2,102 research outputs found

### Analysis of the second order exchange self energy of a dense electron gas

We investigate the evaluation of the six-fold integral representation for the
second order exchange contribution to the self energy of a three dimensional
electron gas at the Fermi surface.Comment: 6 page

### CMB anisotropies in the presence of a stochastic magnetic field

Primordial magnetic fields present since before the epoch of matter-radiation
equality have an effect on the anisotropies of the cosmic microwave background.
The CMB anisotropies due to scalar perturbations are calculated in the gauge
invariant formalism for magnetized adiabatic initial conditions. Furthermore
the linear matter power spectrum is calculated. Numerical solutions are
complemented by a qualitative analysis.Comment: 26 pages, 21 figures; sections 2 and 4 expanded; matches version
published in PR

### Time-dependent quantum transport in a resonant tunnel junction coupled to a nanomechanical oscillator

We present a theoretical study of time-dependent quantum transport in a
resonant tunnel junction coupled to a nanomechanical oscillator within the
non-equilibrium Green's function technique. An arbitrary voltage is applied to
the tunnel junction and electrons in the leads are considered to be at zero
temperature. The transient and the steady state behavior of the system is
considered here in order to explore the quantum dynamics of the oscillator as a
function of time. The properties of the phonon distribution of the
nanomechnical oscillator strongly coupled to the electrons on the dot are
investigated using a non-perturbative approach. We consider both the energy
transferred from the electrons to the oscillator and the Fano factor as a
function of time. We discuss the quantum dynamics of the nanomechanical
oscillator in terms of pure and mixed states. We have found a significant
difference between a quantum and a classical oscillator. In particular, the
energy of a classical oscillator will always be dissipated by the electrons
whereas the quantum oscillator remains in an excited state. This will provide
useful insight for the design of experiments aimed at studying the quantum
behavior of an oscillator.Comment: 24 pages, 10 figure

### Analytic invariant charge and the lattice static quark-antiquark potential

A recently developed model for the QCD analytic invariant charge is compared
with quenched lattice simulation data on the static quark-antiquark potential.
By employing this strong running coupling one is able to obtain the confining
quark-antiquark potential in the framework of the one-gluon exchange model. To
achieve this objective a technique for evaluating the integrals of a required
form is developed. Special attention is paid here to removing the divergences
encountered the calculations. All this enables one to examine the asymptotic
behavior of the potential at both small and large distances with high accuracy.
An explicit expression for the quark-antiquark potential, which interpolates
between these asymptotics, and satisfies the concavity condition, is proposed.
The derived potential coincides with the perturbative results at small
distances, and it is in a good agreement with the lattice data in the
nonperturbative physically-relevant region. An estimation of the parameter
$\Lambda_{QCD}$ is obtained for the case of pure gluodynamics. It is found to
be consistent with all the previous estimations of $\Lambda_{QCD}$ in the
framework of approach in hand.Comment: LaTeX2e, 10 pages with 3 EPS figure

### Critical phenomena and phase sequence in classical bilayer Wigner crystal at zero temperature

We study the ground-state properties of a system of identical classical
Coulombic point particles, evenly distributed between two equivalently charged
parallel plates at distance $d$; the system as a whole is electroneutral. It
was previously shown that upon increasing d from 0 to infinity, five different
structures of the bilayer Wigner crystal become energetically favored, starting
from a hexagonal lattice (phase I, d=0) and ending at a staggered hexagonal
lattice (phase V, d -> infinity). In this paper, we derive new series
representations of the ground-state energy for all five bilayer structures. The
derivation is based on a sequence of transformations for lattice sums of
Coulomb two-particle potentials plus the neutralizing background, having their
origin in the general theory of Jacobi theta functions. The new series provide
convenient starting points for both analytical and numerical progress. Its
convergence properties are indeed excellent: Truncation at the fourth term
determines in general the energy correctly up to 17 decimal digits. The
accurate series representations are used to improve the specification of
transition points between the phases and to solve a controversy in previous
studies. In particular, it is shown both analytically and numerically that the
hexagonal phase I is stable only at d=0, and not in a finite interval of small
distances between the plates as was anticipated before. The expansions of the
structure energies around second-order transition points can be done
analytically, which enables us to show that the critical behavior is of the
Ginzburg-Landau type, with a mean-field critical index beta=1/2 for the growth
of the order parameters

### Braiding of anyonic quasiparticles in the charge transfer statistics of symmetric fractional edge-state Mach-Zehnder interferometer

