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2004-06-01 | Constraints on Resonant Particle Production during Inflation from the Matter and CMB Power Spectra | We analyze the limits on resonant particle production during inflation based
upon the power spectrum of fluctuations in matter and the cosmic microwave
background. We show that such a model is consistent with features observed in
the matter power spectrum deduced from galaxy surveys and damped Lyman-alpha
systems at high redshift. It also provides an alternative explanation for the
excess power observed in the power spectrum of the cosmic microwave background
fluctuations in the range of 1000 < l < 3500. For our best-fit models, epochs
of resonant particle creation reenter the horizon at wave numbers ~ 0.4 and/or
0.2 (h/Mpc). The amplitude and location of these features correspond to the
creation of fermion species of mass ~ 1-2 Mpl during inflation with a coupling
constant between the inflaton field and the created fermion species of near
unity. Although the evidence is marginal, if this interpretation is correct,
this could be one of the first observational hints of new physics at the Planck
scale. | 0406046v2 |
2005-11-28 | Most precise single redshift bound to Delta alpha/alpha | Verification of theoretical predictions of an oscillating behavior of the
fine-structure constant alpha with cosmic time requires high precision Delta
alpha/alpha measurements at individual redshifts, while in earlier studies the
mean Delta alpha/alpha values averaged over wide redshift intervals were
usually reported. This requirement can be met via the single ion differential
alpha measurement (SIDAM) procedure proposed in Levshakov et al. (2005). We
apply the SIDAM to the FeII lines associated with the damped Ly-alpha system
observed at z=1.15 in the spectrum of HE0515-4414. The weighted mean <Delta
alpha/alpha> calculated on base of carefully selected 34 FeII pairs {1608,X} (X
= 2344, 2374, and 2586 A) is <Delta alpha/alpha> = (-0.07+/-0.84) 10^{-6}
(1sigma C.L.). The precision of this estimate improves by a factor 2 the
previous one reported for the same system by Quast et al. (2004). The obtained
result represents an absolute improvement with respect to what has been done in
the measurements of Delta alpha/alpha. | 0511765v1 |
2006-06-08 | Cosmological bounds on dark matter-neutrino interactions | We investigate the cosmological effects of a neutrino interaction with cold
dark matter. We postulate a neutrino that interacts with a ``neutrino
interacting dark matter'' (NIDM) particle with an elastic-scattering cross
section that either decreases with temperature as $T^2$ or remains constant
with temperature. The neutrino--dark-matter interaction results in a
neutrino--dark-matter fluid with pressure, and this pressure results in
diffusion-damped oscillations in the matter power spectrum, analogous to the
acoustic oscillations in the baryon-photon fluid. We discuss the bounds from
the Sloan Digital Sky Survey on the NIDM opacity (ratio of cross section to
NIDM-particle mass) and compare with the constraint from observation of
neutrinos from supernova 1987A. If only a fraction of the dark matter interacts
with neutrinos, then NIDM oscillations may affect current cosmological
constraints from measurements of galaxy clustering. We discuss how detection of
NIDM oscillations would suggest a particle-antiparticle asymmetry in the
dark-matter sector. | 0606190v1 |
2006-07-26 | Matter density perturbations in interacting quintessence models | Models with dark energy decaying into dark matter have been proposed to solve
the coincidence problem in cosmology. We study the effect of such coupling in
the matter power spectrum. Due to the interaction, the growth of matter density
perturbations during the radiation dominated regime is slower compared to
non-interacting models with the same ratio of dark matter to dark energy today.
This effect introduces a damping on the power spectrum at small scales
proportional to the strength of the interaction and similar to the effect
generated by ultrarelativistic neutrinos. The interaction also shifts
matter--radiation equality to larger scales. We compare the matter power
spectrum of interacting quintessence models with the measurments of 2dFGRS. We
particularize our study to models that during radiation domination have a
constant dark matter to dark energy ratio. | 0607604v1 |
2006-11-27 | High-Precision Measurements of Delta alpha/alpha from QSO Absorption Spectra | Precise radial velocity measurements (delta v/c ~ 10^{-7}) of FeII lines in
damped Ly-alpha systems from very high quality VLT/UVES spectra of quasars
HE0515-4414 and Q1101-264 are used to probe cosmological time dependence of the
fine structure constant, alpha. It is found that between two redshifts z1 =
1.15 and z2 = 1.84 the value of Delta alpha/alpha changes at the level of a few
ppm: (alpha_z2 - alpha_z1)/alpha_0 = 5.43 +/- 2.52 ppm. Variations of alpha can
be considered as one of the most reliable method to constrain the dark energy
equation of state and improvements on the accuracy of the wavelength
calibration of QSO spectra are of great importance. | 0611803v1 |
2007-01-24 | The N/O evolution on galaxies:the role played by the star formation history | We study the evolution of nitrogen resulting from a set of spiral and
irregular galaxy models computed for a large number of input mass radial
distributions and with various star formation efficiencies. We show that our
models produce a nitrogen abundance evolution in good agreement with the
observational data. In particular, low N/O values for high-redshift objects,
such as those obtained for Damped Lyman Alpha galaxies can be obtained with our
models simultaneously to higher and constant values of N/O as those observed
for irregular and dwarf galaxies, at the same low oxygen abundances $\rm
12+log(O/H) \sim 7$ dex. The differences in the star formation histories of the
regions and galaxies modeled are essential to reproduce the observational data
in the N/O-O/H plane. | 0701691v1 |
1994-07-22 | X-Ray Scattering Measurements of the Transient Structure of a Driven Charge-Density-Wave | We report time-resolved x-ray scattering measurements of the transient
structural response of the sliding {\bf Q}$_{1}$ charge-density-wave (CDW) in
NbSe$_{3}$ to a reversal of the driving electric field. The observed time scale
characterizing this response at 70K varies from $\sim$ 15 msec for driving
fields near threshold to $\sim$ 2 msec for fields well above threshold. The
position and time-dependent strain of the CDW is analyzed in terms of a
phenomenological equation of motion for the phase of the CDW order parameter.
The value of the damping constant, $\gamma = (3.2 \pm 0.7) \times 10^{-19}$ eV
$\cdot$ seconds $\cdot$ \AA$^{-3}$, is in excellent agreement with the value
determined from transport measurements. As the driving field approaches
threshold from above, the line shape becomes bimodal, suggesting that the CDW
does not depin throughout the entire sample at one well-defined voltage. | 9407094v1 |
1995-07-03 | Fundamental steps of group velocity for slow surface polariton under the quantum hall effect conditions | A new type of collective electromagnetic excitations, namely surface
polaritons (SP) --- in a 2D electronic layer in a high magnetic field under
Quantum Hall Effect (QHE) conditions is predicted. We have found the spectrum,
damping, and polarization of the SP in a wide range of frequencies $\omega$ and
wavevectors $\bf k$. It is shown that near the Cyclotron Resonance (CR)
($\omega\sim\Omega=\displaystyle eB/mc$) the phase velocity of the SP is
drastically slowed down and the group velocity undergoes fundamental steps
defined by the Fine Structure Constant $\alpha=e^2/\hbar c$. In the vicinity of
a CR subharmonic ($\omega\sim 2 \Omega$) the negative (anomalous) dispersion of
the SP occurs. The relaxation of electrons in the 2D layer gives rise to a new
dissipative collective threshold-type mode of the SP. We suggest a method for
calculating the kinetic coefficients for the 2D electronic layer under QHE
condition, using the Wigner distribution function formalism and determine their
spatial and frequency dispersion. Using this method we have calculated the
line-shape of the CR and the d.c. conductance under the QHE condition, which
are in good agreement with experimental data. | 9507001v1 |
1995-08-18 | On the normal phase of 2D Fermi liquid with weak attraction between particles | Proceeding from the simplest field theoretical model of 2D metal, the normal
phase Green functions of the weakly interacting fermions and the order
parameter fluctuations (responsible for the attraction between fermions) are
obtained. It is shown that taking into consideration the fluctuations mentioned
leads to a considerable reduction of the fermion wave function renormalization
constant (quasiparticle weight) as well as to a linear dependence of the
quasiparticle damping on the temperature. A general dependence of 2D Fermi
liquid properties on the fermion density is discussed. The relevance of the
proposed model to the marginal behavior of the Fermi liquid of high--$T_c$
superconductors, in particular, to their linear temperature dependence of the
resistivity is indicated. | 9508076v1 |
1997-12-19 | Spatiotemporal dynamics of discrete sine-Gordon lattices with sinusoidal couplings | The spatiotemporal dynamics of a damped sine-Gordon chain with sinusoidal
nearest-neighbor couplings driven by a constant uniform force are discussed.
The velocity characteristics of the chain versus the external force is shown.
Dynamics in the high- and low-velocity regimes are investigated. It is found
that in the high-velocity regime, the dynamics is dominated by rotating modes,
the velocity shows a branching bifurcation feature, while in the low-velocity
regime, the velocity exhibits step-like dynamical transitions, broken by the
destruction of strong resonances. | 9712226v1 |
1998-11-06 | Intermediate temperature dynamics of one-dimensional Heisenberg antiferromagnets | We present a general theory for the intermediate temperature (T) properties
of Heisenberg antiferromagnets of spin-S ions on p-leg ladders, valid for 2Sp
even or odd. Following an earlier proposal for 2Sp even (Damle and Sachdev,
cond-mat/9711014), we argue that an integrable, classical, continuum model of a
fixed-length, 3-vector applies over an intermediate temperature range; this
range becomes very wide for moderate and large values of 2Sp. The coupling
constants of the effective model are known exactly in terms of the energy gap
above the ground state (for 2Sp even) or a crossover scale (for 2Sp odd).
