bulk
magnon bands and protected magnon edge modes located in the bulk band gaps.
Employing the Landau-Lifshitz-Gilbert phenomenology, we calculate the spin
current driven through a normal metal$|$topological magnon insulator$|$normal
metal heterostructure by a spin accumulation imbalance between the metals, with
and without random lattice defects. We show that bulk and edge transport are
characterized by different length scales. This results in a characteristic
system size where the magnon transport crosses over from being bulk-dominated
for small systems to edge-dominated for larger systems. These findings are
generic and relevant for topological transport in systems of nonconserved
bosons.
|
1710.09998v2
|
2017-10-30
|
Spin-transfer Antiferromagnetic Resonance
|
Currents can induce spin excitations in antiferromagnets, even when they are
insulating. We investigate how spin transfer can cause antiferromagnetic
resonance in bilayers and trilayers that consist of one antiferromagnetic
insulator and one or two metals. An ac voltage applied to the metal generates a
spin Hall current that drives the magnetic moments in the antiferromagnet. We
consider excitation of the macrospin mode and of transverse standing-spin-wave
modes. By solving the Landau-Lifshitz-Gilbert equation in the antiferromagnetic
insulator and the spin-diffusion equation in the normal metal, we derive
analytical expressions for the spin-Hall-magnetoresistance and spin-pumping
inverse-spin-Hall dc voltages. In bilayers, the two contributions compensate
each other and cannot easily be distinguished. We present numerical results for
a MnF$_2|$Pt bilayer. Trilayers facilitate separation of the
spin-Hall-magnetoresistance and spin-pumping voltages, thereby revealing more
information about the spin excitations. We also compute the decay of the pumped
spin current through the antiferromagnetic layer as a function of frequency and
the thickness of the antiferromagnetic layer.
|
1710.10909v1
|
2017-10-31
|
Compact Multi-Class Boosted Trees
|
Gradient boosted decision trees are a popular machine learning technique, in
part because of their ability to give good accuracy with small models. We
describe two extensions to the standard tree boosting algorithm designed to
increase this advantage. The first improvement extends the boosting formalism
from scalar-valued trees to vector-valued trees. This allows individual trees
to be used as multiclass classifiers, rather than requiring one tree per class,
and drastically reduces the model size required for multiclass problems. We
also show that some other popular vector-valued gradient boosted trees
modifications fit into this formulation and can be easily obtained in our
implementation. The second extension, layer-by-layer boosting, takes smaller
steps in function space, which is empirically shown to lead to a faster
convergence and to a more compact ensemble. We have added both improvements to
the open-source TensorFlow Boosted trees (TFBT) package, and we demonstrate
their efficacy on a variety of multiclass datasets. We expect these extensions
will be of particular interest to boosted tree applications that require small
models, such as embedded devices, applications requiring fast inference, or
applications desiring more interpretable models.
|
1710.11547v1
|
2017-11-14
|
Stationary Vacuum Black Holes in 5 Dimensions
|
We study the problem of asymptotically flat bi-axially symmetric stationary
solutions of the vacuum Einstein equations in $5$-dimensional spacetime. In
this setting, the cross section of any connected component of the event horizon
is a prime $3$-manifold of positive Yamabe type, namely the $3$-sphere $S^3$,
the ring $S^1\times S^2$, or the lens space $L(p,q)$. The Einstein vacuum
equations reduce to an axially symmetric harmonic map with prescribed
singularities from $\mathbb{R}^3$ into the symmetric space
$SL(3,\mathbb{R})/SO(3)$. In this paper, we solve the problem for all possible
topologies, and in particular the first candidates for smooth vacuum
non-degenerate black lenses are produced. In addition, a generalization of this
result is given in which the spacetime is allowed to have orbifold
singularities. We also formulate conditions for the absence of conical
singularities which guarantee a physically relevant solution.
