38 research outputs found
Ground state properties and excitation spectra of non-Galilean invariant interacting Bose systems
We study the ground state properties and the excitation spectrum of bosons
which, in addition to a short-range repulsive two body potential, interact
through the exchange of some dispersionless bosonic modes. The latter induces a
time dependent (retarded) boson-boson interaction which is attractive in the
static limit. Moreover the coupling with dispersionless modes introduces a
reference frame for the moving boson system and hence breaks the Galilean
invariance of this system. The ground state of such a system is depleted {\it
linearly} in the boson density due to the zero point fluctuations driven by the
retarded part of the interaction. Both quasiparticle (microscopic) and
compressional (macroscopic) sound velocities of the system are studied. The
microscopic sound velocity is calculated up the second order in the effective
two body interaction in a perturbative treatment, similar to that of Beliaev
for the dilute weakly interacting Bose gas. The hydrodynamic equations are used
to obtain the macroscopic sound velocity. We show that these velocities are
identical within our perturbative approach. We present analytical results for
them in terms of two dimensional parameters -- an effective interaction
strength and an adiabaticity parameter -- which characterize the system. We
find that due the presence of several competing effects, which determine the
speed of the sound of the system, three qualitatively different regimes can be
in principle realized in the parameter space and discuss them on physical
grounds.Comment: 6 pages, 2 figures, to appear in Phys. Rev.
X-ray edge problem of graphene
The X-ray edge problem of graphene with the Dirac fermion spectrum is
studied. At half-filling the linear density of states suppresses the singular
response of the Fermi liquid, while away from half-filling the singular
features of the Fermi liquid reappear. The crossover behavior as a function of
the Fermi energy is examined in detail. The exponent of the power-law
absorption rate depends both on the intra- and inter-valley scattering, and it
changes as a function of the Fermi energy, which may be tested experimentally.Comment: 7 pages, 1 figur
Excitation spectrum in a cylindrical Bose-Einstein gas
Whole excitation spectrum is calculated within the Popov approximation of the
Bogoliubov theory for a cylindrical symmetric Bose-Einstein gas trapped
radially by a harmonic potential. The full dispersion relation and its
temperature dependence of the zero sound mode propagating along the axial
direction are evaluated in a self-consistent manner. The sound velocity is
shown to depend not only on the peak density, but also on the axial area
density. Recent sound velocity experiment on Na atom gas is discussed in light
of the present theory.Comment: 4 pages, 5 eps figure
Superfluidity of bosons on a deformable lattice
We study the superfluid properties of a system of interacting bosons on a
lattice which, moreover, are coupled to the vibrational modes of this lattice,
treated here in terms of Einstein phonon model. The ground state corresponds to
two correlated condensates: that of the bosons and that of the phonons. Two
competing effects determine the common collective soundwave-like mode with
sound velocity , arising from gauge symmetry breaking: i) The sound velocity
(corresponding to a weakly interacting Bose system on a rigid lattice) in
the lowest order approximation is reduced due to reduction of the repulsive
boson-boson interaction, arising from the attractive part of phonon mediated
interaction in the static limit. ii) the second order correction to the sound
velocity is enhanced as compared to the one of bosons on a rigid lattice when
the the boson-phonon interaction is switched on due to the retarded nature of
phonon mediated interaction. The overall effect is that the sound velocity is
practically unaffected by the coupling with phonons, indicating the robustness
of the superfluid state. The induction of a coherent state in the phonon
system, driven by the condensation of the bosons could be of experimental
significance, permitting spectroscopic detections of superfluid properties of
the bosons. Our results are based on an extension of the Beliaev - Popov
formalism for a weakly interacting Bose gas on a rigid lattice to that on a
deformable lattice with which it interacts.Comment: 12 pages, 14 figures, to appear in Phys. Rev.
Polaronic excitations in CMR manganite films
In the colossal magnetoresistance manganites polarons have been proposed as
the charge carrier state which localizes across the metal-insulator transition.