Analytic and numeric results for dynamic and transport properties are obtained,
including some exact results for the spin-wave damping. Numerous quantitative
predictions for neutron scattering and NMR experiments are made. A general
discussion on the nature of T>0 transport in integrable systems is also
presented: an exact solution of a toy model proves that diffusion can exist in
integrable systems, provided proper care is taken in approaching the
thermodynamic limit. | 9811083v2 |
1999-02-25 | Mobility of Bloch Walls via the Collective Coordinate Method | We have studied the problem of the dissipative motion of Bloch walls
considering a totally anisotropic one dimensional spin chain in the presence of
a magnetic field. Using the so-called "collective coordinate method" we
construct an effective Hamiltonian for the Bloch wall coupled to the magnetic
excitations of the system. It allows us to analyze the Brownian motion of the
wall in terms of the reflection coefficient of the effective potential felt by
the excitations due to the existence of the wall. We find that for finite
values of the external field the wall mobility is also finite. The spectrum of
the potential at large fields is investigated and the dependence of the damping
constant on temperature is evaluated. As a result we find the temperature and
magnetic field dependence of the wall mobility. | 9902330v1 |
1999-04-06 | Non-Fermi-liquid behavior in the Kondo lattices induced by peculiarities of magnetic ordering and spin dynamics | A scaling consideration of the Kondo lattices is performed with account of
singularities in the spin excitation spectral function. It is shown that a
non-Fermi-liquid (NFL) behavior between two critical values of the bare $s-f$
coupling constant occurs naturally for complicated magnetic structures with
several magnon branches. This may explain the fact that a NFL behavior takes
place often in the heavy-fermion systems with peculiar spin dynamics. Another
kind of a NFL-like state (with different critical exponents) can occur for
simple antiferromagnets with account of magnon damping, and for paramagnets,
especially with two-dimensional character of spin fluctuations. The mechanisms
proposed lead to some predictions about behavior of specific heat, resistivity,
magnetic susceptibility, and anisotropy parameter, which can be verified
experimentally. | 9904072v3 |
1999-11-23 | Collisionless dynamics of dilute Bose gases: Role of quantum and thermal fluctuations | We study the low-energy collective oscillations of a dilute Bose gas at
finite temperature in the collisionless regime. By using a time-dependent
mean-field scheme we derive for the dynamics of the condensate and
noncondensate components a set of coupled equations, which we solve
perturbatively to second order in the interaction coupling constant. This
approach is equivalent to the finite-temperature extension of the Beliaev
approximation and includes corrections to the Gross-Pitaevskii theory due both
to quantum and thermal fluctuations. For a homogeneous system we explicitly
calculate the temperature dependence of the velocity of propagation and damping
rate of zero sound. In the case of harmonically trapped systems in the
thermodynamic limit, we calculate, as a function of temperature, the frequency
shift of the low-energy compressional and surface modes. | 9911377v1 |
2000-04-07 | Coherent Atomic Oscillations and Resonances between Coupled Bose-Einstein Condensates with Time-Dependent Trapping Potential | We study the quantum coherent-tunneling between two Bose-Einstein condensates
separated through an oscillating trap potential. The cases of slowly and
rapidly varying in time trap potential are considered. In the case of a slowly
varying trap we study the nonlinear resonances and chaos in the oscillations of
the relative atomic population. Using the Melnikov function approach, we find
the conditions for chaotic macroscopic quantum-tunneling phenomena to exists.
Criteria for the onset of chaos are also given. We find the values of frequency
and modulation amplitude which lead to chaos on oscillations in the relative
population, for any given damping and the nonlinear atomic interaction. In the
case of a rapidly varying trap we use the multiscale expansion method in the
parameter epsilon = 1/Omega, where Omega is the frequency of modulations and we
derive the averaged system of equations for the modes. The analysis of this
system shows that new macroscopic quantum self trapping regions, in comparison
with the constant trap case, exist. | 0004117v1 |
2000-05-02 | Gravity-driven Dense Granular Flows | We report and analyze the results of numerical studies of dense granular
flows in two and three dimensions, using both linear damped springs and
Hertzian force laws between particles. Chute flow generically produces a
constant density profile that satisfies scaling relations suggestive of a
Bagnold grain inertia regime. The type of force law has little impact on the
behavior of the system. Bulk and surface flows differ in their failure criteria
and flow rheology, as evidenced by the change in principal stress directions
near the surface. Surface-only flows are not observed in this geometry. | 0005051v1 |
2000-08-03 | Kinetic Friction due to Ohm's Law Heating | Using both a recent calculation by Bruch of the damping of the motion of a
monolayer nitrogen film oscillating harmonically on a metallic surface due to
Ohm's law heating and a Thomas-Fermi approximation treatment of the Ohm's law
heating mechanism, which accounts for the nonzero thickness of the surface
region of a metal, it is argued that this mechanism for friction is able to
account for recent measurements of the drop in the friction for anitrogen film
sliding over a lead substrate as it goes below its superconducting transition
temperature. Bruch's calculation is also made more transparent by re-doing the
calculation for a film sliding at constant speed, instead of oscillating. Using
this treatment, it is easily shown that Bruch's calculation is equivalent to
integrating Boyer's solution of the problem of a charge sliding over a metallic
surface over the charge density of the monolayer nitrogen film. | 0008063v3 |
2001-11-07 | Study of Magnetic Excitation in Singlet-Ground-State Magnets CsFeCl$_3$ and RbFeCl$_3$ by Nuclear Magnetic Relaxation | The temperature dependences of spin-lattice relaxation time $T_1$ of
$^{133}$Cs in CsFeCl$_3$ and $^{87}$Rb in RbFeCl$_3$ were measured in the
temperature range between 1.5 K and 22 K, at various fields up to 7 T applied
parallel (or perpendicular) to the c-axis, and the analysis was made on the
basis of the DCEFA. The mechanism of the nuclear magnetic relaxation is
interpreted in terms of the magnetic fluctuations which are characterized by
the singlet ground state system. In the field region where the phase transition
occurs, $T_1^{-1}$ exhibited the tendency of divergence near $T_{\rm N}$, and
this feature was ascribed to the transverse spin fluctuation associated with
the mode softening at the $K$-point. It was found that the damping constant of
the soft mode is remarkably affected by the occurrence of the magnetic ordering
at lower temperature, and increases largely in the field region where the phase
transition occurs. | 0111097v2 |
2003-02-10 | The differential sum rule for the relaxation rate in dirty superconductors | We consider the differential sum rule for the effective scattering rate $%
1/\tau (\omega)$ and optical conductivity $\sigma_{1}(\omega) $ in a dirty BCS
superconductor, for arbitrary ratio of the superconducting gap $% \Delta$ and
the normal state constant damping rate $1/\tau$. We show that if $\tau$ is
independent of $T$, the area under $1/\tau (\omega)$ does not change between
the normal and the superconducting states, i.e., there exists an exact
differential sum rule for the scattering rate. For \textit{any} value of the
dimensionless parameter $\Delta\tau $, the sum rule is exhausted at frequencies
controlled by $\Delta$. %but the numerical convergence is weak. We show that in
the dirty limit the convergence of the differential sum rule for the scattering
rate is much faster then the convergence of the $f-$sum rule, but slower then
the convergence of the differential sum rule for conductivity. | 0302191v1 |
2003-02-12 | Electronic-vibrational coupling in single-molecule devices | Experiments studying vibrational effects on electronic transport through
single molecules have observed several seemingly inconsistent behaviors,
ranging from up to 30 harmonics of a vibrational frequency in one experiment,
to an absence of higher-harmonic peaks in another. We study the different
manifestations of electronic-vibrational coupling in inelastic and elastic
electron transport through single molecules. For the case of inelastic
transport, higher harmonics are shown to be damped by additional small factors
beyond powers of the electron-vibration coupling constant $\lambda$. Two
mechanisms greatly increase the size of secondary peaks in inelastic transport:
coupling between electron transport and spatial motion of the molecule, and the
``pumping'' of higher vibrational modes of the molecule when vibrational
excitations do not completely relax between electron transits. | 0302222v2 |
2003-04-09 | Plastic Flow in Two-Dimensional Solids | A time-dependent Ginzburg-Landau model of plastic deformation in
two-dimensional solids is presented. The fundamental dynamic variables are the
displacement field $\bi u$ and the lattice velocity ${\bi v}=\p {\bi u}/\p t$.
Damping is assumed to arise from the shear viscosity in the momentum equation.
The elastic energy density is a periodic function of the shear and tetragonal
strains, which enables formation of slips at large strains. In this work we
neglect defects such as vacancies, interstitials, or grain boundaries. The
simplest slip consists of two edge dislocations with opposite Burgers vectors.
The formation energy of a slip is minimized if its orientation is parallel or
perpendicular to the flow in simple shear deformation and if it makes angles of
$\pm \pi/4$ with respect to the stretched direction in uniaxial stretching.
High-density dislocations produced in plastic flow do not disappear even if
the flow is stopped. Thus large applied strains give rise to metastable,
structurally disordered states. We divide the elastic energy into an elastic
part due to affine deformation and a defect part. The latter represents degree
of disorder and is nearly constant in plastic flow under cyclic straining. | 0304209v1 |
2004-02-16 | Superconducting Qubits and the Physics of Josephson Junctions | We describe in this paper how the nonlinear Josephson inductance is the
crucial circuit element for all Josephson qubits. We discuss the three types of
qubit circuits, and show how these circuits use this nonlinearity in unique
manners. We give a brief derivation of the BCS theory, highlighting the
appearance of the macroscopic phase parameter. The Josephson equations are
derived using standard first and second order perturbation theory that describe
quasiparticle and Cooper-pair tunneling. An exact calculation of the Josephson
effect then follows using the quasiparticle bound-state theory, and then expand
upon this theory to describe quasiparticle excitations as transitions from the
ground to excited bound states from nonadiabatic changes in the bias. Although
quasiparticle current is typically calculated only for a constant DC voltage,
the advantage to this approach is seen where we qualitatively describe
quasiparticle tunneling with AC voltage excitations, as appropriate for the
qubit state. This section describes how the Josephson qubit is typically
insensitive to quasiparticle damping, even to the extent that a phase qubit can
be constructed from microbridge junctions. | 0402415v1 |
2004-04-24 | On the Modulational Instability of the Nonlinear Schrödinger Equation with Dissipation | The modulational instability of spatially uniform states in the nonlinear
Schr\"odinger equation is examined in the presence of higher-order dissipation.
The study is motivated by results on the effects of three-body recombination in
Bose-Einstein condensates, as well as by the important recent work of Segur et
al. on the effects of linear damping in NLS settings. We show how the presence
of even the weakest possible dissipation suppresses the instability on a longer
time scale. However, on a shorter scale, the instability growth may take place,
and a corresponding generalization of the MI criterion is developed. The
analytical results are corroborated by numerical simulations. The method is
valid for any power-law dissipation form, including the constant dissipation as
a special case. | 0404597v1 |
2005-01-04 | Local spectroscopy and atomic imaging of tunneling current, forces and dissipation on graphite | Theory predicts that the currents in scanning tunneling microscopy (STM) and
the attractive forces measured in atomic force microscopy (AFM) are directly
related. Atomic images obtained in an attractive AFM mode should therefore be
redundant because they should be \emph{similar} to STM. Here, we show that
while the distance dependence of current and force is similar for graphite,
constant-height AFM- and STM images differ substantially depending on distance
and bias voltage. We perform spectroscopy of the tunneling current, the
frequency shift and the damping signal at high-symmetry lattice sites of the
graphite (0001) surface. The dissipation signal is about twice as sensitive to
distance as the frequency shift, explained by the Prandtl-Tomlinson model of
atomic friction. | 0501045v1 |
2005-03-01 | Discrete models of dislocations and their motion in cubic crystals | A discrete model describing defects in crystal lattices and having the
standard linear anisotropic elasticity as its continuum limit is proposed. The
main ingredients entering the model are the elastic stiffness constants of the
material and a dimensionless periodic function that restores the translation
invariance of the crystal and influences the Peierls stress. Explicit
expressions are given for crystals with cubic symmetry: sc, fcc and bcc.