|
1711.05229v2
|
2017-11-27
|
Voltage induced switching of an antiferromagnetically ordered topological Dirac semimetal
|
An antiferromagnetic semimetal has been recently identified as a new member
of topological semimetals that may host three-dimensional symmetry-protected
Dirac fermions. A reorientation of the N\'{e}el vector may break the underlying
symmetry and open a gap in the quasi-particle spectrum, inducing the
(semi)metal-insulator transition. Here, we predict that such transition may be
controlled by manipulating the chemical potential location of the material. We
perform both analytical and numerical analysis on the thermodynamic potential
of the model Hamiltonian and find that the gapped spectrum is preferred when
the chemical potential is located at the Dirac point. As the chemical potential
deviates from the Dirac point, the system shows a possible transition from the
gapped to the gapless phase and switches the corresponding N\'{e}el vector
configuration. We perform density functional theory calculations to verify our
analysis using a realistic material and discuss a two terminal transport
measurement as a possible route to identify the voltage induced switching of
the N\'{e}el vector.
|
1711.09926v2
|
2017-12-11
|
Convergent tangent plane integrators for the simulation of chiral magnetic skyrmion dynamics
|
We consider the numerical approximation of the Landau-Lifshitz-Gilbert
equation, which describes the dynamics of the magnetization in ferromagnetic
materials. In addition to the classical micromagnetic contributions, the energy
comprises the Dzyaloshinskii-Moriya interaction, which is the most important
ingredient for the enucleation and the stabilization of chiral magnetic
skyrmions. We propose and analyze three tangent plane integrators, for which we
prove (unconditional) convergence of the finite element solutions towards a
weak solution of the problem. The analysis is constructive and also establishes
existence of weak solutions. Numerical experiments demonstrate the
applicability of the methods for the simulation of practically relevant problem
sizes.
|
1712.03795v3
|
2017-12-31
|
Proper dissipative torques in antiferromagnetic dynamics
|
There is little doubt that the magnetization dynamics of ferromagnetic
systems is governed by the Landau-Lifshitz-Gilbert equation or its
generalization with various spin torques. In contrast, there are several sets
of dynamic equations for two-sublattice antiferromagnets (AFMs) in literature
that have different forms of dissipative torques and no proper dynamic
equations for multi-sublattice AFMs and ferrimagnets in general. Here we
introduce the general Rayleigh dissipation functional into the Lagrange
equation and derive the proper form of the dissipative torques in the
phenomenological equations for the AFMs with multiple sublattices. A new type
of dissipative torque arising from inter-sublattice drag effect is discovered
that has important influences on magnon lifetime and domain wall motion. In
particular, our theory unifies different dynamic equations of AFMs in
literature.
|
1801.00217v1
|
2018-01-12
|
Optimal Streaming Codes for Channels with Burst and Arbitrary Erasures
|
This paper considers transmitting a sequence of messages (a streaming source)
over a packet erasure channel. In each time slot, the source constructs a
packet based on the current and the previous messages and transmits the packet,
which may be erased when the packet travels from the source to the destination.
Every source message must be recovered perfectly at the destination subject to
a fixed decoding delay. We assume that the channel loss model introduces either
one burst erasure or multiple arbitrary erasures in any fixed-sized sliding
window. Under this channel loss assumption, we fully characterize the maximum
achievable rate by constructing streaming codes that achieve the optimal rate.
In addition, our construction of optimal streaming codes implies the full
characterization of the maximum achievable rate for convolutional codes with
any given column distance, column span and decoding delay. Numerical results
demonstrate that the optimal streaming codes outperform existing streaming
codes of comparable complexity over some instances of the Gilbert-Elliott
channel and the Fritchman channel.
|
1801.04241v2
|
2018-01-16
|
Detecting Electron Density Fluctuations from Cosmic Microwave Background Polarization using a Bispectrum Approach
|
Recent progress in high sensitivity Cosmic Microwave Background (CMB)
polarization experiments opens up a window on large scale structure (LSS), as
CMB polarization fluctuations on small angular scales can arise from a
combination of LSS and ionization fluctuations in the late universe.
