12,759 research outputs found

### Quantized Non-Abelian Monopoles on S^3

A possible electric-magnetic duality suggests that the confinement of
non-Abelian electric charges manifests itself as a perturbative quantum effect
for the dual magnetic charges. Motivated by this possibility, we study vacuum
fluctuations around a non-Abelian monopole-antimonopole pair treated as point
objects with charges g=\pm n/2 (n=1,2,...), and placed on the antipodes of a
three sphere of radius R. We explicitly find all the fluctuation modes by
linearizing and solving the Yang-Mills equations about this background field on
a three sphere. We recover, generalize and extend earlier results, including
those on the stability analysis of non-Abelian magnetic monopoles. We find that
for g \ge 1 monopoles there is an unstable mode that tends to squeeze magnetic
flux in the angular directions. We sum the vacuum energy contributions of the
fluctuation modes for the g=1/2 case and find oscillatory dependence on the
cutoff scale. Subject to certain assumptions, we find that the contribution of
the fluctuation modes to the quantum zero point energy behaves as -R^{-2/3} and
hence decays more slowly than the classical -R^{-1} Coulomb potential for large
R. However, this correction to the zero point energy does not agree with the
linear growth expected if the monopoles are confined.Comment: 18 pages, 5 figures. Minor changes, reference list update

### Heat wave propagation in a nonlinear chain

We investigate the propagation of temperature perturbations in an array of
coupled nonlinear oscillators at finite temperature. We evaluate the response
function at equilibrium and show how the memory effects affect the diffusion
properties. A comparison with nonequilibrium simulations reveals that the
telegraph equation provides a reliable interpretative paradigm for describing
quantitatively the propagation of a heat pulse at the macroscopic level. The
results could be of help in understanding and modeling energy transport in
individual nanotubes.Comment: Revised version, 1 fig. adde

### Faraday patterns in dipolar Bose-Einstein condensates

Faraday patterns can be induced in Bose-Einstein condensates by a periodic
modulation of the system nonlinearity. We show that these patterns are
remarkably different in dipolar gases with a roton-maxon excitation spectrum.
Whereas for non-dipolar gases the pattern size decreases monotonously with the
driving frequency, patterns in dipolar gases present, even for shallow roton
minima, a highly non trivial frequency dependence characterized by abrupt
pattern size transitions, which are especially pronounced when the dipolar
interaction is modulated. Faraday patterns constitute hence an optimal tool for
revealing the onset of the roton minimum, a major key feature of dipolar gases.Comment: 4 pages, 10 figure

### Comment on ``Sound velocity and multibranch Bogoliubov spectrum of an elongated Fermi superfluid in the BEC-BCS crossover"

The work by T. K. Ghosh and K. Machida [cond-mat/0510160 and Phys. Rev. A 73,
013613 (2006)] on the sound velocity in a cylindrically confined Fermi
superfluid obeying a power-law equation of state is shown to make use of an
improper projection of the sound wave equation. This inaccuracy fully accounts
for the difference between their results and those previously reported by
Capuzzi et al. [cond-mat/0509323 and Phys. Rev. A 73, 021603(R) (2006)]. In
this Comment we show that both approaches lead exactly to the same result when
the correct weight function is used in the projection. Plots of the correct
behavior of the phonon and monopole-mode spectra in the BCS, unitary, and BEC
limits are also shown.Comment: Comment on cond-mat/051016

### Non-Hermitian Adiabatic Quantum Optimization

We propose a novel non-Hermitian adiabatic quantum optimization algorithm.
One of the new ideas is to use a non-Hermitian auxiliary "initial'' Hamiltonian
that provides an effective level repulsion for the main Hamiltonian. This
effect enables us to develop an adiabatic theory which determines ground state
much more efficiently than Hermitian methods.Comment: Minor corrections, 1 figure, 9 page