Numerical simulations of this model with conservative or damped dynamics
illustrate static and moving edge and screw dislocations and describe their
cores and profiles. Dislocation loops and dipoles are also numerically
observed. Cracks can be created and propagated by applying a sufficient load to
a dipole formed by two edge dislocations. | 0503020v1 |
2005-05-24 | Quasi-Elastic Scattering, Random Fields and phonon-coupling effects in PbMg1/3Nb2/3O3 | The low-energy part of the vibration spectrum in PbMg$_{1/3}$Nb$_{2/3}$O$_3$
(PMN) relaxor ferroelectric has been studied by neutron scattering above and
below the Burns temperature, T$_d$. The transverse acoustic and the lowest
transverse optic phonons are strongly coupled and we have obtained a model for
this coupling. We observe that the lowest optic branch is always underdamped. A
resolution-limited central peak and quasi-elastic scattering appear in the
vicinity of the Burns temperature. It is shown that it is unlikely that the
quasi-elastic scattering originates from the combined effects of coupling
between TA and TO phonons with an increase of the damping of the TO phonon
below T$_d$. The quasi-elastic scattering has a peak as a function of
temperature close to the peak in the dielectric constant while the intensity of
the central peak scattering increases strongly below this temperature. These
results are discussed in terms of a random field model for relaxors. | 0505584v1 |
2005-12-26 | Spin waves in a band ferromagnet: spin-rotationally symmetric study with self-energy and vertex corrections | First-order quantum corrections to the transverse spin-fluctuation propagator
are obtained within a systematic inverse-degeneracy 1/N expansion, which
provides a spin-rotationally symmetric scheme for including self-energy and
vertex corrections while preserving the Goldstone mode. An expression is
obtained for the spin-wave stiffness constant including all first-order quantum
corrections, and the dominant contribution is shown to yield a strong reduction
due to a correlation-induced enhancement in the exchange-energy gain upon spin
twisting. The quantum reduction factor U/W highlights the subtlety in the
characteristic competition in a band ferromagnet between interaction U and
bandwidth W. Quantum corrections also yield an intrinsic spin-wave damping
mechanism due to coupling between spin and charge fluctuations. | 0512648v2 |
2006-02-18 | Optical phonons in new ordered perovskite Sr2Cu(Re0.69Ca0.31) Oy system observed by infrared reflectance spectroscopy | We report infrared reflectivity spectra for a new correlated cupric oxide
system Sr2Cu(Re0.69Ca0.31)Oy with y ~ 0.6 at several temperatures ranging
between 8 and 380 K. The reflectivity spectrum at 300 K comprises of several
optical phonons. A couple of residual bands located around 315 and 653 cm-1
exhibit exceptionally large intensity as compared to the other ones. The
overall reflectivity spectrum lifts up slightly with increasing temperature.
The energy and damping factor of transverse-optical phonons are determined by
fitting the imaginary dielectric constant by Lorentz oscillator model and
discussed as a function of temperature in terms of lattice anharmonicity. | 0602438v1 |
2006-05-03 | Non equilibrium inertial dynamics of colloidal systems | We consider the properties of a one dimensional fluid of brownian inertial
hard-core particles, whose microscopic dynamics is partially damped by a
heat-bath. Direct interactions among the particles are represented as binary,
instantaneous elastic collisions. Collisions with the heath bath are accounted
for by a Fokker-Planck collision operator, whereas direct collisions among the
particles are treated by a well known method of kinetic theory, the Revised
Enskog Theory. By means of a time multiple time-scale method we derive the
evolution equation for the average density. Remarkably, for large values of the
friction parameter and/or of the mass of the particles we obtain the same
equation as the one derived within the dynamic density functional theory (DDF).
In addition, at moderate values of the friction constant, the present method
allows to study the inertial effects not accounted for by DDF method. Finally,
a numerical test of these corrections is provided. | 0605094v1 |
2006-08-30 | Current-Driven Domain-Wall Dynamics in Curved Ferromagnetic Nanowires | The current-induced motion of a domain wall in a semicircle nanowire with
applied Zeeman field is investigated. Starting from a micromagnetic model we
derive an analytical solution which characterizes the domain-wall motion as a
harmonic oscillation. This solution relates the micromagnetic material
parameters with the dynamical characteristics of a harmonic oscillator, i.e.,
domain-wall mass, resonance frequency, damping constant, and force acting on
the wall. For wires with strong curvature the dipole moment of the wall as well
as its geometry influence the eigenmodes of the oscillator. Based on these
results we suggest experiments for the determination of material parameters
which otherwise are difficult to access. Numerical calculations confirm our
analytical solution and show its limitations. | 0608680v1 |
2007-02-23 | Electronic viscosity in a quantum well: A test for the local density approximation | In the local density approximation (LDA) for electronic time-dependent
current-density functional theory (TDCDFT) many-body effects are described in
terms of the visco-elastic constants of the homogeneous three-dimensional
electron gas. In this paper we critically examine the applicability of the
three-dimensional LDA to the calculation of the viscous damping of
1-dimensional collective oscillations of angular frequency $\omega$ in a quasi
2-dimensional quantum well. We calculate the effective viscosity
$\zeta(\omega)$ from perturbation theory in the screened Coulomb interaction
and compare it with the commonly used three-dimensional LDA viscosity
$Y(\omega)$. Significant differences are found. At low frequency $Y(\omega)$ is
dominated by a shear term, which is absent in $\zeta(\omega)$. At high
frequency $\zeta(\omega)$ and $Y(\omega)$ exhibit different power law behaviors
($\omega^{-3}$ and $\omega^{-5/2}$ respectively), reflecting different spectral
densities of electron-hole excitations in two and three dimensions. These
findings demonstrate the need for better approximations for the
exchange-correlation stress tensor in specific systems where the use of the
three-dimensional functionals may lead to unphysical results. | 0702538v1 |
2007-02-28 | Bond Stiffening in Nanoclusters and its Consequences | We have used density functional perturbation theory to investigate the
stiffness of interatomic bonds in small clusters of Si, Sn and Pb. As the
number of atoms in a cluster is decreased, there is a marked shortening and
stiffening of bonds. The competing factors of fewer but stiffer bonds in
clusters result in softer elastic moduli but higher (average) frequencies as
size is decreased, with clear signatures of universal scaling relationships. A
significant role in understanding trends is played by the coordination number
of the bulk structure: the higher this is, the lesser is the relative softening
of elastic constants, and the greater the relative damping of vibrational
amplitudes, for clusters compared to the bulk. Our results could provide a
framework for understanding recent reports that some clusters remain solid
above the bulk melting temperature. | 0702677v1 |
2007-03-12 | Velocity dependence of friction and Kramers relaxation rates | We study the influence of the velocity dependence of friction on the escape
of a Brownian particle from the deep potential well ($E_{b} \gg k_{B}T$,
$E_{b}$ is the barrier height, $k_{B}$ is the Boltzmann constant, $T$ is the
bath temperature). The bath-induced relaxation is treated within the Rayleigh
model (a heavy particle of mass $M$ in the bath of light particles of mass
$m\ll M$) up to the terms of the order of $O(\lambda^{4})$,
$\lambda^{2}=m/M\ll1$. The term $\sim 1$ is equivalent to the Fokker-Planck
dissipative operator, and the term $\sim \lambda^{2}$ is responsible for the
velocity dependence of friction. As expected, the correction to the Kramers
escape rate in the overdamped limit is proportional to $\lambda^{2}$ and is
small. The corresponding correction in the underdamped limit is proportional to
$\lambda^{2}E_{b}/(k_{B}T)$ and is not necessarily small. We thus suggest that
the effects due to the velocity-dependent friction may be of considerable
importance in determining the rate of escape of an under- and moderately damped
Brownian particle from a deep potential well, while they are of minor
importance for an overdamped particle. | 0703312v1 |
1995-04-12 | STABLE CLOCKS AND GENERAL RELATIVITY | We survey the role of stable clocks in general relativity. Clock comparisons
have provided important tests of the Einstein Equivalence Principle, which
underlies metric gravity. These include tests of the isotropy of clock
comparisons (verification of local Lorentz invariance) and tests of the
homogeneity of clock comparisons (verification of local position invariance).
Comparisons of atomic clocks with gravitational clocks test the Strong
Equivalence Principle by bounding cosmological variations in Newton's constant.
Stable clocks also play a role in the search for gravitational radiation:
comparision of atomic clocks with the binary pulsar's orbital clock has
verified gravitational-wave damping, and phase-sensitive detection of waves
from inspiralling compact binaries using laser interferometric gravitational
observatories will facilitate extraction of useful source information from the
data. Stable clocks together with general relativity have found important
practical applications in navigational systems such as GPS. | 9504017v1 |
2000-11-18 | Third post-Newtonian dynamics of compact binaries: Noetherian conserved quantities and equivalence between the harmonic-coordinate and ADM-Hamiltonian formalisms | A Lagrangian from which derive the third post-Newtonian (3PN) equations of
motion of compact binaries (neglecting the radiation reaction damping) is
obtained. The 3PN equations of motion were computed previously by Blanchet and
Faye in harmonic coordinates. The Lagrangian depends on the harmonic-coordinate
positions, velocities and accelerations of the two bodies. At the 3PN order,
the appearance of one undetermined physical parameter \lambda reflects an
incompleteness of the point-mass regularization used when deriving the
equations of motion. In addition the Lagrangian involves two unphysical
(gauge-dependent) constants r'_1 and r'_2 parametrizing some logarithmic terms.