Gravitational lensing effects can be extracted from CMB datasets with quadratic
estimators but reconstructions of electron density fluctuations (EDFs) with
quadratic estimators are found to be significantly biased by the much larger
lensing effects in the secondary CMB fluctuations. In this paper we establish a
bispectrum formalism using tracers of LSS to extract the subdominant EDFs from
CMB polarization data. We find that this bispectrum can effectively reconstruct
angular band-powers of cross correlation between EDFs and LSS tracers. Next
generation CMB polarization experiments in conjunction with galaxy surveys and
cosmic infrared background experiments can detect signatures of EDFs with high
significance.
|
1801.05396v1
|
2018-01-18
|
Magpy: A C++ accelerated Python package for simulating magnetic nanoparticle stochastic dynamics
|
Magpy is a C++ accelerated Python package for modelling and simulating the
magnetic dynamics of nano-sized particles. Nanoparticles are modelled as a
system of three-dimensional macrospins and simulated with a set of coupled
stochastic differential equations (the Landau-Lifshitz-Gilbert equation), which
are solved numerically using explicit or implicit methods. The results of the
simulations may be used to compute equilibrium states, the dynamic response to
external magnetic fields, and heat dissipation. Magpy is built on a C++
library, which is optimised for serial execution, and exposed through a Python
interface utilising an embarrassingly parallel strategy. Magpy is free,
open-source, and available on github under the 3-Clause BSD License.
|
1801.06073v2
|
2018-01-22
|
On the List Decodability of Self-orthogonal Rank Metric Codes
|
V. Guruswami and N. Resch prove that the list decodability of
$\mathbb{F}_q$-linear rank metric codes is as good as that of random rank
metric codes in~\cite{venkat2017}. Due to the potential applications of
self-orthogonal rank metric codes, we focus on list decoding of them. In this
paper, we prove that with high probability, an $\F_q$-linear self-orthogonal
rank metric code over $\mathbb{F}_q^{n\times m}$ of rate
$R=(1-\tau)(1-\frac{n}{m}\tau)-\epsilon$ is shown to be list decodable up to
fractional radius $\tau\in(0,1)$ and small $\epsilon\in(0,1)$ with list size
depending on $\tau$ and $q$ at most $O_{\tau, q}(\frac{1}{\epsilon})$. In
addition, we show that an $\mathbb{F}_{q^m}$-linear self-orthogonal rank metric
code of rate up to the Gilbert-Varshamov bound is $(\tau n, \exp(O_{\tau,
q}(\frac{1}{\epsilon})))$-list decodable.
|
1801.07033v1
|
2018-01-25
|
Smoothing Algorithms for Computing the Projection onto a Minkowski Sum of Convex Sets
|
In this paper, the problem of computing the projection, and therefore the
minimum distance, from a point onto a Minkowski sum of general convex sets is
studied. Our approach is based on the minimum norm duality theorem originally
stated by Nirenberg and the Nesterov smoothing techniques. It is shown that
projection points onto a Minkowski sum of sets can be represented as the sum of
points on constituent sets so that, at these points, all of the sets share the
same normal vector which is the negative of the dual solution. The proposed
NESMINO algorithm improves the theoretical bound on number of iterations from
$O(\frac{1}{\epsilon})$ by Gilbert [SIAM J. Contr., vol. 4, pp. 61--80, 1966]
to $O\left(\frac{1}{\sqrt{\epsilon}}\ln(\frac{1}{\epsilon})\right)$, where
$\epsilon$ is the desired accuracy for the objective function. Moreover, the
algorithm also provides points on each component sets such that their sum is
equal to the projection point.
