8,908 research outputs found
Semiclassical quantization of the Bogoliubov spectrum
We analyze the Bogoliubov spectrum of the 3-sites Bose-Hubbard model with
finite number of Bose particles by using a semiclassical approach. The
Bogoliubov spectrum is shown to be associated with the low-energy regular
component of the classical Hubbard model. We identify the full set of the
integrals of motions of this regular component and, quantizing them, obtain the
energy levels of the quantum system. The critical values of the energy, above
which the regular Bogoliubov spectrum evolves into a chaotic spectrum, is
indicated as well.Comment: 4.1 pages, 3 figure
Three-dimensional Roton-Excitations and Supersolid formation in Rydberg-excited Bose-Einstein Condensates
We study the behavior of a Bose-Einstein condensate in which atoms are weakly
coupled to a highly excited Rydberg state. Since the latter have very strong
van der Waals interactions, this coupling induces effective, nonlocal
interactions between the dressed ground state atoms, which, opposed to dipolar
interactions, are isotropically repulsive. Yet, one finds partial attraction in
momentum space, giving rise to a roton-maxon excitation spectrum and a
transition to a supersolid state in three-dimensional condensates. A detailed
analysis of decoherence and loss mechanisms suggests that these phenomena are
observable with current experimental capabilities.Comment: 4 pages, 5 figure
Monte Carlo simulations of , a classical Heisenberg antiferromagnet in two-dimensions with dipolar interaction
We study the phase diagram of a quasi-two dimensional magnetic system with Monte Carlo simulations of a classical Heisenberg spin
Hamiltonian which includes the dipolar interactions between
spins. Our simulations reveal an Ising-like antiferromagnetic phase at low
magnetic fields and an XY phase at high magnetic fields. The boundary between
Ising and XY phases is analyzed with a recently proposed finite size scaling
technique and found to be consistent with a bicritical point at T=0. We discuss
the computational techniques used to handle the weak dipolar interaction and
the difference between our phase diagram and the experimental results.Comment: 13 pages 18 figure
First-Matsubara-frequency rule in a Fermi liquid. Part II: Optical conductivity and comparison to experiment
Motivated by recent optical measurements on a number of strongly correlated
electron systems, we revisit the dependence of the conductivity of a Fermi
liquid, \sigma(\Omega,T), on the frequency \Omega and temperature T. Using the
Kubo formalism and taking full account of vertex corrections, we show that the
Fermi liquid form Re\sigma^{-1}(\Omega,T)\propto \Omega^2+4\pi^2T^2 holds under
very general conditions, namely in any dimensionality above one, for a Fermi
surface of an arbitrary shape (but away from nesting and van Hove
singularities), and to any order in the electron-electron interaction. We also
show that the scaling form of Re\sigma^{-1}(\Omega,T) is determined by the
analytic properties of the conductivity along the Matsubara axis. If a system
contains not only itinerant electrons but also localized degrees of freedom
which scatter electrons elastically, e.g., magnetic moments or resonant levels,
the scaling form changes to Re\sigma^{-1}(\Omega,T)\propto \Omega^2+b\pi^2T^2,
with 1\leq b<\infty. For purely elastic scattering, b =1. Our analysis implies
that the value of b\approx 1, reported for URu_2Si_2 and some rare-earth based
doped Mott insulators, indicates that the optical conductivity in these
materials is controlled by an elastic scattering mechanism, whereas the values
of b\approx 2.3 and b\approx 5.6, reported for underdoped cuprates and
organics, correspondingly, imply that both elastic and inelastic mechanisms
contribute to the optical conductivity.Comment: 18 pages, 10 figure
An Induction Accelerator of Cosmic Rays on the Axis of an Accretion Disk
The structure and magnitude of the electric field created by a rotating
accretion disk with a poloidal magnetic field is found for the case of a vacuum
approximation along the axis. The accretion disk is modeled as a torus filled
with plasma and the frozen-in magnetic field. The dimensions and location of
the maximum electric field are found, as well as the energy of the accelerated
particles. The gravitational field is assumed to be weak.Comment: 10 pages, 4 figure
Effect of disorder studied with ferromagnetic resonance for arrays of tangentially magnetized sub-micron Permalloy discs fabricated by nanosphere lithography
Tangentially magnetized trigonal arrays of sub-micron Permalloy discs are
characterized with ferromagnetic resonance to determine the possible
contributions to frequency and linewidth from array disorder. Each array is
fabricated by a water-surface self-assembly lithographic technique, and
consists of a large trigonal array of 700 nm diameter magnetic discs. Each
array is characterized by a different degree of ordering. Two modes are present
in the ferromagnetic resonance spectra: a large amplitude, `fundamental' mode
and a lower amplitude mode at higher field. Angular dependence of the resonance
field in a very well ordered array is found to be negligible for both modes.
The relationship between resonance frequency and applied magnetic field is
found to be uncorrelated with array disorder. Linewidth is found to increase
with increasing array disorder
Edge dislocations in crystal structures considered as traveling waves of discrete models
The static stress needed to depin a 2D edge dislocation, the lower dynamic
stress needed to keep it moving, its velocity and displacement vector profile
are calculated from first principles. We use a simplified discrete model whose
far field distortion tensor decays algebraically with distance as in the usual
elasticity. An analytical description of dislocation depinning in the strongly
overdamped case (including the effect of fluctuations) is also given. A set of
parallel edge dislocations whose centers are far from each other can depin
a given one provided , where is the average inter-dislocation
distance divided by the Burgers vector of a single dislocation. Then a limiting
dislocation density can be defined and calculated in simple cases.Comment: 10 pages, 3 eps figures, Revtex 4. Final version, corrected minor
error
Resonance Damping in Ferromagnets and Ferroelectrics
The phenomenological equations of motion for the relaxation of ordered phases
of magnetized and polarized crystal phases can be developed in close analogy
with one another. For the case of magnetized systems, the driving magnetic
field intensity toward relaxation was developed by Gilbert. For the case of
polarized systems, the driving electric field intensity toward relaxation was
developed by Khalatnikov. The transport times for relaxation into thermal
equilibrium can be attributed to viscous sound wave damping via
magnetostriction for the magnetic case and electrostriction for the
polarization case.Comment: 5 pages no figures ReVTeX
General-Relativistic Curvature of Pulsar Vortex Structure
The motion of a neutron superfluid condensate in a pulsar is studied. Several
theorems of general-relativistic hydrodynamics are proved for a superfluid. The
average density distribution of vortex lines in pulsars and their
general-relativistic curvature are derived.Comment: 18 pages, 1 figure
Environmental Decoherence versus Intrinsic Decoherence
We review the difference between standard environmental decoherence and
'intrinsic decoherence', which is taken to be an ineluctable process of Nature.
Environmental decoherence is typically modeled by spin bath or oscillator modes
- we review some of the unanswered questions not captured by these models, and
also the application of them to experiments. Finally, a sketch is given of a
new theoretical approach to intrinsic decoherence, and this scheme is applied
to the discussion of gravitational decoherence.Comment: to be published in Phil Trans Roy Soc
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