The expressions of the ten Noetherian conserved quantities, associated with the
invariance of the Lagrangian under the Poincar\'e group, are computed. By
performing an infinitesimal ``contact'' transformation of the motion, we prove
that the 3PN harmonic-coordinate Lagrangian is physically equivalent to the 3PN
Arnowitt-Deser-Misner Hamiltonian obtained recently by Damour, Jaranowski and
Sch\"afer. | 0011063v2 |
2006-10-06 | New Insights into Uniformly Accelerated Detector in a Quantum Field | We obtained an exact solution for a uniformly accelerated Unruh-DeWitt
detector interacting with a massless scalar field in (3+1) dimensions which
enables us to study the entire evolution of the total system, from the initial
transient to late-time steady state. We find that the Unruh effect as derived
from time-dependent perturbation theory is valid only in the transient stage
and is totally invalid for cases with proper acceleration smaller than the
damping constant. We also found that, unlike in (1+1)D results, the (3+1)D
uniformly accelerated Unruh-DeWitt detector in a steady state does emit a
positive radiated power of quantum nature at late-times, but it is not
connected to the thermal radiance experienced by the detector in the Unruh
effect proper. | 0610024v1 |
1995-09-29 | Thermal Fermionic Dispersion Relations in a Magnetic Field | The thermal self-energy of an electron in a static uniform magnetic field $B$
is calculated to first order in the fine structure constant $\alpha $ and to
all orders in $eB$. We use two methods, one based on the Furry picture and
another based on Schwinger's proper-time method. As external states we consider
relativistic Landau levels with special emphasis on the lowest Landau level. In
the high-temperature limit we derive self-consistent dispersion relations for
particle and hole excitations, showing the chiral asymmetry caused by the
external field. For weak fields, earlier results on the ground- state energy
and the anomalous magnetic moment are discussed and compared with the present
analysis. In the strong-field limit the appearance of a field-independent
imaginary part of the self-energy, related to Landau damping in the
$e^{+}e^{-}$ plasma, is pointed out. | 9509418v1 |
1999-12-22 | Chaotic inflation on the brane | We consider slow-roll inflation in the context of recently proposed
four-dimensional effective gravity induced on the world-volume of a three-brane
in five-dimensional Einstein gravity. We find significant modifications of the
simplest chaotic inflationary scenario when the five-dimensional Planck scale
is below about 10^{17} GeV. We use the comoving curvature perturbation, which
remains constant on super-Hubble scales, in order to calculate the spectrum of
adiabatic density perturbations generated. Modifications to the Friedmann
constraint equation lead to a faster Hubble expansion at high energies and a
more strongly damped evolution of the scalar field. This assists slow-roll,
enhances the amount of inflation obtained in any given model, and drives the
perturbations towards an exactly scale-invariant Harrison-Zel'dovich spectrum.
In chaotic inflation driven by a massive scalar field we show that inflation
can occur at field values far below the four-dimensional Planck scale, though
above the five-dimensional fundamental scale. | 9912464v3 |
2003-07-05 | Hard Loops, Soft Loops, and High Density Effective Field Theory | We study several issues related to the use of effective field theories in QCD
at large baryon density. We show that the power counting is complicated by the
appearance of two scales inside loop integrals. Hard dense loops involve the
large scale $\mu^2$ and lead to phenomena such as screening and damping at the
scale $g\mu$. Soft loops only involve small scales and lead to superfluidity
and non-Fermi liquid behavior at exponentially small scales. Four-fermion
operators in the effective theory are suppressed by powers of $1/\mu$, but they
get enhanced by hard loops. As a consequence their contribution to the pairing
gap is only suppressed by powers of the coupling constant, and not powers of
$1/\mu$. We determine the coefficients of four-fermion operators in the
effective theory by matching quark-quark scattering amplitudes. Finally, we
introduce a perturbative scheme for computing corrections to the gap parameter
in the superfluid phase | 0307074v1 |
2004-04-21 | Rho - Omega Splitting and Mixing in Nuclear Matter | We investigate the splitting and mixing of $\rho$ and $\omega$ mesons in
nuclear matter. The calculations were performed on the basis of QCD sum rules
and include all operators up to mass dimension-6 twist-4 and up to first order
in the coupling constants. Special attention is devoted to the impact of the
scalar 4-quark condensates on both effects. In nuclear matter the Landau
damping governs the $\rho - \omega$ mass splitting while the scalar 4-quark
condensates govern the strenght of individual mass shifts. A strong in-medium
mass splitting causes the disappearance of the $\rho - \omega$ mixing. | 0404176v5 |
2007-02-13 | Universality of QCD traveling-waves with running coupling | The Balitsky-Kovchegov QCD equation for rapidity evolution describing
saturation effects at high energy admits universal asymptotic traveling-wave
solutions when the nonlinear damping becomes effective. The asymptotic
solutions fall in universality classes depending only on some specific
properties of the solution of the associated linear equation. We derive these
solutions for the recent QCD formulations of the Balitsky-Kovchegov equation
with running coupling constant obtained from quark-loop calculation. While the
associated linear solutions depend in different ways with observables and
higher-order effects, we show that the asymptotic traveling-wave solutions all
belong to the same universality class whose solutions are given. Hence the
influence of saturation stabilizes the QCD evolution with respect to higher
order effects and leads to universal features at high enough rapidity, such as
the form of the traveling waves, the intercept of the saturation scale and
geometric scaling in square-root of the rapidity. | 0702131v2 |
2006-11-23 | A coherent-state-based path integral for quantum mechanics on the Moyal plane | Inspired by a recent work that proposes using coherent states to evaluate the
Feynman kernel in noncommutative space, we provide an independent formulation
of the path-integral approach for quantum mechanics on the Moyal plane, with
the transition amplitude defined between two coherent states of mean position
coordinates. In our approach, we invoke solely a representation of the of the
noncommutative algebra in terms of commutative variables. The kernel expression
for a general Hamiltonian was found to contain gaussian-like damping terms, and
it is non-perturbative in the sense that it does not reduce to the commutative
theory in the limit of vanishing $\theta$ - the noncommutative parameter. As an
example, we studied the free particle's propagator which turned out to be
oscillating with period being the product of its mass and $\theta$. Further, it
satisfies the Pauli equation for a charged particle with its spin aligned to a
constant, orthogonal $B$ field in the ordinary Landau problem, thus providing
an interesting evidence of how noncommutativity can induce spin-like effects at
the quantum mechanical level. | 0611254v1 |
2004-11-30 | Development of singularities for the compressible Euler equations with external force in several dimensions | We consider solutions to the Euler equations in the whole space from a
certain class, which can be characterized, in particular, by finiteness of
mass, total energy and momentum. We prove that for a large class of right-hand
sides, including the viscous term, such solutions, no matter how smooth
initially, develop a singularity within a finite time. We find a sufficient
condition for the singularity formation, "the best sufficient condition", in
the sense that one can explicitly construct a global in time smooth solution
for which this condition is not satisfied "arbitrary little". Also compactly
supported perturbation of nontrivial constant state is considered. We
generalize the known theorem by Sideris on initial data resulting in
singularities. Finally, we investigate the influence of frictional damping and
rotation on the singularity formation. | 0411652v2 |
2006-03-04 | Monotonicity properties of blow-up time for nonlinear Schrödinger equation: numerical tests | We consider the focusing nonlinear Schr\"{o}dinger equation, in the
$L^2$-critical and supercritical cases. We investigate numerically the
dependence of the blow-up time on a parameter in three cases: dependence upon
the coupling constant, when the initial data are fixed; dependence upon the
strength of a quadratic oscillation in the initial data when the equation and
the initial profile are fixed; finally, dependence upon a damping factor when
the initial data are fixed. It turns out that in most situations monotonicity
in the evolution of the blow-up time does not occur. In the case of quadratic
oscillations in the initial data, with critical nonlinearity, monotonicity
holds; this is proven analytically. | 0603107v2 |
2003-06-23 | Stochastic 'fuzzy confinement' of intrinsic localised modes | The long time diffusive behaviour of intrinsic localised modes (discrete
breathers) in the discrete damped-driven sine-Gordon chain under Gaussian white
noise (to simulate temperature) is studied. We present a theoretical model for
an approximate description of the diffusion, derive an expression for the
diffusion constant and compare with results from simulations. It turns out that
an increase of the temperature inhibits the diffusive motion in such a way that
the breather, propagating with a well-defined velocity in the noise-free case,
is almost pinned. As all physical processes in the real world occur at
temperatures T > 0, these results also have a bearing on the experimental
detection of mobile breathers, e.g. in parallel arrays of Josephson junctions. | 0306043v2 |
2005-06-08 | Travelling kinks in discrete phi^4 models | In recent years, three exceptional discretizations of the phi^4 theory have
been discovered [J.M. Speight and R.S. Ward, Nonlinearity 7, 475 (1994); C.M.
Bender and A. Tovbis, J. Math. Phys. 38, 3700 (1997); P.G. Kevrekidis, Physica
D 183, 68 (2003)] which support translationally invariant kinks, i.e. families
of stationary kinks centred at arbitrary points between the lattice sites. It
has been suggested that the translationally invariant stationary kinks may
persist as 'sliding kinks', i.e. discrete kinks travelling at nonzero
velocities without experiencing any radiation damping. The purpose of this
study is to check whether this is indeed the case. By computing the Stokes
constants in beyond-all-order asymptotic expansions, we prove that the three
exceptional discretizations do not support sliding kinks for most values of the
velocity - just like the standard, one-site, discretization. There are,
however, isolated values of velocity for which radiationless kink propagation
becomes possible. There is one such value for the discretization of Speight and
Ward and three 'sliding velocities' for the model of Kevrekedis. | 0506019v2 |
2006-09-21 | Covariant response theory beyond RPA and its application | The covariant particle-vibration coupling model within the time blocking
approximation is employed to supplement the Relativistic Random Phase
Approximation (RRPA) with coupling to collective vibrations. The Bethe-Salpeter
equation in the particle-hole channel with an energy dependent residual
particle-hole (p-h) interaction is formulated and solved in the shell-model
Dirac basis as well as in the momentum space. The same set of the coupling
constants generates the Dirac-Hartree single-particle spectrum, the static part
of the residual p-h interaction and the particle-phonon coupling amplitudes.
This approach is applied to quantitative description of damping phenomenon in
even-even spherical nuclei with closed shells $^{208}$Pb and $^{132}$Sn. Since
the phonon coupling enriches the RRPA spectrum with a multitude of
ph$\otimes$phonon states a noticeable fragmentation of giant monopole and
dipole resonances is obtained in the examined nuclei. The results are compared
with experimental data and with results of the non-relativistic approach. | 0609061v1 |
2005-11-17 | Distance dependence of force and dissipation in non-contact atomic force microscopy on Cu(100) and Al(111) | The dynamic characteristics of a tip oscillating in the nc-AFM mode in close
vicinity to a Cu(100)-surface are investigated by means of phase variation
experiments in the constant amplitude mode. The change of the quality factor
upon approaching the surface deduced from both frequency shift and excitation
versus phase curves yield to consistent values. The optimum phase is found to
be independent of distance. The dependence of the quality factor on distance is
related to 'true' damping, because artefacts related to phase misadjustment can
be excluded. The experimental results, as well as on-resonance measurements at
different bias voltages on an Al(111) surface, are compared to Joule
dissipation and to a model of dissipation in which long-range forces lead to
viscoelastic deformations. | 0511151v1 |
1998-08-22 | Quantum Coherence Oscillations in Antiferromagnetic Chains | Macroscopic quantum coherence oscillations in mesoscopic antiferromagnets may
appear when the anisotropy potential creates a barrier between the
antiferromagnetic states with opposite orientations of the Neel vector. This
phenomenon is studied for the physical situation of the nuclear spin system of
eight Xe atoms arranged on a magnetic surface along a chain. The oscillation
period is calculated as a function of the chain constant. The environmental
decoherence effects at finite temperature are accounted assuming a dipole
coupling between the spin chain and the fluctuating magnetic field of the
surface. The numerical calculations indicate that the oscillations are damped
by a rate $\sim (N-1)/ \tau$, where $N$ is the number of spins and $\tau$ is
the relaxation time of a single spin. | 9808041v1 |
1998-12-09 | The 1/N-expansion, quantum-classical correspondence and nonclassical states generation in dissipative higher-order anharmonic oscillators | We develop a method for the determination of thecdynamics of dissipative
quantum systems in the limit of large number of quanta N, based on the
1/N-expansion of Heidmann et al. [ Opt. Commun. 54, 189 (1985) ] and the
quantum-classical correspondence. Using this method, we find analytically the
dynamics of nonclassical states generation in the higher-order anharmonic
dissipative oscillators for an arbitrary temperature of a reservoir. We show
that the quantum correction to the classical motion increases with time
quadratically up to some maximal value, which is dependent on the degree of
nonlinearity and a damping constant, and then it decreases. Similarities and
differences with the corresponding behavior of the quantum corrections to the
classical motion in the Hamiltonian chaotic systems are discussed. We also
compare our results obtained for some limiting cases with the results obtained
by using other semiclassical tools and discuss the conditions for validity of
our approach. | 9812019v2 |
2001-06-08 | Modelling the Recoherence of Mesoscopic Superpositions in Dissipative Environments | A model is presented to describe the recently proposed experiment (J.