|
1801.08285v1
|
2018-01-25
|
Finite momentum Cooper pairing in 3D topological insulator Josephson junctions
|
Unconventional superconductivity arising from the interplay between strong
spin-orbit coupling and magnetism is an intensive area of research. One form of
unconventional superconductivity arises when Cooper pairs subjected to a
magnetic exchange coupling acquire a finite momentum. Here, we report on a
signature of finite momentum Cooper pairing in the 3D topological insulator
Bi2Se3. We apply in-plane and out-of-plane magnetic fields to proximity-coupled
Bi2Se3 and find that the in-plane field creates a spatially oscillating
superconducting order parameter in the junction as evidenced by the emergence
of an anomalous Fraunhofer pattern. We describe how the anomalous Fraunhofer
patterns evolve for different device parameters, and we use this to understand
the microscopic origin of the oscillating order parameter. The agreement
between the experimental data and simulations shows that the finite momentum
pairing originates from the coexistence of the Zeeman effect and Aharonov-Bohm
flux.
|
1801.08504v1
|
2018-01-31
|
Numerical analytic continuation of Euclidean data
|
In this work we present a direct comparison of three different numerical
analytic continuation methods: the Maximum Entropy Method, the Backus-Gilbert
method and the Schlessinger point or Resonances Via Pad\'{e} method. First, we
perform a benchmark test based on a model spectral function and study the
regime of applicability of these methods depending on the number of input
points and their statistical error. We then apply these methods to more
realistic examples, namely to numerical data on Euclidean propagators obtained
from a Functional Renormalization Group calculation, to data from a lattice
Quantum Chromodynamics simulation and to data obtained from a tight-binding
model for graphene in order to extract the electrical conductivity.
|
1801.10348v2
|
2018-02-03
|
Stochastic dynamics of planar magnetic moments in a three-dimensional environment
|
We study the stochastic dynamics of a two-dimensional magnetic moment
embedded in a three-dimensional environment, described by means of the
stochastic Landau-Lifshitz-Gilbert (sLLG) equation. We define a covariant
generalization of this equation, valid in the "generalized Stratonovich
discretization prescription". We present a path integral formulation that
allows to compute any $n-$point correlation function, independently of the
stochastic calculus used. Using this formalism, we show the equivalence between
the cartesian formulation with vectorial noise, with the polar formulation with
just one scalar fluctuation term. In particular, we show that, for isotropic
fluctuations, the system is represented by an {\em additive stochastic
process}, despite of the multiplicative terms appearing in the original
formulation of the sLLG equation, but, for anisotropic fluctuations the noise
turns out to be truly multiplicative.
|
1802.01027v2
|
2018-02-07
|
Asymptotically Locally Euclidean/Kaluza-Klein Stationary Vacuum Black Holes in 5 Dimensions
|
We produce new examples, both explicit and analytical, of bi-axisymmetric
stationary vacuum black holes in 5 dimensions. A novel feature of these
solutions is that they are asymptotically locally Euclidean in which spatial
cross-sections at infinity have lens space $L(p,q)$ topology, or asymptotically
Kaluza-Klein so that spatial cross-sections at infinity are topologically
$S^1\times S^2$. These are nondegenerate black holes of cohomogeneity 2, with
any number of horizon components, where the horizon cross-section topology is
any one of the three admissible types: $S^3$, $S^1\times S^2$, or $L(p,q)$.
Uniqueness of these solutions is also established. Our method is to solve the
relevant harmonic map problem with prescribed singularities, having target
symmetric space $SL(3,\mathbb{R})/SO(3)$. In addition, we analyze the
possibility of conical singularities and find a large family for which
geometric regularity is guaranteed.