### Magnetoconductance of carbon nanotube p-n junctions

The magnetoconductance of p-n junctions formed in clean single wall carbon
nanotubes is studied in the noninteracting electron approximation and
perturbatively in electron-electron interaction, in the geometry where a
magnetic field is along the tube axis. For long junctions the low temperature
magnetoconductance is anomalously large: the relative change in the conductance
becomes of order unity even when the flux through the tube is much smaller than
the flux quantum. The magnetoconductance is negative for metallic tubes. For
semiconducting and small gap tubes the magnetoconductance is nonmonotonic;
positive at small and negative at large fields.Comment: 5 pages, 2 figure

### Simple one-dimensional quantum-mechanical model for a particle attached to a surface

We present a simple one-dimensional quantum-mechanical model for a particle
attached to a surface. We solve the Schr\"odinger equation in terms of Weber
functions and discuss the behavior of the eigenvalues and eigenfunctions. We
derive the virial theorem and other exact relationships as well as the
asymptotic behaviour of the eigenvalues. We calculate the zero-point energy for
model parameters corresponding to H adsorbed on Pd(100) and also outline the
application of the Rayleigh-Ritz variational method

### Piezoconductivity of gated suspended graphene

We investigate the conductivity of graphene sheet deformed over a gate. The
effect of the deformation on the conductivity is twofold: The lattice
distortion can be represented as pseudovector potential in the Dirac equation
formalism, whereas the gate causes inhomogeneous density redistribution. We use
the elasticity theory to find the profile of the graphene sheet and then
evaluate the conductivity by means of the transfer matrix approach. We find
that the two effects provide functionally different contributions to the
conductivity. For small deformations and not too high residual stress the
correction due to the charge redistribution dominates and leads to the
enhancement of the conductivity. For stronger deformations, the effect of the
lattice distortion becomes more important and eventually leads to the
suppression of the conductivity. We consider homogeneous as well as local
deformation. We also suggest that the effect of the charge redistribution can
be best measured in a setup containing two gates, one fixing the overall charge
density and another one deforming graphene locally

### Two-photon Double Ionization of H$_2$ in Intense Femtosecond Laser Pulses

Triple-differential cross sections for two-photon double ionization of
molecular hydrogen are presented for a central photon energy of 30 eV. The
calculations are based on a fully {\it ab initio}, nonperturbative, approach to
the time-dependent Schroedinger equation in prolate spheroidal coordinates,
discretized by a finite-element discrete-variable-representation. The wave
function is propagated in time for a few femtoseconds using the short,
iterative Lanczos method to study the correlated response of the two
photoelectrons to short, intense laser radiation. The current results often lie
in between those of Colgan {\it et al} [J. Phys. B {\bf 41} (2008) 121002] and
Morales {\it et al} [J. Phys. B {\bf 41} (2009) 134013]. However, we argue that
these individual predictions should not be compared directly to each other, but
preferably to experimental data generated under well-defined conditions.Comment: 4 pages, 4 figure

### Resonance modes in a 1D medium with two purely resistive boundaries: calculation methods, orthogonality and completeness

Studying the problem of wave propagation in media with resistive boundaries
can be made by searching for "resonance modes" or free oscillations regimes. In
the present article, a simple case is investigated, which allows one to
enlighten the respective interest of different, classical methods, some of them
being rather delicate. This case is the 1D propagation in a homogeneous medium
having two purely resistive terminations, the calculation of the Green function
being done without any approximation using three methods. The first one is the
straightforward use of the closed-form solution in the frequency domain and the
residue calculus. Then the method of separation of variables (space and time)
leads to a solution depending on the initial conditions. The question of the
orthogonality and completeness of the complex-valued resonance modes is
investigated, leading to the expression of a particular scalar product. The
last method is the expansion in biorthogonal modes in the frequency domain, the
modes having eigenfrequencies depending on the frequency. Results of the three
methods generalize or/and correct some results already existing in the
literature, and exhibit the particular difficulty of the treatment of the
constant mode

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