Raimond,
M. Brune and S. Haroche Phys. Rev. Lett {\bf 79}, 1964 (1997)) where a
mesoscopic superposition of radiation states is prepared in a high-Q cavity
which is coupled to a similar resonator. The dynamical coherence loss of such
state in the absence of dissipation is reversible and can in principle be
observed. We show how this picture is modified due to the presence of the
environmental couplings. Analytical expressions for the experimental
conditional probabilities and the linear entropy are given. We conclude that
the phenomenon can still be observed provided the ratio between the damping
constant and the inter-cavities coupling does not exceed about a few percent.
This observation is favored for superpositions of states with large overlap. | 0106044v1 |
2001-12-20 | Classical and Quantum-like approaches to Charged-Particle Fluids in a Quadrupole | A classical description of the dynamics of a dissipative charged-particle
fluid in a quadrupole-like device is developed. It is shown that the set of the
classical fluid equations contains the same information as a complex function
satisfying a Schrodinger-like equation in which Planck's constant is replaced
by the time-varying emittance, which is related to the time-varying temperature
of the fluid. The squared modulus and the gradient of the phase of this complex
function are proportional to the fluid density and to the current velocity,
respectively. Within this framework, the dynamics of an electron bunch in a
storage ring in the presence of radiation damping and quantum-excitation is
recovered. Furthermore, both standard and generalized (including dissipation)
coherent states that may be associated with the classical particle fluids are
fully described in terms of the above formalism. | 0112123v1 |
2002-03-25 | Field quantization for open optical cavities | We study the quantum properties of the electromagnetic field in optical
cavities coupled to an arbitrary number of escape channels. We consider both
inhomogeneous dielectric resonators with a scalar dielectric constant
$\epsilon({\bf r})$ and cavities defined by mirrors of arbitrary shape. Using
the Feshbach projector technique we quantize the field in terms of a set of
resonator and bath modes. We rigorously show that the field Hamiltonian reduces
to the system--and--bath Hamiltonian of quantum optics. The field dynamics is
investigated using the input--output theory of Gardiner and Collet. In the case
of strong coupling to the external radiation field we find spectrally
overlapping resonator modes. The mode dynamics is coupled due to the damping
and noise inflicted by the external field. For wave chaotic resonators the mode
dynamics is determined by a non--Hermitean random matrix. Upon including an
amplifying medium, our dynamics of open-resonator modes may serve as a starting
point for a quantum theory of random lasing. | 0203122v2 |
2007-01-29 | Two-Mode Squeezed States and Entangled States of Two Mechanical Resonators | We study a device consisting of a dc-SQUID with two sections of its loop
acting as two mechanical resonators. An analog of the parametric
down-conversion process in quantum optics can be realized with this device. We
show that a two-mode squeezed state can be generated for two overdamped
mechanical resonators, where the damping constants of the two mechanical
resonators are larger than the coupling strengths between the dc-SQUID and the
two mechanical resonators. Thus we show that entangled states of these two
mechanical resonators can be generated. | 0701209v3 |
2007-04-26 | Type I singularities and the Phantom Menace | We consider the future dynamics of a transient phantom dominated phase of the
universe in LQC and in the RS braneworld, which both have a non-standard
Friedmann equation. We find that for a certain class of potentials, the Hubble
parameter oscillates with simple harmonic motion in the LQC case and therefore
avoids any future singularity. For more general potentials we find that damping
effects eventually lead to the Hubble parameter becoming constant. On the other
hand in the braneworld case we find that although the type I singularity can be
avoided, the scale factor still diverges at late times. | 0704.3606v4 |
2007-05-08 | Particle-vibration coupling within covariant density functional theory | Covariant density functional theory, which has so far been applied only
within the framework of static and time dependent mean field theory is extended
to include Particle-Vibration Coupling (PVC) in a consistent way. Starting from
a conventional energy functional we calculate the low-lying collective
vibrations in Relativistic Random Phase Approximation (RRPA) and construct an
energy dependent self-energy for the Dyson equation. The resulting
Bethe-Salpeter equation in the particle-hole ($ph$) channel is solved in the
Time Blocking Approximation (TBA). No additional parameters are used and double
counting is avoided by a proper subtraction method. The same energy functional,
i.e. the same set of coupling constants, generates the Dirac-Hartree
single-particle spectrum, the static part of the residual $ph$-interaction and
the particle-phonon coupling vertices. Therefore a fully consistent description
of nuclear excited states is developed. This method is applied for an
investigation of damping phenomena in the spherical nuclei with closed shells
$^{208}$Pb and $^{132}$Sn. Since the phonon coupling terms enrich the RRPA
spectrum with a multitude of $ph\otimes$phonon components a noticeable
fragmentation of the giant resonances is found, which is in full agreement with
experimental data and with results of the semi-phenomenological
non-relativistic approach. | 0705.1044v1 |
2007-06-15 | Loschmidt echo and stochastic-like quantum dynamics of nano-particles | We investigate time evolution of prepared vibrational state (system) coupled
to a reservoir with dense spectrum of its vibrational states. We assume that
the reservoir has an equidistant spectrum, and the system - reservoir coupling
matrix elements are independent of the reservoir states. The analytical
solution manifests three regimes of the evolution for the system: (I) weakly
damped oscillations; (II) multicomponent Loschmidt echo in recurrence cycles;
(III) overlapping recurrence cycles. We find the characteristic critical values
of the system - reservoir coupling constant for the transitions between these
regimes. Stochastic dynamics occurs in the regime (III) due to inevoidably in
any real system coarse graining of time or energy measurements, or initial
condition uncertainty. Even though a specific toy model is investigated here,
when properly interpreted it yields quite reasonable description for a variety
of physically relevant phenomena. | 0706.2333v1 |
2007-08-13 | Ricci curvature and geodesic flows stability in Riemannian twisted flux tubes | Ricci and sectional curvatures of twisted flux tubes in Riemannian manifold
are computed to investigate the stability of the tubes. The geodesic equations
are used to show that in the case of thick tubes, the curvature of planar
(Frenet torsion-free) tubes have the effect ct of damping the flow speed along
the tube. Stability of geodesic flows in the Riemannian twisted thin tubes
(almost filaments), against constant radial perturbations is investigated by
using the method of negative sectional curvature for unstable flows. No special
form of the flow like Beltrami flows is admitted, and the proof is general for
the case of thin tubes. It is found that for positive perturbations and angular
speed of the flow, instability is achieved, since the sectional Ricci curvature
of the twisted tube metric is negative. | 0708.1644v1 |
2007-08-21 | Phase effects on synchronization by dynamical relaying in delay-coupled systems | Synchronization in an array of mutually coupled systems with a finite
time-delay in coupling is studied using Josephson junction as a model system.
The sum of the transverse Lyapunov exponents is evaluated as a function of the
parameters by linearizing the equation about the synchronization manifold. The
dependence of synchronization on damping parameter, coupling constant and
time-delay is studied numerically. The change in the dynamics of the system due
to time-delay and phase difference between the applied fields is studied. The
case where a small frequency detuning between the applied fields is also
discussed. | 0708.2759v4 |
2007-09-03 | Transport properties controlled by a thermostat: An extended dissipative particle dynamics thermostat | We introduce a variation of the dissipative particle dynamics (DPD)
thermostat that allows for controlling transport properties of molecular
fluids. The standard DPD thermostat acts only on a relative velocity along the
interatomic axis. Our extension includes the damping of the perpendicular
components of the relative velocity, yet keeping the advantages of conserving
Galilei invariance and within our error bar also hydrodynamics. This leads to a
second friction parameter for tuning the transport properties of the system.
Numerical simulations of a simple Lennard-Jones fluid and liquid water
demonstrate a very sensitive behaviour of the transport properties, e.g.,
viscosity, on the strength of the new friction parameter. We envisage that the
new thermostat will be very useful for the coarse-grained and adaptive
resolution simulations of soft matter, where the diffusion constants and
viscosity of the coarse-grained models are typically too high/low,
respectively, compared to all-atom simulations. | 0709.0276v1 |
2007-09-07 | Broadband dielectric response of CaCu3Ti4O12: From dc to the electronic transition regime | We report on phonon properties and electronic transitions in CaCu3Ti4O12, a
material which reveals a colossal dielectric constant at room temperature
without any ferroelectric transition. The results of far- and mid-infrared
measurements are compared to those obtained by broadband dielectric and
millimeter-wave spectroscopy on the same single crystal. The unusual
temperature dependence of phonon eigenfrequencies, dampings and ionic plasma
frequencies of low lying phonon modes are analyzed and discussed in detail.
