3,714 research outputs found
Temperature Dependence of the Cyclotron Mass in n-Type CdS
Recent cyclotron resonance experiments in n-type CdS at ultra-high magnetic
fields have revealed a pronounced maximum of the electron cyclotron mass as a
function of temperature. In order to interpret these data, we calculate the
magneto-absorption spectra of polarons in n-CdS using the arbitrary-coupling
approach. We show that in high magnetic fields the polaron effects beyond the
weak-coupling approximation clearly reveal themselves in the magneto-optical
absorption even at relatively small values of the Froehlich coupling constant.
In particular, those effects result in a non-monotonous behaviour of the
cyclotron mass as a function of temperature. We extend the theory to take into
account a combined effect of several scattering mechanisms on the
magneto-absorption spectra. The extended theory allows us to interpret
quantitatively the experimentally observed behaviour of the cyclotron mass in
CdS.Comment: 4 pages, 3 figures, E-mail addresses: [email protected],
[email protected]
Optical spectra of quantum dots: effects of non-adiabaticity
It is shown that in many cases an adequate description of optical spectra of
semiconductor quantum dots requires a treatment beyond the commonly used
adiabatic approximation. We have developed a theory of phonon-assisted optical
transitions in semiconductor quantum dots, which takes into account
non-adiabaticity of the exciton-phonon system. Effects of non-adiabaticity lead
to a mixing of different exciton and phonon states that provides a key to the
understanding of surprisingly high intensities of phonon satellites observed in
photoluminescence spectra of quantum dots. A breakdown of the adiabatic
approximation gives an explanation also for discrepancies between the serial
law, observed in multi-phonon optical spectra of some quantum dots, and the
Franck-Condon progression, prescribed by the adiabatic approach.Comment: 4 pages, 3 figures, E-mail addresses: [email protected],
[email protected], [email protected], [email protected],
[email protected]
Cooper pairing and superconductivity on a spherical surface
Electrons in a multielectron bubble in helium form a spherical,
two-dimensional system coupled to the ripplons at the bubble surface. The
electron-ripplon coupling, known to lead to polaronic effects, is shown to give
rise also to Cooper pairing. A Bardeen-Cooper-Schrieffer (BCS) Hamiltonian
arises from the analysis of the electron-ripplon interaction in the bubble, and
values of the coupling strength are obtained for different bubble
configurations. The BCS Hamiltonian on the sphere is analysed using the
Richardson method. We find that although the typical ripplon energies are
smaller than the splitting between electronic levels, a redistribution of the
electron density over the electronic levels is energetically favourable as
pairing correlations can be enhanced. The density of states of the system with
pairing correlations is derived. No gap is present, but the density of states
reveals a strong step-like increase at the pair-breaking energy. This feature
of the density of states should enable the unambiguous detection of the
proposed state with pairing correlations in the bubble, through either
capacitance spectroscopy or tunneling experiments, and allow to map out the
phase diagram of the electronic system in the bubble.Comment: 25 pages, 7 figures, 1 tabl
Vortices in nonequilibrium photon condensates
We present a theoretical study of vortices in arrays of photon condensates.
Even when interactions are negligible, as is the case in current experiments,
pumping and losses can lead to a finite vortex core size. While some properties
of photon condensate vortices, such as their self-acceleration and the
generation of vortex pairs by a moving vortex, resemble those in interacting
polariton condensates far from equilibrium, in several aspects they differ from
previously studied systems: the vortex core size is determined by the balance
between pumping and tunneling, the core appears oblate in the direction of its
motion and new vortex pairs can spontaneously nucleate in the core region.Comment: 5 pages plus supplementary figure
Magnetic susceptibility of ultra-small superconductor grains
For assemblies of superconductor nanograins, the magnetic response is
analyzed as a function of both temperature and magnetic field. In order to
describe the interaction energy of electron pairs for a huge number of
many-particle states, involved in calculations, we develop a simple
approximation, based on the Richardson solution for the reduced BCS Hamiltonian
and applicable over a wide range of the grain sizes and interaction strengths
at arbitrary distributions of single-electron energy levels in a grain. Our
study is focused upon ultra-small grains, where both the mean value of the
nearest-neighbor spacing of single-electron energy levels in a grain and
variations of this spacing from grain to grain significantly exceed the
superconducting gap in bulk samples of the same material. For these ultra-small
superconductor grains, the overall profiles of the magnetic susceptibility as a
function of magnetic field and temperature are demonstrated to be qualitatively
different from those for normal grains. We show that the analyzed signatures of
pairing correlations are sufficiently stable with respect to variations of the
average value of the grain size and its dispersion over an assembly of
nanograins. The presence of these signatures does not depend on a particular
choice of statistics, obeyed by single-electron energy levels in grains.Comment: 40 pages, 12 figures, submitted to Phys. Rev. B, E-mail addresses:
[email protected], [email protected], [email protected]
Vortices on a superconducting nanoshell: phase diagram and dynamics
In superconductors, the search for special vortex states such as giant
vortices focuses on laterally confined or nanopatterned thin superconducting
films, disks, rings, or polygons. We examine the possibility to realize giant
vortex states and states with non-uniform vorticity on a superconducting
spherical nanoshell, due to the interplay of the topology and the applied
magnetic field. We derive the phase diagram and identify where, as a function
of the applied magnetic field, the shell thickness and the shell radius, these
different vortex phases occur. Moreover, the curved geometry allows these
states (or a vortex lattice) to coexist with a Meissner state, on the same
curved film. We have examined the dynamics of the decay of giant vortices or
states with non-uniform vorticity into a vortex lattice, when the magnetic
field is adapted so that a phase boundary is crossed.Comment: 21 pages, 9 figure
Co-existence of the Meissner and vortex-state on a superconducting spherical shell
We show that on superconducting spherical nanoshells, the co-existence of the
Meissner state with a variety of vortex patterns drives the phase transition to
higher magnetic fields. The spherical geometry leads to a Magnus-Lorentz force
pushing the nucleating vortices and antivortices towards the poles, overcoming
local pinning centers, preventing vortex-antivortex recombination and leading
to the appearance of a Meissner belt around the sphere's equator. In
sufficiently small and thin spherical shells paramagnetic vortex states can be
stable, enabling spatial separation of freely moving shells with different
radii and vorticity in an inhomogeneous external magnetic field.Comment: 11 pages, 5 figures (higher res gif version of fig.5 included in
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