25 research outputs found
Anisotropic conductivity of disordered 2DEGs due to spin-orbit interactions
We show that the conductivity tensor of a disordered two-dimensional electron
gas becomes anisotropic in the presence of both Rashba and Dresselhaus
spin-orbit interactions (SOI). This anisotropy is a mesoscopic effect and
vanishes with vanishing charge dephasing time. Using a diagrammatic approach
including zero, one, and two-loop diagrams, we show that a consistent
calculation needs to go beyond a Boltzmann equation approach. In the absence of
charge dephasing and for zero frequency, a finite anisotropy \sigma_{xy}
e^2/lhpf arises even for infinitesimal SOI.Comment: 6+ page
Theory of Bose-Einstein condensation and superfluidity of two-dimensional polaritons in an in-plane harmonic potential
Recent experiments have shown that it is possible to create an in-plane
harmonic potential trap for a two-dimensional (2D) gas of exciton-polaritons in
a microcavity structure, and evidence has been reported of Bose-Einstein
condensation of polaritons accumulated in this type of trap. We present here
the theory of Bose-Einstein condensation (BEC) and superfluidity of the exciton
polaritons in a harmonic potential trap. Along the way, we determine a general
method for defining the superfluid fraction in a 2D trap, in terms of angular
momentum representation. We show that in the continuum limit, as the trap
becomes shallower the superfluid fraction approaches the 2D Kosterlitz-Thouless
limit, while the condensate fraction approaches zero, as expected.Comment: 14 pages, 5 figures. Accepted for publication by Physical review
Superfluidity of dipole excitons in two layers of gapped graphene
A study of the formation of excitons as a problem of two Dirac particles
confined in two-layer graphene sheets separated by a dielectric when gaps are
opened and they interact via a Coulomb potential is presented. We propose to
observe Bose-Einstein condensation and superfluidity of quasi-two-dimensional
dipole excitons in double layer graphene in the presence of band gaps. The
energy spectrum of the collective excitations, the sound spectrum, and the
effective exciton mass are functions of the energy gaps, density and interlayer
separation. The superfluid density ns and temperature of the
Kosterlitz-Thouless phase transition Tc are decreasing functions of the energy
gaps as well as the interlayer separation, and therefore, could be controlled
by these parameters.Comment: 12 pages, 3 figure
Bose-Einstein condensation and Superfluidity of magnetoexcitons in Graphene
We propose experiments to observe Bose-Einstein condensation (BEC) and
superfluidity of quasi-two-dimensional (2D) spatially indirect magnetoexcitons
in bilayer graphene. The magnetic field is assumed strong. The energy
spectrum of collective excitations, the sound spectrum as well as the effective
magnetic mass of magnetoexcitons are presented in the strong magnetic field
regime. The superfluid density and the temperature of the
Kosterlitz-Thouless phase transition are shown to be increasing functions
of the excitonic density but decreasing functions of and the interlayer
separation . Numerical results are presented from these calculations.Comment: 5 pages, 1 figur
Collective Properties of Excitons in Presence of a Two-Dimensional Electron Gas
We have studied the collective properties of two-dimensional (2D) excitons
immersed within a quantum well which contains 2D excitons and a two-dimensional
electron gas (2DEG). We have also analyzed the excitations for a system of 2D
dipole excitons with spatially separated electrons and holes in a pair of
quantum wells (CQWs) when one of the wells contains a 2DEG. Calculations of the
superfluid density and the Kosterlitz-Thouless (K-T) phase transition
temperature for the 2DEG-exciton system in a quantum well have shown that the
K-T transition temperature increases with increasing exciton density and that
it might be possible to have fast long range transport of excitons. The
superfluid density and the K-T transition temperature for dipole excitons in
CQWs in the presence of a 2DEG in one of the wells increases with increasing
inter-well separation.Comment: 10 pages, 1 figure. accepted by Solid State Communication
Bose-Einstein condensation of trapped polaritons in 2D electron-hole systems in a high magnetic field
The Bose-Einstein condensation (BEC) of magnetoexcitonic polaritons in
two-dimensional (2D) electron-hole system embedded in a semiconductor
microcavity in a high magnetic field is predicted. There are two physical
realizations of 2D electron-hole system under consideration: a graphene layer
and quantum well (QW). A 2D gas of magnetoexcitonic polaritons is considered in
a planar harmonic potential trap. Two possible physical realizations of this
trapping potential are assumed: inhomogeneous local stress or harmonic electric
field potential applied to excitons and a parabolic shape of the semiconductor
cavity causing the trapping of microcavity photons. The effective Hamiltonian
of the ideal gas of cavity polaritons in a QW and graphene in a high magnetic
field and the BEC temperature as functions of magnetic field are obtained. It
is shown that the effective polariton mass increases with