Electronic excitations below 4 eV are identified as transitions between full
and empty hybridized oxygen-copper bands and between oxygen-copper and
unoccupied Ti 3d bands. The unusually small band gap determined from the
dc-conductivity (~200 meV) compares well with the optical results. | 0709.1065v1 |
2007-10-07 | Decays in Quantum Hierarchical Models | We study the dynamics of a simple model for quantum decay, where a single
state is coupled to a set of discrete states, the pseudo continuum, each
coupled to a real continuum of states. We find that for constant matrix
elements between the single state and the pseudo continuum the decay occurs via
one state in a certain region of the parameters, involving the Dicke and
quantum Zeno effects. When the matrix elements are random several cases are
identified. For a pseudo continuum with small bandwidth there are weakly damped
oscillations in the probability to be in the initial single state. For
intermediate bandwidth one finds mesoscopic fluctuations in the probability
with amplitude inversely proportional to the square root of the volume of the
pseudo continuum space. They last for a long time compared to the non-random
case. | 0710.1403v1 |
2007-12-27 | Dynamics of emergent Cooper pairing at finite temperatures | We study the time evolution of a system of fermions with pairing interactions
at a finite temperature. The dynamics is triggered by an abrupt increase of the
BCS coupling constant. We show that if initially the fermions are in a normal
phase, the amplitude of the BCS order parameter averaged over the Boltzman
distribution of initial states exhibits damped oscillations with a relatively
short decay time. The latter is determined by the temperature, the
single-particle level spacing, and the ground state value of the BCS gap for
the new coupling. In contrast, the decay is essentially absent when the system
was in a superfluid phase before the coupling increase. | 0712.4280v4 |
2008-04-14 | Micromagnetics of single and double point contact spin torque oscillators | In this paper we numerically conduct micromagnetic modelling to optimize
computational boundaries of magnetic thin-film elements applicable to single
and double point contact spin torque nano-oscillators. Different boundary
conditions have been introduced to compensate spin waves reflections at
boundaries that are based on extended layers, absorbing boundaries, and focal
point methods and are compared with a technique based on scattering theory. A
surface roughness boundary model is presented which is modelled according to
the Rayleigh criterion to minimize specular reflections at computational
boundaries. It is shown that the surface roughness model disperses the
reflected spin waves and improves the signal to background noise ratio. The
model is tested in comparison to conventional approaches such as extended layer
systems, variable damping constant and focal point methods for double point
contacts. The surface roughness model gives solutions that are stable in time,
in qualitative agreement with experiments and capable to reproduce phenomena
such as phase locking in double point contacts. | 0804.2119v1 |
2008-06-15 | Stochastic acceleration of solitons for the nonlinear Schrödinger equation | The effective dynamics of solitons for the generalized nonlinear
Schr\"odinger equation in a random potential is rigorously studied. It is shown
that when the external potential varies slowly in space compared to the size of
the soliton, the dynamics of the center of the soliton is almost surely
described by Hamilton's equations for a classical particle in the random
potential, plus error terms due to radiation damping. Furthermore, a limit
theorem for the dynamics of the center of mass of the soliton in the
weak-coupling and space-adiabatic limit is proven in two and higher dimensions:
Under certain mixing hypotheses for the potential, the momentum of the center
of mass of the soliton converges in law to a diffusion process on a sphere of
constant momentum. Moreover, in three and higher dimensions, the trajectory of
the center of mass of the soliton converges to a spatial Brownian motion. | 0806.2439v1 |
2008-06-30 | Beam Coupling Impedance Measurement and Mitigation for a TOTEM Roman Pot | The longitudinal and transverse beam coupling impedance of the first final
TOTEM Roman Pot unit has been measured in the laboratory with the wire method.
For the evaluation of transverse impedance the wire position has been kept
constant, and the insertions of the RP were moved asymmetrically. With the
original configuration of the RP, resonances with fairly high Q values were
observed. In order to mitigate this problem, RF-absorbing ferrite plates were
mounted in appropriate locations. As a result, all resonances were sufficiently
damped to meet the stringent LHC beam coupling impedance requirements. | 0806.4974v1 |
2009-02-25 | B-mode CMB Polarization from Patchy Screening during Reionization | B-modes in CMB polarization from patchy reionization arise from two effects:
generation of polarization from scattering of quadrupole moments by
reionization bubbles, and fluctuations in the screening of E-modes from
recombination. The scattering contribution has been studied previously, but the
screening contribution has not yet been calculated. We show that on scales
smaller than the acoustic scale (l>300), the B-mode power from screening is
larger than the B-mode power from scattering. The ratio approaches a constant
~2.5 below the damping scale (l>2000). On degree scales relevant for
gravitational waves (l<100), screening B-modes have a white noise tail and are
subdominant to the scattering effect. These results are robust to uncertainties
in the modeling of patchy reionization. | 0902.4413v2 |
2009-03-10 | Phonon-phonon interactions and phonon damping in carbon nanotubes | We formulate and study the effective low-energy quantum theory of interacting
long-wavelength acoustic phonons in carbon nanotubes within the framework of
continuum elasticity theory. A general and analytical derivation of all three-
and four-phonon processes is provided, and the relevant coupling constants are
determined in terms of few elastic coefficients. Due to the low dimensionality
and the parabolic dispersion, the finite-temperature density of noninteracting
flexural phonons diverges, and a nonperturbative approach to their interactions
is necessary. Within a mean-field description, we find that a dynamical gap
opens. In practice, this gap is thermally smeared, but still has important
consequences. Using our theory, we compute the decay rates of acoustic phonons
due to phonon-phonon and electron-phonon interactions, implying upper bounds
for their quality factor. | 0903.1771v2 |
2009-03-24 | Chaotic thermalization in Yang-Mills-Higgs theory on a spacial lattice | We analyze the Hamiltonian time evolution of classical SU(2) Yang-Mills-Higgs
theory with a fundamental Higgs doublet on a spacial lattice. In particular, we
study energy transfer and equilibration processes among the gauge and Higgs
sectors, calculate the maximal Lyapunov exponents under randomized initial
conditions in the weak-coupling regime, where one expects them to be related to
the high-temperature plasmon damping rate, and investigate their energy and
coupling dependence. We further examine finite-time and finite-size errors,
study the impact of the Higgs fields on the instability of constant non-Abelian
magnetic fields, and comment on the implications of our results for the
thermalization properties of hot gauge fields in the presence of matter. | 0903.3990v2 |
2009-05-18 | Long-term evolution and gravitational wave radiation of neutron stars with differential rotation induced by r-modes | In a second-order r-mode theory, S'a & Tom'e found that the r-mode
oscillation in neutron stars (NSs) could induce stellar differential rotation,
which leads to a saturation state of the oscillation spontaneously. Based on a
consideration of the coupling of the r-modes and the stellar spin and thermal
evolutions, we carefully investigate the influences of the r-mode-induced
differential rotation on the long-term evolutions of isolated NSs and NSs in
low-mass X-ray binaries, where the viscous damping of the r-modes and its
resultant effects are taken into account. The numerical results show that, for
both kinds of NSs, the differential rotation can prolong the duration of the
r-mode saturation state significantly. As a result, the stars can keep nearly
constant temperature and angular velocity over a thousand years. Moreover, due
to the long-term steady rotation of the stars, persistent quasi-monochromatic
gravitational wave radiation could be expected, which increases the
detectibility of gravitational waves from both nascent and accreting old NSs. | 0905.2804v1 |
2009-05-22 | Quantum dissipative Brownian motion and the Casimir effect | We explore an analogy between the thermodynamics of a free dissipative
quantum particle and that of an electromagnetic field between two mirrors of
finite conductivity. While a free particle isolated from its environment will
effectively be in the high-temperature limit for any nonvanishing temperature,
a finite coupling to the environment leads to quantum effects ensuring the
correct low-temperature behavior. Even then, it is found that under appropriate
circumstances the entropy can be a nonmonotonic function of the temperature.
Such a scenario with its specific dependence on the ratio of temperature and
damping constant also appears for the transverse electric mode in the Casimir
effect. The limits of vanishing dissipation for the quantum particle and of
infinite conductivity of the mirrors in the Casimir effect both turn out to be
noncontinuous. | 0905.3608v1 |
2009-06-24 | Suppression of friction by mechanical vibrations | Mechanical vibrations are known to affect frictional sliding and the
associated stick-slip patterns causing sometimes a drastic reduction of the
friction force. This issue is relevant for applications in nanotribology and to
understand earthquake triggering by small dynamic perturbations .
We study the dynamics of repulsive particles confined between a horizontally
driven top plate and a vertically oscillating bottom plate. Our numerical
results show a suppression of the high dissipative stick-slip regime in a well
defined range of frequencies that depends on the vibrating amplitude, the
normal applied load, the system inertia and the damping constant. We propose a
theoretical explanation of the numerical results and derive a phase diagram
indicating the region of parameter space where friction is suppressed. Our
results allow to define better strategies for the mechanical control of
friction. | 0906.4504v1 |
2009-10-17 | Zero and First Sound in Normal Fermi Systems | On the basis of a moment method, general solutions of a linearized Boltzmann
equation for a normal Fermi system are investigated. In particular, we study
the sound velocities and damping rates as functions of the temperature and the
coupling constant. In the extreme limits of collisionless and hydrodynamic
regimes, eigenfrequency of sound mode obtained from the moment equations
reproduces the well-known results of zero sound and first sound. In addition,
the moment method can describe crossover between those extreme limits at finite
temperatures. Solutions of the moment equations also involve a thermal
diffusion mode. From solutions of these equations, we discuss excitation
spectra corresponding to the particle-hole continuum as well as collective
excitations. We also discuss a collective mode in a weak coupling case. | 0910.3283v1 |
2010-02-11 | Spin drag in an ultracold Fermi gas on the verge of a ferromagnetic instability | Recent experiments [Jo et al., Science 325, 1521 (2009)] have presented
evidence of ferromagnetic correlations in a two-component ultracold Fermi gas
with strong repulsive interactions. Motivated by these experiments we consider
spin drag, i.e., frictional drag due to scattering of particles with opposite
spin, in such systems. We show that when the ferromagnetic state is approached
from the normal side, the spin drag relaxation rate is strongly enhanced near
the critical point. We also determine the temperature dependence of the spin
diffusion constant. In a trapped gas the spin drag relaxation rate determines
the damping of the spin dipole mode, which therefore provides a precursor
signal of the ferromagnetic phase transition that may be used to experimentally
determine the proximity to the ferromagnetic phase. | 1002.2371v2 |
2010-03-12 | Longitudinal dielectric permettivity of quantum Maxwell collisional plasmas | The kinetic equation of Wigner -- Vlasov -- Boltzmann with collision integral
in relaxation BGK (Bhatnagar, Gross and Krook) form in coordinate space for
quantum non--degenerate (Maxwellian) collisional plasma is used. Exact
expression (within the limits of considered model) is found. The analysis of
longitudinal dielectric permeability is done. It is shown that in the limit
when Planck's constant tends to zero of expression for dielectric permettivity
transforms into the classical case of dielectric permettivity. At small values
of wave number it has been received the solution of the dispersion equation.
Damping of plasma oscillations has been analized. The analytical comparison
with the dielectric Mermin' function received with the use of the kinetic
equation in momentum space is done. Graphic comparison of the real and
imaginary parts of dielectric permettivity of quantum and classical plasma is
done also. | 1003.2531v1 |
2010-07-01 | Directed motion of domain walls in biaxial ferromagnets under the influence of periodic external magnetic fields | Directed motion of domain walls (DWs) in a classical biaxial ferromagnet
placed under the influence of periodic unbiased external magnetic fields is
investigated. Using the symmetry approach developed in this article the
necessary conditions for the directed DW motion are found. This motion turns
out to be possible if the magnetic field is applied along the most easy axis.