|
1802.02457v2
|
2018-02-16
|
A Reallocation Algorithm for Online Split Packing of Circles
|
The Split Packing algorithm \cite{splitpacking_ws, splitpackingsoda,
splitpacking} is an offline algorithm that packs a set of circles into
triangles and squares up to critical density. In this paper, we develop an
online alternative to Split Packing to handle an online sequence of insertions
and deletions, where the algorithm is allowed to reallocate circles into new
positions at a cost proportional to their areas. The algorithm can be used to
pack circles into squares and right angled triangles. If only insertions are
considered, our algorithm is also able to pack to critical density, with an
amortised reallocation cost of $O(c\log \frac{1}{c})$ for squares, and
$O(c(1+s^2)\log_{1+s^2}\frac{1}{c})$ for right angled triangles, where $s$ is
the ratio of the lengths of the second shortest side to the shortest side of
the triangle, when inserting a circle of area $c$. When insertions and
deletions are considered, we achieve a packing density of $(1-\epsilon)$ of the
critical density, where $\epsilon>0$ can be made arbitrarily small, with an
amortised reallocation cost of $O(c(1+s^2)\log_{1+s^2}\frac{1}{c} +
c\frac{1}{\epsilon})$.
|
1802.05873v3
|
2018-02-22
|
Super-Resolution 1H Magnetic Resonance Spectroscopic Imaging utilizing Deep Learning
|
Magnetic resonance spectroscopic imaging (SI) is a unique imaging technique
that provides biochemical information from in vivo tissues. The 1H spectra
acquired from several spatial regions are quantified to yield metabolite
concentrations reflective of tissue metabolism. However, since these
metabolites are found in tissues at very low concentrations, SI is often
acquired with limited spatial resolution. In this work we test the hypothesis
that deep learning is able to upscale low resolution SI, together with the
T1-weighted (T1w) image, to reconstruct high resolution SI. We report a novel
densely connected Unet (D-Unet) architecture capable of producing
super-resolution spectroscopic images. The inputs for the D-UNet are the T1w
image and the low resolution SI image while the output is the high resolution
SI. The results of the D-UNet are compared both qualitatively and
quantitatively to simulated and in vivo high resolution SI. It is found that
this deep learning approach can produce high quality spectroscopic images and
reconstruct entire 1H spectra from low resolution acquisitions, which can
greatly advance the current SI workflow.
|
1802.07909v3
|
2018-02-26
|
Devil's Staircases in SFS Josephson Junctions
|
We study the effect of coupling between the superconducting current and
magnetization in the superconductor/ferromagnet/superconductor Josephson
junction under an applied circularly polarized magnetic field. Manifestation of
ferromagnetic resonance in the frequency dependence of the amplitude of the
magnetization and the average critical current density is demonstrated
numerically. The IV-characteristics show subharmonic steps that form devil's
staircases, following a continued fraction algorithm. The origin of the found
steps is related to the effect of the magnetization dynamics on the phase
difference in the Josephson junction. The dynamics of our system is described
by a generalized RCSJ model coupled to the Landau-Lifshitz-Gilbert equation. In
the suplement we justify analytically the appearance of the fractional steps in
IV-characteristics of the superconductor/ferromagnet/superconductor Josephson
junction.
|
1802.09212v2
|
2018-02-26
|
Controlled creation and stability of kπ-skyrmions on a discrete lattice
|
We determine sizes and activation energies of k{\pi}-skyrmions on a discrete
lattice using the Landau- Lifshitz-Gilbert equation and the geodesic nudged
elastic band method. The employed atomic material parameters are based on the
skyrmionic material system Pd/Fe/Ir(111). We find that the critical magnetic
fields for collapse of the 2{\pi}-skyrmion and 3{\pi}-skyrmion are very close
to each other and considerably lower than the critical field of the
1{\pi}-skyrmion. The activation energy protecting the structures does not
strictly decrease with increasing k as it can be larger for the 3{\pi}-skyrmion
than for the 2{\pi}-skyrmion depending on the applied magnetic field.
Furthermore, we propose a method of switching the skyrmion order k by a
reversion of the magnetic field direction in samples of finite size.
|
1802.09257v1
|
2018-03-14
|
Subnanosecond magnetization reversal of magnetic nanoparticle driven by chirp microwave field pulse
|
We investigate the magnetization reversal of single-domain magnetic
nanoparticle driven by linear down-chirp microwave magnetic field pulse.