magnetic field as . The BEC critical temperature
decreases as and increases with the spring constant of the parabolic
trap. The Rabi splitting related to the creation of a magnetoexciton in a high
magnetic field in graphene and QW is obtained. It is shown that Rabi splitting
in graphene can be controlled by the external magnetic field since it is
proportional to , while in a QW the Rabi splitting does not depend on
the magnetic field when it is strong.Comment: 16 pages, 6 figures. accepted in Physical Review
Critical temperature modification of low dimensional superconductors by spin doping
Ion implantation of Fe and Mn into Al thin films was used for effective
modification of Al superconductive properties. Critical temperature of the
transition to superconducting state was found to decrease gradually with
implanted Fe concentration. it was found that suppression by Mn implantation
much stronger compared to Fe. At low concentrations of implanted ions,
suppression of the critical temperature can be described with reasonable
accuracy by existing models, while at concentrations above 0.1 at.% a
pronounced discrepancy between the models and experiments is observed.Comment: 7 figures, 18 page
Anomalous far infrared monochromatic transmission through a film of type-II superconductor in magnetic field
Anomalous far infrared monochromatic transmission through a lattice of
Abrikosov vortices in a type-II superconducting film is found and reported. The
transmitted frequency corresponds to the photonic mode localized by the defects
of the Abrokosov lattice. These defects are formed by extra vortices placed out
of the nodes of the ideal Abrokosov lattice. The extra vortices can be pinned
by crystal lattice defects of a superconductor. The corresponding frequency is
studied as a function of magnetic field and temperature in the framework of the
Dirac-type two-band model. While our approach is valid for all type-II
superconductors, the specific calculations have been performed for the
YBaCuO (YBCO). The control of the transmitted
frequency by varying magnetic field and/or temperature is analyzed. It is
suggested that found anomalously transmitted localized mode can be utilized in
the far infrared monochromatic filters.Comment: 9 pages, 2 figure
Bose-Einstein condensation of quasiparticles in graphene
The collective properties of different quasiparticles in various graphene
based structures in high magnetic field have been studied. We predict
Bose-Einstein condensation (BEC) and superfluidity of 2D spatially indirect
magnetoexcitons in two-layer graphene. The superfluid density and the
temperature of the Kosterlitz-Thouless phase transition are shown to be
increasing functions of the excitonic density but decreasing functions of
magnetic field and the interlayer separation. The instability of the ground
state of the interacting 2D indirect magnetoexcitons in a slab of superlattice
with alternating electron and hole graphene layers (GLs) is established. The
stable system of indirect 2D magnetobiexcitons, consisting of pair of indirect
excitons with opposite dipole moments, is considered in graphene superlattice.
The superfluid density and the temperature of the Kosterlitz-Thouless phase
transition for magnetobiexcitons in graphene superlattice are obtained.
Besides, the BEC of excitonic polaritons in GL embedded in a semiconductor
microcavity in high magnetic field is predicted. While superfluid phase in this
magnetoexciton polariton system is absent due to vanishing of
magnetoexciton-magnetoexciton interaction in a single layer in the limit of
high magnetic field, the critical temperature of BEC formation is calculated.
The essential property of magnetoexcitonic systems based on graphene (in
contrast, e.g., to a quantum well) is stronger influence of magnetic field and
weaker influence of disorder. Observation of the BEC and superfluidity of 2D
quasiparticles in graphene in high magnetic field would be interesting
confirmation of the phenomena we have described.Comment: 13 pages, 5 figure
Thermodynamics of Two - Band Superconductors: The Case of MgB
Thermodynamic properties of the multiband superconductor MgB have often
been described using a simple sum of the standard BCS expressions corresponding
to - and -bands. Although, it is \textit{a priori} not clear if
this approach is working always adequately, in particular in cases of strong
interband scattering. Here we compare the often used approach of a sum of two
independent bands using BCS-like -model expressions for the specific
heat, entropy and free energy to the solution of the full Eliashberg equations.
The superconducting energy gaps, the free energy, the entropy and the heat
capacity for varying interband scattering rates are calculated within the
framework of two-band Eliashberg theory. We obtain good agreement between the
phenomenological two-band -model with the Eliashberg results, which
delivers for the first time the theoretical verification to use the
-model as a useful tool for a reliable analysis of heat capacity data.
For the thermodynamic potential and the entropy we demonstrate that only the
sum over the contributions of the two bands has physical meaning.Comment: 27 pages, 10 figures, 1 table, submitted to Phys. Rev.