The symmetry approach prohibits the directed DW motion if the magnetic field is
applied along any of the hard axes. With the help of the soliton perturbation
theory and numerical simulations, the average DW velocity as a function of
different system parameters such as damping constant, amplitude, and frequency
of the external field, is computed. | 1007.0090v2 |
2010-08-23 | Perturbations of Dark Solitons | A method for approximating dark soliton solutions of the nonlinear
Schrodinger equation under the influence of perturbations is presented. The
problem is broken into an inner region, where core of the soliton resides, and
an outer region, which evolves independently of the soliton. It is shown that a
shelf develops around the soliton which propagates with speed determined by the
background intensity. Integral relations obtained from the conservation laws of
the nonlinear Schrodinger equation are used to approximate the shape of the
shelf. The analysis is developed for both constant and slowly evolving
backgrounds. A number of problems are investigated including linear and
nonlinear damping type perturbations. | 1008.3756v1 |
2010-08-30 | Directed transport in equilibrium | We investigate a symmetry broken dimer constrained to move in a particular
direction when in contact with a heat-bath at a constant temperature. The dimer
is not driven by any external force. The system gains kinetic energy from the
heat-bath. The symmetry broken system can use this energy in directed
transport. At the hard core collision limit between the particles of the dimer,
we show by exact analytic calculations and complementary numerical results that
the dimer undergoes steady directed transport. Our observation, being
consistent with the {\it second law of thermodynamics}, {\it detailed balance}
etc leads to new physical understanding to which much attention has not been
paid. | 1008.4992v8 |
2010-08-30 | Scalar graviton in the healthy extension of Hořava-Lifshitz theory | In this note we study the linear dynamics of scalar graviton in a de Sitter
background in the infrared limit of the healthy extension of
Ho\v{r}ava-Lifshitz gravity with the dynamical critical exponent $z=3$. Both
our analytical and numerical results show that the non-zero Fourier modes of
scalar graviton oscillate with an exponentially damping amplitude on the
sub-horizon scale, while on the super-horizon scale, the phases are frozen and
they approach to some asymptotic values. In addition, as the case of the
non-zero modes on super-horizon scale, the zero mode also initially decays
exponentially and then approaches to an asymptotic constant value. | 1008.5048v3 |
2011-02-07 | Buckling and longterm dynamics of a nonlinear model for the extensible beam | This work is focused on the longtime behavior of a non linear evolution
problem describing the vibrations of an extensible elastic homogeneous beam
resting on a viscoelastic foundation with stiffness k>0 and positive damping
constant. Buckling of solutions occurs as the axial load exceeds the first
critical value, \beta_c, which turns out to increase piecewise-linearly with k.
Under hinged boundary conditions and for a general axial load P, the existence
of a global attractor, along with its characterization, is proved by exploiting
a previous result on the extensible viscoelastic beam. As P<\beta_c, the
stability of the straight position is shown for all values of k. But, unlike
the case with null stiffness, the exponential decay of the related energy is
proved if P<\bar\beta(k), where \bar\beta(k) < \beta_c(k) and the equality
holds only for small values of k. | 1102.1259v1 |
2011-02-17 | Effects of disorder on magnetic vortex gyration | A vortex gyrating in a magnetic disk has two regimes of motion in the
presence of disorder. At large gyration amplitudes, the vortex core moves
quasi-freely through the disorder potential. As the amplitude decreases, the
core can become pinned at a particular point in the potential and precess with
a significantly increased frequency. In the pinned regime, the amplitude of the
gyration decreases more rapidly than it does at larger precession amplitudes in
the quasi-free regime. In part, this decreased decay time is due to an increase
in the effective damping constant and in part due to geometric distortion of
the vortex. A simple model with a single pinning potential illustrates these
two contributions. | 1102.3711v1 |
2011-05-02 | Ultrasoft Fermionic Mode in Yukawa Theory at High Temperature | We explore whether an ultrasoft fermionic mode exists at extremely high
temperature in Yukawa theory with massless fermion (coupling constant is g). We
find that the fermion propagator has a pole at \omega = \pm p/3-i\zeta, for
ultrasoft momentum p, where \zeta \sim g^4T ln 1/g, and the residue is Z \sim
g^2. It is shown that one needs to take into account the asymptotic masses and
the damping rate of hard particles to get a sensible result for such an
ultrasoft fermionic mode; possible vertex correction turns out unnecessary for
the scalar coupling in contrast to the gauge coupling. | 1105.0423v2 |
2011-07-17 | Spin current induced magnetization oscillations in a paramagnetic disc | When electron spins are injected uniformly into a paramagnetic disc, they can
precess along the demagnetizing field induced by the resulting magnetic moment.
Normally this precession damps out by virtue of the spin relaxation which is
present in paramagnetic materials. We propose a new mechanism to excite a
steady-state form of this dynamics by injecting a constant spin current into
this paramagnetic disc. We show that the rotating magnetic field generated by
the eddy currents provide a torque which makes this possible. Unlike the
ferromagnetic equivalent, the spin-torque-oscillator, the oscillation frequency
is fixed and determined by the dimensions and intrinsic parameters of the
paramagnet. The system possesses an intrinsic threshold for spin injection
which needs to be overcome before steady-state precession is possible. The
additional application of a magnetic field lowers this threshold. We discuss
the feasibility of this effect in modern materials. Transient analysis using
pump-probe techniques should give insight in the physical processes which
accompany this effect. | 1107.3288v1 |
2011-11-09 | Numerical stability of the Z4c formulation of general relativity | We study numerical stability of different approaches to the discretization of
a conformal decomposition of the Z4 formulation of general relativity. We
demonstrate that in the linear, constant coefficient regime a novel
discretization for tensors is formally numerically stable with a method of
lines time-integrator. We then perform a full set of apples with apples tests
on the non-linear system, and thus present numerical evidence that both the new
and standard discretizations are, in some sense, numerically stable in the
non-linear regime. The results of the Z4c numerical tests are compared with
those of BSSNOK evolutions. We typically do not employ the Z4c constraint
damping scheme and find that in the robust stability and gauge wave tests the
Z4c evolutions result in lower constraint violation at the same resolution as
the BSSNOK evolutions. In the gauge wave tests we find that the Z4c evolutions
maintain the desired convergence factor over many more light-crossing times
than the BSSNOK tests. The difference in the remaining tests is marginal. | 1111.2177v1 |
2011-11-16 | Evolution of the spectrum and the metal-insulator transition in local approximations for many-electron models | In the framework of the many-electron s-d exchange model and Hubbard model,
self-consistent equations are derived for the one-particle retarded Green's
function in the many-electron Hubbard X-operator representation. We analyze the
general structure of the single-site approximations and their connection with
the coherent potential approximation (CPA) and dynamic effective field theory
(DMFT). Using the self-consistent approximation, we examine in detail the
picture of the evolution of the electron spectrum with the model parameters
(coupling constants, the concentration of charge carriers). The influence of
various factors (Kondo many-electron scattering, smearing due to damping,
dynamics of localized moment subsystem) on the shape of the density of states
N(E) in the interacting system is investigated. It is shown that the use of the
locator representation allows to avoid in some cases the non-analyticity in
approximate expressions for the Green's functions. Our approach enables one to
reproduce, at certain values of the parameters, three-peak structure of N(E)
near the metal-insulator transition. | 1111.3751v1 |
2011-11-22 | On the Kramers-Kronig transform with logarithmic kernel for the reflection phase in the Drude model | We use the Kramers-Kronig transform (KKT) with logarithmic kernel to obtain
the reflection phase and, subsequently, the complex refractive index of a bulk
mirror from reflectance. However, there remains some confusion regarding the
formulation for this analysis. Assuming the damped Drude model for the
dielectric constant and the oblique incidence case, we calculate the additional
terms: phase at zero frequency and Blashke factor and we propose a reformulated
KKT within this model. Absolute reflectance in the s-polarization case of a
gold film is measured between 40 and 350 eV for various glancing angles using
synchrotron radiation and its complex refractive index is deduced using the
reformulated KKT that we propose. The results are discussed with respect to the
data available in the literature. | 1111.5139v1 |
2011-12-22 | A continuous model for turbulent energy cascade | In this paper we introduce a new PDE model in frequency space for the
inertial energy cascade that reproduces the classical scaling laws of
Kolmogorov's theory of turbulence. Our point of view is based upon studying the
energy flux through a continuous range of scales rather than the discrete set
of dyadic scales. The resulting model is a variant of Burgers equation on the
half line with a boundary condition which represents a constant energy input at
integral scales. The viscous dissipation is modeled via a damping term. We show
existence of a unique stationary solution, both in the viscous and inviscid
cases, which replicates the classical dissipation anomaly in the limit of
vanishing viscosity.
A survey of recent developments in the deterministic approach to the laws of
turbulence, and in particular, to Onsager's conjecture is given. | 1112.5376v1 |
2012-01-30 | Non-contact Friction and Relaxational Dynamics of Surface Defects | Motion of cantilever near sample surfaces exhibits additional friction even
before two bodies come into mechanical contact. Called non-contact friction
(NCF), this friction is of great practical importance to the ultrasensitive
force detection measurements. Observed large NCF of a micron-scale cantilever
found anomalously large damping that exceeds theoretical predictions by 8-11
orders of magnitude. This finding points to contribution beyond fluctuating
electromagnetic fields within van der Waals approach. Recent experiments
reported by Saitoh et al. (Phys. Rev. Lett. 105, 236103 (2010)) also found
nontrivial distance dependence of NCF. Motivated by these observations, we
propose a mechanism based on the coupling of cantilever to the relaxation
dynamics of surface defects. We assume that the surface defects couple to the
cantilever tip via spin-spin coupling and their spin relaxation dynamics gives
rise to the backaction terms and modifies both the friction coefficient and the
spring constant. We explain the magnitude, as well as the distance dependence
of the friction due to these backaction terms. Reasonable agreement is found
with the experiments. | 1201.6378v1 |
2012-03-08 | A Self-Consistent Marginally Stable State for Parallel Ion Cyclotron Waves | We derive an equation whose solutions describe self-consistent states of
marginal stability for a proton-electron plasma interacting with
parallel-propagating ion cyclotron waves. Ion cyclotron waves propagating
through this marginally stable plasma will neither grow nor damp. The
dispersion relation of these waves, {\omega} (k), smoothly rises from the usual
MHD behavior at small |k| to reach {\omega} = {\Omega}p as k \rightarrow
\pm\infty. The proton distribution function has constant phase-space density
along the characteristic resonant surfaces defined by this dispersion relation.