Numerical simulations based on the Landau-Lifshitz-Gilbert equation reveal that
solely down-chirp pulse is capable of inducing subnanosecond magnetization
reversal. With a certain range of initial frequency and chirp rate, the
required field amplitude is much smaller than that of constant-frequency
microwave field. The fast reversal is because the down-chirp microwave field
acts as an energy source and sink for the magnetic particle before and after
crossing over the energy barrier, respectively. Applying a spin-polarized
current additively to the system further reduces the microwave field amplitude.
Our findings provide a new way to realize low-cost and fast magnetization
reversal.
|
1803.05261v1
|
2018-03-14
|
Dynamics of distorted skyrmions in strained chiral magnets
|
In this work, we study the microscopic dynamics of distorted skyrmions in
strained chiral magnets [K. Shibata et al., Nat. Nanotech. 10, 589 (2015)]
under gradient magnetic field or electric current by Landau-Lifshitz-Gilbert
simulations of the anisotropic spin model. It is observed that the dynamical
responses are also anisotropic, and the velocities of the distorted skyrmions
are periodically dependent on the directions of the external stimuli.
Furthermore, in addition to the uniform motion, our work also demonstrates
anti-phase harmonic vibrations of the two skyrmions in nanostripes, and the
frequencies are mainly determined by the exchange anisotropy. The simulated
results are well explained by Thiele theory, which may provide useful
information in understanding the dynamics of the distorted skyrmions in
strained chiral magnets.
|
1803.05298v1
|
2018-03-19
|
Dynamics and Stability of Meshed Multiterminal HVDC Networks
|
This paper investigates the existence of an equilibrium point in
multiterminal HVDC (MT-HVDC) grids, assesses its uniqueness and defines
conditions to ensure its stability. An offshore MT-HVDC system including two
wind farms is selected as application test case. At first, a generalized
dynamic model of the network is proposed, using hypergraph theory. Such model
captures the frequency dependence of transmission lines and cables, it is
non-linear due to the constant power behavior of the converter terminals using
droop regulation, and presents a suitable degree of simplifications of the MMC
converters, under given conditions, to allow system level studies over
potentially large networks. Based on this model, the existence and uniqueness
of the equilibrium point is demonstrated by returning the analysis to a
load-flow problem and using the Banach fixed point theorem. Additionally, the
stability of the equilibrium is analyzed by obtaining a Lyapunov function by
the Krasovskii's theorem. Computational results obtained for the selected 4
terminals MT-HVDC grid corroborate the requirement for the existence and
stability of the equilibrium point.
|
1803.06892v2
|
2018-03-16
|
Motion of vortices in ferromagnetic spin-1 BEC
|
The paper investigates dynamics of nonsingular vortices in a ferromagnetic
spin-1 BEC, where spin and mass superfluidity coexist in the presence of
uniaxial anisotropy (linear and quadratic Zeeman effect). The analysis is based
on hydrodynamics following from the Gross-Pitaevskii theory. Cores of
nonsingular vortices are skyrmions with charge, which is tuned by uniaxial
anisotropy and can have any fractal value between 0 and 1. There are
circulations of mass and spin currents around these vortices. The results are
compared with the equation of vortex motion derived earlier in the
Landau-Lifshitz-Gilbert theory for magnetic vortices in easy-plane
ferromagnetic insulators. In the both cases the transverse gyrotropic force
(analog of the Magnus force in superfluid and classical hydrodynamics) is
proportional to the charge of skyrmions in vortex cores.
