363 research outputs found
Optical conductivity of the Frohlich polaron
We present accurate results for optical conductivity of the three dimensional
Frohlich polaron in all coupling regimes. The systematic-error free
diagrammatic quantum Monte Carlo method is employed where the Feynman graphs
for the momentum-momentum correlation function in imaginary time are summed up.
The real-frequency optical conductivity is obtained by the analytic
continuation with stochastic optimization. We compare numerical data with
available perturbative and non-perturbative approaches to the optical
conductivity and show that the picture of sharp resonances due to relaxed
excited states in the strong coupling regime is ``washed out''by large
broadening of these states. As a result, the spectrum contains only a
single-maximum broad peak with peculiar shape and a shoulder.Comment: 4 pages, 6 ps-figure
Interface superconductivity in LaAlO-SrTiO heterostructures
The interface superconductivity in LaAlO-SrTiO heterostructures
reveals a non-monotonic behavior of the critical temperature as a function of
the two-dimensional density of charge carriers. We develop a theoretical
description of interface superconductivity in strongly polar heterostructures,
based on the dielectric function formalism. The density dependence of the
critical temperature is calculated accounting for all phonon branches including
different types of optical (interface and half-space) and acoustic phonons. The
LO- and acoustic-phonon-mediated electron-electron interaction is shown to be
the dominating mechanism governing the superconducting phase transition in the
heterostructure.Comment: 7 pages, 2 figures, accepted in Physical Review
Microscopic mechanisms for the Fermi-liquid behavior of Nb-doped strontium titanate
The relaxation rate in Nb-doped strontium titanate involving different
scattering channels is investigated theoretically. It is demonstrated that the
total relaxation rate in SrTiO_{3} is provided mainly by two mechanisms. The
Baber electron-electron scattering with participation of both Coulomb and
phonon-mediated electron-electron interactions provides the T^{2}-dependence of
the relaxation rate. The scattering on the potential landscape caused by
impurities is responsible for the residual relaxation rate at low temperatures.
A good agreement with experiment is achieved accounting for all phonon branches
in strontium titanate, both the optical and acoustic phonons. It is shown that
the effective electron-electron interaction can be attractive in strontium
titanate, and provides superconductivity at low temperatures and Fermi-liquid
response in a wide range of temperatures. Thus our microscopic model supports
the notion that superconductivity and Fermi-liquid properties of n-type
SrTiO_{3} have a common origin.Comment: 22 pages, 6 figures, 1 tabl
Quantum theory of intersubband polarons
We present a microscopic quantum theory of intersubband polarons,
quasiparticles originated from the coupling between intersubband transitions
and longitudinal optical phonons. To this aim we develop a second quantized
theory taking into account both the Fr\"ohlich interaction between phonons and
intersubband transitions and the Coulomb interaction between the intersubband
transitions themselves. Our results show that the coupling between the phonons
and the intersubband transitions is extremely intense, thanks both to the
collective nature of the intersubband excitations and to the natural tight
confinement of optical phonons. Not only the coupling is strong enough to
spectroscopically resolve the resonant splitting between the modes (strong
coupling regime), but it can become comparable to the bare frequency of the
excitations (ultrastrong coupling regime). We thus predict the possibility to
exploit intersubband polarons both for applied optoelectronic research, where a
precise control of the phonon resonances is needed, and also to observe
fundamental quantum vacuum physics, typical of the ultrastrong coupling regime
Nucleation of superconductivity in mesoscopic star-shaped superconductors
We study the phase transition of a star-shaped superconductor, which covers
smoothly the range from zero to two dimensions with respect to the
superconducting coherence length. Detailed measurements and numerical
calculations show that the nucleation of superconductivity in this device is
very inhomogeneous, resulting in rich structure in the superconducting
transition as a function of temperature and magnetic field. The superconducting
order parameter is strongly enhanced and mostly robust in regions close to
multiple boundaries.Comment: 4 pages, 5 figures, E-mail addresses:
[email protected] (V. Chandrasekhar), [email protected]
(J. T. Devreese
Optical Absorption of an Interacting Many-Polaron Gas
The optical absorption of a many (continuum) polaron gas is derived in the
framework of a variational approach at zero temperature and weak or
intermediate electron-phonon coupling strength. We derive a compact formula for
the optical conductivity of the many-polaron system taking into account
many-body effects in the electron or hole system. Within the method presented
here, these effects are contained completely in the dynamical structure factor
of the electron or hole system. This allows to build on well-established
studies of the interacting electron gas. Based on this approach a novel feature
in the absorption spectrum of the many-polaron gas, related to the emission of
a plasmon together with a phonon, is identified. As an application and
illustration of the technique, we compare the theoretical many-polaron optical
absorption spectrum as derived in the present work with the `d-band' absorption
feature in NdCuO. Similarities are shown between the theoretically
and the experimentally derived first frequency moment of the optical absorption
of a family of differently doped NdCeCuO materials.Comment: 24 pages, 5 figures; revised and expanded versio
On the validity of the Franck-Condon principle in the optical spectroscopy: optical conductivity of the Fr\"{o}hlich polaron
The optical absorption of the Fr\"{o}hlich polaron model is obtained by an
approximation-free Diagrammatic Monte Carlo method and compared with two new
approximate approaches that treat lattice relaxation effects in different ways.
We show that: i) a strong coupling expansion, based on the the Franck-Condon
principle, well describes the optical conductivity for large coupling strengths
(); ii) a Memory Function Formalism with phonon broadened levels
reproduces the optical response for weak coupling strengths ()
taking the dynamic lattice relaxation into account. In the coupling regime
the optical conductivity is a rapidly changing superposition of
both Franck-Condon and dynamic contributions.Comment: accepted for publication in PR
Path Integral of the Two Dimensional Su-Schrieffer-Heeger Model
The equilibrium thermodynamics of the two dimensional Su-Schrieffer-Heeger
Model is derived by means of a path integral method which accounts for the
variable range of the electronic hopping processes. While the lattice degrees
of freedom are classical functions of time and are integrated out exactly, the
electron particle paths are treated quantum mechanically. The free energy of
the system and its temperature derivatives are computed by summing at any
over the ensemble of relevant particle paths which mainly contribute to the
total partition function. In the low regime, the {\it heat capacity over T}
ratio shows un upturn peculiar of a glassy like behavior. This feature is more
sizeable in the square lattice than in the linear chain as the overall hopping
potential contribution to the total action is larger in higher dimensionality.Comment: Phys.Rev.B vol.71 (2005
Photoemission spectroscopy and sum rules in dilute electron-phonon systems
A family of exact sum rules for the one-polaron spectral function in the
low-density limit is derived. An algorithm to calculate energy moments of
arbitrary order of the spectral function is presented. Explicit expressions are
given for the first two moments of a model with general electron-phonon
interaction, and for the first four moments of the Holstein polaron. The sum
rules are linked to experiments on momentum-resolved photoemission
spectroscopy. The bare electronic dispersion and the electron-phonon coupling
constant can be extracted from the first and second moments of spectrum. The
sum rules could serve as constraints in analytical and numerical studies of
electron-phonon models.Comment: 4 page
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