3,633 research outputs found
Driving quantum walk spreading with the coin operator
We generalize the discrete quantum walk on the line using a time dependent
unitary coin operator. We find an analytical relation between the long-time
behaviors of the standard deviation and the coin operator. Selecting the coin
time sequence allows to obtain a variety of predetermined asymptotic
wave-function spreadings: ballistic, sub-ballistic, diffusive, sub-diffusive
and localized.Comment: 6 pages, 3 figures, appendix added. to appear in PR
On the dissipative effects in the electron transport through conducting polymer nanofibers
Here, we study the effects of stochastic nuclear motions on the electron
transport in doped polymer fibers assuming the conducting state of the
material. We treat conducting polymers as granular metals and apply the quantum
theory of conduction in mesoscopic systems to describe the electron transport
between the metalliclike granules. To analyze the effects of nuclear motions we
mimic them by the phonon bath, and we include the electron-phonon interactions
in consideration. Our results show that the phonon bath plays a crucial part in
the intergrain electron transport at moderately low and room temperatures
suppressing the original intermediate state for the resonance electron
tunneling, and producing new states which support the electron transport.Comment: 6 pages, 4 figures, minor changes are made in the Fig. 3, accepted
for publication in J. of Chem. Phys
Stationary point approach to the phase transition of the classical XY chain with power-law interactions
The stationary points of the Hamiltonian H of the classical XY chain with
power-law pair interactions (i.e., decaying like r^{-{\alpha}} with the
distance) are analyzed. For a class of "spinwave-type" stationary points, the
asymptotic behavior of the Hessian determinant of H is computed analytically in
the limit of large system size. The computation is based on the Toeplitz
property of the Hessian and makes use of a Szeg\"o-type theorem. The results
serve to illustrate a recently discovered relation between phase transitions
and the properties of stationary points of classical many-body Hamiltonian
functions. In agreement with this relation, the exact phase transition energy
of the model can be read off from the behavior of the Hessian determinant for
exponents {\alpha} between zero and one. For {\alpha} between one and two, the
phase transition is not manifest in the behavior of the determinant, and it
might be necessary to consider larger classes of stationary points.Comment: 9 pages, 6 figure
Kekule-distortion-induced Exciton instability in graphene
Effects of a Kekule distortion on exciton instability in single-layer
graphene are discussed. In the framework of quantum electrodynamics the mass of
the electron generated dynamically is worked out using a Schwinger-Dyson
equation. For homogeneous lattice distortion it is shown that the generated
mass is independent of the amplitude of the lattice distortion at the one-loop
approximation. Formation of excitons induced by the homogeneous Kekule
distortion could appear only through direct dependence of the lattice
distortion.Comment: 6 pages, 1 figur
Soluble Models of Strongly Interacting Ultracold Gas Mixtures in Tight Waveguides
A generalized Fermi-Bose mapping method is used to determine the exact ground
states of several models of mixtures of strongly interacting ultracold gases in
tight waveguides, which are generalizations of the Tonks-Girardeau (TG) gas (1D
Bose gas with point hard cores) and fermionic Tonks-Girardeau (FTG) gas (1D
spin-aligned Fermi gas with infinitely strong zero-range attractions). We
detail the case of a Bose-Fermi mixture with TG boson-boson (BB) and
boson-fermion (BF) interactions. Exact results are given for density profiles
in a harmonic trap, single-particle density matrices, momentum distributions,
and density-density correlations. Since the ground state is highly degenerate,
we analyze the splitting of the ground manifold for large but finite BB and BF
repulsions.Comment: Revised to discuss splitting of degenerate ground manifold for large
but finite BB and BF repulsions; accepted by 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
Diffusive Boundary Layers in the Free-Surface Excitable Medium Spiral
Spiral waves are a ubiquitous feature of the nonequilibrium dynamics of a
great variety of excitable systems. In the limit of a large separation in
timescale between fast excitation and slow recovery, one can reduce the spiral
problem to one involving the motion of a free surface separating the excited
and quiescent phases. In this work, we study the free surface problem in the
limit of small diffusivity for the slow field variable. Specifically, we show
that a previously found spiral solution in the diffusionless limit can be
extended to finite diffusivity, without significant alteration. This extension
involves the creation of a variety of boundary layers which cure all the
undesirable singularities of the aforementioned solution. The implications of
our results for the study of spiral stability are briefly discussed.Comment: 6 pages, submitted to PRE Rapid Com
A note on the Casimir energy of a massive scalar field in positive curvature space
We re-evaluate the zero point Casimir energy for the case of a massive scalar
field in space, allowing also for
deviations from the standard conformal value , by means of zero
temperature zeta function techniques. We show that for the problem at hand this
approach is equivalent to the high temperature regularization of the vacuum
energy, as conjectured in a previous publication. Two different, albeit equally
valid, ways of doing the analytic continuation are described.Comment: 6 pages, no figure
Tensor Microwave Background Fluctuations for Large Multipole Order
We present approximate formulas for the tensor BB, EE, TT, and TE multipole
coefficients for large multipole order l. The error in using the approximate
formula for the BB multipole coefficients is less than cosmic variance for
l>10. These approximate formulas make various qualitative properties of the
calculated multipole coefficients transparent: specifically, they show that,
whatever values are chosen for cosmological parameters, the tensor EE multipole
coefficients will always be larger than the BB coefficients for all l>15, and
that these coefficients will approach each other for l<<100. These
approximations also make clear how these multipole coefficients depend on
cosmological parameters.Comment: 19 pages, 9 figures, accepted for publication in Phys. Rev. D,
references and comments on earlier work on the subject added, cosmetic
modification of figure
An integral formula for L^2-eigenfunctions of a fourth order Bessel-type differential operator
We find an explicit integral formula for the eigenfunctions of a fourth order
differential operator against the kernel involving two Bessel functions. Our
formula establishes the relation between K-types in two different realizations
of the minimal representation of the indefinite orthogonal group, namely the
L^2-model and the conformal model
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