The character of the polarons is still under debate. We present an assessment
of measurements which identify polarons in the metallic state of
La{2/3}Sr{1/3}MnO{3} (LSMO) and La{2/3}Ca{1/3}MnO{3} (LCMO) thin films. We
focus on optical spectroscopy in these films which displays a pronounced
resonance in the mid-infrared. The temperature dependent resonance has been
previously assigned to polaron excitations. These polaronic resonances are
qualitatively distinct in LSMO and LCMO and we discuss large and small polaron
scenarios which have been proposed so far. There is evidence for a large
polaron excitation in LSMO and small polarons in LCMO. These scenarios are
examined with respect to further experimental probes, specifically charge
carrier mobility (Hall-effect measurements) and high-temperature
dc-resistivity.Comment: 16 pages, 10 figure
Effects of Disorder in a Dilute Bose Gas
We discuss the effects of a weak random external potential on the properties
of the dilute Bose gas at zero temperature. The results recently obtained by
Huang and Meng for the depletion of the condensate and of the superfluid
density are recovered. Results for the shift of the velocity of sound as well
as for its damping due to collisions with the external field are presented. The
damping of phonons is calculated also for dense superfluids. (submitted to
Phys.Rev.B)Comment: 21 pages, Plain Tex, 2 figures available upon request, preprint UTF
31
Infrared Behavior of Interacting Bosons at Zero Temperature
We exploit the symmetries associated with the stability of the superfluid
phase to solve the long-standing problem of interacting bosons in the presence
of a condensate at zero temperature. Implementation of these symmetries poses
strong conditions on the renormalizations that heal the singularities of
perturbation theory. The renormalized theory gives: For d>3 the Bogoliubov
quasiparticles as an exact result; for 1<d<=3 a nontrivial solution with the
exact exponent for the singular longitudinal correlation function, with phonons
again as low-lying excitations.Comment: Minor Changes. 4 pages, RevTeX, no figures, uses multicol.sty e-mail:
[email protected]
Solution of the X-ray edge problem for 2D electrons in a magnetic field
The absorption and emission spectra of transitions between a localized level
and a two-dimensional electron gas, subjected to a weak magnetic field, are
calculated analytically. Adopting the Landau level bosonization technique
developed in previous papers, we find an exact expression for the relative
intensities of spectral lines. Their envelope function, governed by the
interaction between the electron gas and the core hole, is reminescent of the
famous Fermi edge singularity, which is recovered in the limit of a vanishing
magnetic field.Comment: 4 pages, 1 figur
Renormalization Group Approach to the Infrared Behavior of a Zero-Temperature Bose System
We exploit the renormalization-group approach to establish the {\em exact}
infrared behavior of an interacting Bose system at zero temperature. The
local-gauge symmetry in the broken-symmetry phase is implemented through the
associated Ward identities, which reduce the number of independent running
couplings to a single one. For this coupling the -expansion can be
controlled to all orders in (). For spatial dimensions the Bogoliubov fixed point is unstable towards a different fixed point
characterized by the divergence of the longitudinal correlation function. The
Bogoliubov linear spectrum, however, is found to be independent from the
critical behavior of this correlation function, being exactly constrained by
Ward identities. The new fixed point properly gives a finite value of the
coupling among transverse fluctuations, but due to virtual intermediate
longitudinal fluctuations the effective coupling affecting the transverse
correlation function flows to zero. As a result, no transverse anomalous
dimension is present. This treatment allows us to recover known results for the
quantum Bose gas in the context of a unifying framework and also to reveal the
non-trivial skeleton structure of its perturbation theory.Comment: 21 page
Properties of Nambu-Goldstone Bosons in a Single-Component Bose-Einstein Condensate
We theoretically study the properties of Nambu-Goldstone bosons in an
interacting single-component Bose-Einstein condensate (BEC). We first point out
that the proofs of Goldstone's theorem by Goldstone, et al. [Phys. Rev. {\bf
127} (1962) 965] may be relevant to distinct massless modes of the BEC: whereas
the first proof deals with the poles of the single-particle Green's function
, the second one concerns those of the two-particle Green's function.
Thus, there may be multiple Nambu-Goldstone bosons even in the single-component
BEC with broken U(1) symmetry. The second mode turns out to have an infinite
lifetime in the long-wavelength limit in agreement with the conventional
viewpoint. In contrast, the first mode from , i.e., the Bogoliubov
mode in the weak-coupling regime, is shown to be a "bubbling" mode fluctuating
temporally out of and back into the condensate. The substantial lifetime
originates from an "improper" structure of the self-energy inherent in the BEC,
which has been overlooked so far and will be elucidated here, and removes
various infrared divergences pointed out previously.Comment: 9 pages, 6 gigure