Our equation contains a free function describing the variation of the proton
phase-space density across these surfaces. Taking this free function to be a
simple "box function", we obtain specific solutions of the marginally stable
state for a range of proton parallel betas. The phase speeds of these waves are
larger than those given by the cold plasma dispersion relation, and the
characteristic surfaces are more sharply peaked in the v\bot direction. The
threshold anisotropy for generation of ion cyclotron waves is also larger than
that given by estimates which assume bi-Maxwellian proton distributions. | 1203.1938v1 |
2012-03-14 | QSO 0347-383 and the invariance of m_p/m_e in the course of cosmic time | The variation of the dimensionless fundamental physical constant mu = m_p/m_e
(the proton to electron mass ratio) can be constrained via observation of Lyman
and Werner lines of molecular hydrogen in the spectra of damped Lyman alpha
systems (DLAs) in the line of sight to distant QSOs. Drawing on VLT-UVES high
resolution data sets of QSO 0347-383 and its DLA obtained in 2009 our analysis
yields dmu/mu = (4.3 +/- 7.2) * 10^-6 at z_abs =3.025. We apply corrections for
the observed offsets between discrete spectra and for the first time we find
indications for inter-order distortions. Current analyses tend to underestimate
the impact of systematic errors. Based on the scatter of the measured redshifts
and the corresponding low significance of the redshift-sensitivity correlation
we estimate the limit of accuracy of line position measurements to about 220
m/s, consisting of roughly 150 m/s due to the uncertainty of the absorption
line fit and about 150 m/s allocated to systematics related to instrumentation
and calibration. | 1203.3193v1 |
2012-03-26 | Local electric current correlation function in an exponentially decaying magnetic field | The effect of an exponentially decaying magnetic field on the dynamics of
Dirac fermions in 3+1 dimensions is explored. The spatially decaying magnetic
field is assumed to be aligned in the third direction, and is defined by
{\mathbf{B}}(x)=B(x){\mathbf{e}}_{z}, with B(x)=B_{0}e^{-\xi\ x/\ell_{B}}.
Here, \xi\ is a dimensionless damping factor and \ell_{B}=(eB_{0})^{-1/2} is
the magnetic length. As it turns out, the energy spectrum of fermions in this
inhomogeneous magnetic field can be analytically determined using the Ritus
method. Assuming the magnetic field to be strong, the chiral condensate and the
\textit{local} electric current correlation function are computed in the lowest
Landau level (LLL) approximation and the results are compared with those
arising from a strong homogeneous magnetic field. Although the constant
magnetic field B_{0} can be reproduced by taking the limit of \xi-> 0 and/or
x-> 0 from B(x), these limits turn out to be singular once the quantum
corrections are taken into account. | 1203.5634v2 |
2012-07-26 | Transient noise spectra in resonant tunneling setups: Exactly solvable models | We investigate the transient evolution of finite-frequency current noise
after abrupt switching on of the tunneling coupling in two paradigmatic,
exactly solvable models of mesoscopic physics: the resonant level model and the
Majorana resonant level model, which emerges as an effective model for a Kondo
quantum dot at the Toulouse point. We find a parameter window in which the
transient noise can become negative, a property it shares with the transient
current. However, in contrast to the transient current, which approaches the
steady state exponentially fast, we observe an algebraic decay in time of the
transient noise for a system at zero temperature. This behaviour is dominant
for characteristic parameter regimes in both models. At finite temperature the
decay is altered from an algebraic to an exponential one with a damping
constant proportional to temperature. | 1207.6222v2 |
2012-12-06 | Time-dependent spherically symmetric covariant Galileons | We study spherically symmetric solutions of the cubic covariant Galileon
model in curved spacetime in presence of a matter source, in the test scalar
field approximation. We show that a cosmological time evolution of the Galileon
field gives rise to an induced matter-scalar coupling, due to the
Galileon-graviton kinetic braiding, therefore the solution for the Galileon
field is non trivial even if the bare matter-scalar coupling constant is set to
zero. The local solution crucially depends on the asymptotic boundary
conditions, and in particular, Minkowski and de Sitter asymptotics correspond
to different branches of the solution. We study the stability of these
solutions, namely, the well-posedness of the Cauchy problem and the positivity
of energy for scalar and tensor perturbations, by diagonalizing the kinetic
terms of the spin-2 and spin-0 degrees of freedom. In addition, we find that in
presence of a cosmological time evolution of the Galileon field, its kinetic
mixing with the graviton leads to a friction force, resulting to efficient
damping of scalar perturbations within matter. | 1212.1394v2 |
2012-12-13 | Astrophysical tests of atomic data important for stellar Mg abundance determinations | Magnesium abundances of cool stars with different metallicities are important
for understanding the galactic chemical evolution. This study tests atomic data
used in stellar magnesium abundance analyses. We evaluate non-local
thermodynamical equilibrium (NLTE) line formation for Mg I using the most
up-to-date theoretical and experimental atomic data available so far and check
the Mg abundances from individual lines in the Sun, four well studied A-type
stars, and three reference metal-poor stars. With the adopted gf-values, NLTE
abundances derived from the Mg I 4703 A, 5528 A, and Mg Ib lines are consistent
within 0.05 dex for each A-type star. The same four Mg I lines in the solar
spectrum give consistent NLTE abundances at $\log N_{\rm Mg}/N_{\rm H} =
-4.45$, when correcting the van der Waals damping constants inferred from the
perturbation theory. Inelastic Mg+H collisions as treated by Barklem, Belyaev,
Spielfiedel, Guitou, and Feautrier serve as efficient thermalizing process for
the statistical equilibrium of Mg I in the atmospheres of metal-poor stars. The
use of the Mg+H collision data improves Mg abundance determinations for HD
84937 and HD 122563, though does not remove completely the differences between
different lines. | 1212.3192v1 |
2013-01-14 | Universal Properties of the Higgs Resonance in (2+1)-Dimensional U(1) Critical Systems | We present spectral functions for the magnitude squared of the order
parameter in the scaling limit of the two-dimensional superfluid to Mott
insulator quantum phase transition at constant density, which has emergent
particle-hole symmetry and Lorentz invariance. The universal functions for the
superfluid, Mott insulator, and normal liquid phases reveal a low-frequency
resonance which is relatively sharp and is followed by a damped oscillation (in
the first two phases only) before saturating to the quantum critical plateau.
The counterintuitive resonance feature in the insulating and normal phases
calls for deeper understanding of collective modes in the strongly coupled
(2+1)-dimensional relativistic field theory. Our results are derived from
analytically continued correlation functions obtained from path-integral Monte
Carlo simulations of the Bose-Hubbard model. | 1301.3139v2 |
2013-03-05 | Dynamics and relaxation in spin nematics | We study dynamics and relaxation of elementary excitations (magnons) in the
spin nematic (quadrupole ordered) phase of S=1 magnets. We develop a general
phenomenological theory of spin dynamics and relaxation for spin-1 systems.
Results of the phenomenological approach are compared to those obtained by
microscopic calculations for the specific S=1 model with isotropic bilinear and
biquadratic exchange interactions. This model exhibits a rich behavior
depending on the ratio of bilinear and biquadratic exchange constants,
including several points with an enhanced symmetry. It is shown that symmetry
plays an important role in relaxation. Particularly, at the SU(3) ferromagnetic
point the magnon damping $\Gamma$ depends on its wavevector k as $\Gamma\propto
k^{4}$, while a deviation from the high-symmetry point changes the behavior of
the leading term to $\Gamma\propto k^{2}$. We point out a similarity between
the behavior of magnon relaxation in spin nematics to that in an isotropic
ferromagnet. | 1303.1194v2 |
2013-03-11 | Ultrasoft fermion mode and off-diagonal Boltzmann equation in quark-gluon plasma at high temperature | We derive the generalized Boltzmann equation (GBE) near equilibrium from the
Kadanoff-Baym equation for quark excitation with ultrasoft momentum (~g^2T, g:
coupling constant, T: temperature) in quantum chromodynamics (QCD) at extremely
high T, and show that the equation is equivalent to the self-consistent
equation derived in the resummed perturbation scheme used to analyze the quark
propagator. We obtain the expressions of the dispersion relation, the damping
rate, and the strength of a quark excitation with ultrasoft momentum by solving
the GBE. We also show that the GBE enables us to obtain the equation
determining the n-point function containing a pair of quarks and (n-2) gluon
external lines whose momenta are ultrasoft. | 1303.2684v1 |
2013-03-27 | Superluminal Waves and the Structure of Pulsar Wind Termination Shocks | The termination shock of a pulsar wind is located roughly where the ram
pressure matches that of the surrounding medium. Downstream of the shock, MHD
models of the diffuse nebular emission suggest the plasma is weakly magnetized.
However, the transition from a Poynting-dominated MHD wind to a
particle-dominated flow is not well understood. We discuss a solution of this
"sigma problem" in which a striped wind converts into a strong, superluminal
electromagnetic wave. This mode slows down as it propagates radially, and its
ram pressure tends to a constant value at large radius, a property we use to
match the solution to the surrounding nebula. The wave thus forms a pre-cursor
to the termination shock, which occurs at the point where the wave dissipates.
Possible damping and dissipation mechanisms are discussed qualitatively. | 1303.6781v1 |
2013-04-26 | Strong magnon softening in tetragonal FeCo compounds | Magnons play an important role in fast precessional magnetization reversal
processes serving as a heat bath for dissipation of the Zeeman energy and thus
being responsible for the relaxation of magnetization. Employing \emph{ab
initio} many-body perturbation theory we studied the magnon spectra of the
tetragonal FeCo compounds considering three different experimental $c/a$
ratios, $c/a=$1.13, 1.18, and 1.24 corresponding to FeCo grown on Pd, Ir, and
Rh, respectively. We find that for all three cases the short-wave-length
magnons are strongly damped and tetragonal distortion gives rise to a
significant magnon softening. The magnon stiffness constant $D$ decreases
almost by a factor of two from FeCo/Pd to FeCo/Rh. The combination of soft
magnons together with the giant magnetic anisotropy energy suggests FeCo/Rh to
be a promising material for perpendicular magnetic recording applications. | 1304.7091v1 |
2013-07-01 | Quasinormal modes of charged dilaton black holes and their entropy spectra | In this study, we employ the scalar perturbations of the charged dilaton
black hole (CDBH) found by Chan, Horne and Mann (CHM), and described with an
action which emerges in the low-energy limit of the string theory. A CDBH is
neither asymptotically flat (AF) nor non-asymptotically flat (NAF) spacetime.
Depending on the value of its dilaton parameter "a", it has both Schwarzschild
and linear dilaton black hole (LDBH) limits. We compute the complex frequencies
of the quasinormal modes (QNMs) of the CDBH by considering small perturbations
around its horizon. By using the highly damped QNMs in the process prescribed
by Maggiore, we obtain the quantum entropy and area spectra of these BHs.
Although the QNM frequencies are tuned by "a", we show that the quantum spectra
do not depend on "a", and they are equally spaced. On the other hand, the
obtained value of undetermined dimensionless constant {\epsilon} is the double
of Bekenstein's result. The possible reason of this discrepancy is also
discussed. | 1307.0340v2 |
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