We have studied the zero-temperature statistics of the charge transfer
between the two edges of Quantum Hall liquids of, in general, different filling
factors, $\nu_{0,1}=1/(2 m_{0,1}+1)$, with $m_0 \geq m_1\geq 0$, forming
Mach-Zehnder interferometer. General expression for the cumulant generating
function in the large-time limit is obtained for symmetric interferometer with
equal propagation times along the two edges between the contacts and constant
bias voltage. The low-voltage limit of the generating function can be
interpreted in terms of the regular Poisson process of electron tunneling,
while its leading large-voltage asymptotics is proven to coincide with the
solution of kinetic equation describing quasiparticle transitions between the
$m$ states of the interferometer with different effective flux through it,
where $m\equiv 1+m_{0}+m_{1}$. For $m>1$, this dynamics reflects both the
fractional charge $e/m$ and the fractional statistical angle $\pi /m$ of the
tunneling quasiparticles. Explicit expressions for the second (shot noise) and
third cumulants are obtained, and their voltage dependence is analyzed.Comment: 11 two-column pages, 4 figure

### Quantum backreaction in evolving FLRW spacetimes

Quantum fluctuations of a nonminimally coupled scalar field in D-dimensional
homogeneous and isotropic background are calculated within the operator
formalism in curved models with time evolutions of the scale factor that allow
smooth transitions between contracting and expanding and between decelerating
and accelerating regimes. The coincident propagator is derived and used to
compute the one-loop backreaction from the scalar field. The inflationary
infrared divergences are absent in Bunch-Davies vacuum when taking into account
a preceding cosmological era or spatial curvature which can be either positive
or negative. It is found that asymptotically, the backreaction energy density
in the minimally coupled case grows logarithmically with the scale factor in
quasi-de Sitter space, and in a class of models decays in slow-roll inflation
and grows as a power-law during super-inflation. The backreaction increases
generically in a contracting phase or in the presence of a negative nonminimal
coupling. The effects of the coupling and renormalization scale upon the
quantum fluctuations together with the novel features due to nontrivial time
evolution and spatial curvature are clarified with exact solutions and
numerical examples.Comment: 23 pages, 6 figure

### Observable Dependent Quasi-Equilibrium in Slow Dynamics

We present examples demonstrating that quasi-equilibrium
fluctuation-dissipation behavior at short time differences is not a generic
feature of systems with slow non-equilibrium dynamics. We analyze in detail the
non-equilibrium fluctuation-dissipation ratio X(t,tw) associated with a
defect-pair observable in the Glauber-Ising spin chain. It turns out that $X
\neq 1$ throughout the short-time regime and in particular X(tw,tw) = 3/4 for
$tw \to \infty$. The analysis is extended to observables detecting defects at a
finite distance from each other, where similar violations of quasi-equilibrium
behaviour are found. We discuss our results in the context of metastable
states, which suggests that a violation of short-time quasi-equilibrium
behavior could occur in general glassy systems for appropriately chosen
observables.Comment: 17 pages, 5 figures; substantially improved version of
cond-mat/040571

### Multiphoton resonance in a three-level system with nearly degenerate excited states

An analytic study is presented of the efficient multiphoton excitation and
strong harmonic generation in three-level systems specified by a pair of nearly
degenerate, strongly dipole-coupled excited states. Such systems are physically
formed by the three lowest states in, e.g., the hydrogen atom or evenly charged
homonuclear diatomic molecular ions under reasonably chosen laser intensities.
As a detailed analytic result, we found that the laser pulse of photon energy
$2{.}05\text{eV}$, duration $0{.}23\text{ps}$ and intensity $5\cdot
10^{13}\,\frac{\text{W}}{\text{cm}^2}$ is able to produce complete inversion of
the initial population in the hydrogen atom through the 5-photon excitation. At
the same photon energy, the pulse of duration $0{.}41\text{ps}$ and intensity
$3{.}44\cdot 10^{14}\,\frac{\text{W}}{\text{cm}^2}$ was found to produce the
same effect in the molecular ion but through the 9-photon excitation. We show
that the accompanying scattering of light has very rich spectrum differing
substantially from that of the two-level system.Comment: 9 pages, 5 figures,submitted to Phys. Rev. A, comments welcom

### Nonlinear screening and ballistic transport in a graphene p-n junction

We study the charge density distribution, the electric field profile, and the
resistance of an electrostatically created lateral p-n junction in graphene. We
show that the electric field at the interface of the electron and hole regions
is strongly enhanced due to limited screening capacity of Dirac quasiparticles.
Accordingly, the junction resistance is lower than estimated in previous
literature.Comment: 4 pages, 2 figures. (v1) Original version (v2) Introduction largely
rewritten, minor typos fixed throughou

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