|
1803.06939v1
|
2018-03-22
|
Mapping ideals of quantum group multipliers
|
We study the dual relationship between quantum group convolution maps
$L^1(\mathbb{G})\rightarrow L^{\infty}(\mathbb{G})$ and completely bounded
multipliers of $\widehat{\mathbb{G}}$. For a large class of locally compact
quantum groups $\mathbb{G}$ we completely isomorphically identify the mapping
ideal of row Hilbert space factorizable convolution maps with
$M_{cb}(L^1(\widehat{\mathbb{G}}))$, yielding a quantum Gilbert representation
for completely bounded multipliers. We also identify the mapping ideals of
completely integral and completely nuclear convolution maps, the latter case
coinciding with $\ell^1(\widehat{b\mathbb{G}})$, where $b\mathbb{G}$ is the
quantum Bohr compactification of $\mathbb{G}$. For quantum groups whose dual
has bounded degree, we show that the completely compact convolution maps
coincide with $C(b\mathbb{G})$. Our techniques comprise a mixture of operator
space theory and abstract harmonic analysis, including Fubini tensor products,
the non-commutative Grothendieck inequality, quantum Eberlein
compactifications, and a suitable notion of quasi-SIN quantum group, which we
introduce and exhibit examples from the bicrossed product construction. Our
main results are new even in the setting of group von Neumann algebras $VN(G)$
for quasi-SIN locally compact groups $G$.
|
1803.08342v2
|
2018-03-28
|
Low-temperature ageing of zirconia-toughened alumina ceramics and its implication in biomedical implants
|
Changes in crystalline phases resulting from low-temperature ageing of
different yttria doped and non-doped zirconia-toughened alumina composites and
nanocomposites were investigated under controlled humidity and temperature
conditions in autoclave. A classical powder mixing processing route and a new
modified colloidal processing route were used to process the composites.
Different compositions ranging from 2.5 wt.% zirconia in a matrix of alumina to
pure zirconia (3Y-TZP) were studied. It was observed that Al2O3+yttria
stabilised ZrO2 composites exhibited significant ageing. However, ageing was
much slower than traditionally observed for Y-TZP ceramics, due to the presence
of the alumina matrix. Ageing was clearly limited for zirconia content beyond
25 wt.%. On the other side of the spectrum, Al2O3+2.5 wt.% ZrO2 initially
presented a monoclinic fraction but did not show any ageing degradation. These
composites seem to represent the best choice between slow crack growth and
ageing resistance.
|
1803.10465v1
|
2018-03-28
|
Accelerated Aging in 3 mol%-Yttria-Stabilized Tetragonal Zirconia Ceramics Sintered in Reducing Conditions
|
The aging behavior of 3-mol%-yttria-stabilized tetragonal zirconia (3Y-TZP)
ceramics sintered in air and in reducing conditions was investigated at
140{\deg}C in water vapor. It was observed by X-ray diffraction (XRD) that
3Y-TZP samples sintered in reducing conditions exhibited significantly higher
tetragonal-to-monoclinic transformation than samples with similar density and
average grain size values but obtained by sintering in air. This fact is
explained by the increase of the oxygen vacancy concentration and by the
presence at the grain boundary region of a new aggregate phase formed because
of the exolution of Fe2+ ions observed by X-ray photoelectron spectroscopy.
|
1803.10580v1
|
2018-03-30
|
Atomic force microscopy study of the surface degradation mechanisms of zirconia based ceramics
|
Atomic force microscopy (AFM) can be used to characterise several aspects of
the surface degradation and reinforcement mechanisms of zirconia based
ceramics, such as crack propagation, martensitic relief formation, grains
pull-out and transformation toughening. AFM can also be used to quantify
precisely the transformation and provide reliable parameters for long term
degradation prediction. In particular, the tetragonal to monoclinic (t-m) phase
transformation of zirconia has been the object of extensive investigations of
the last twenty years, and is now recognised as being of martensitic nature.
New strong evidences supporting the martensitic nature of the transformation
are reported here. These observations, considering their scale and precision,
are a new step toward the understanding of the t-m phase transformation of
zirconia and related degradation mechanisms.
|
1804.00002v1
|
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