427 research outputs found

### Transport of magnetoexcitons in single and coupled quantum wells

The transport relaxation time $\tau (P)$ and the mean free path of
magnetoexcitons in single and coupled quantum wells are calculated ($P$ is the
magnetic momentum of the magnetoexciton). We present the results for
magnetoexciton scattering in a random field due to (i) quantum well width
fluctuations, (ii) composite fluctuations and (iii) ionized impurities. The
time $\tau(P)$ depends nonmonotonously on $P$ in the case (ii) and in the cases
(i), (iii) for $D/l$ smaller than some critical value ($D$ is the interwell
separation, $l=\sqrt{\hbar c/eH}$ is the magnetic length). For $D/l\gg 1$ the
transport relaxation time increases monotonously with $P$. The magnetoexciton
mean free path $\lambda (P)$ has a maximum at $P\ne 0$ in the cases (i), (iii).
It decreases with increasing $D/l$. The mean free path calculated for the case
(ii) may have two maxima. One of them disappears with the variation of the
random fields parameters. The maximum of $\lambda (P)$ increases with $H$ for
types (i,iii) of scattering processes and decreases in the case (ii).Comment: 13 pages, 8 figures in EPS format; Physica Scripta (in print

### Cooper pairing of electrons and holes in graphene bilayer: Correlation effects

Cooper pairing of spatially separated electrons and holes in graphene bilayer
is studied beyond the mean-field approximation. Suppression of the screening at
large distances, caused by appearance of the gap, is considered
self-consistently. A mutual positive feedback between appearance of the gap and
enlargement of the interaction leads to a sharp transition to correlated state
with greatly increased gap above some critical value of the coupling strength.
At coupling strength below the critical, this correlation effect increases the
gap approximately by a factor of two. The maximal coupling strength achievable
in experiments is close to the critical value. This indicated importance of
correlation effects in closely-spaced graphene bilayers at weak substrate
dielectric screening. Another effect beyond mean-field approximation considered
is an influence of vertex corrections on the pairing, which is shown to be very
weak.Comment: 6 pages, 5 figures; some references were adde

### Phases of a bilayer Fermi gas

We investigate a two-species Fermi gas in which one species is confined in
two parallel layers and interacts with the other species in the
three-dimensional space by a tunable short-range interaction. Based on the
controlled weak coupling analysis and the exact three-body calculation, we show
that the system has a rich phase diagram in the plane of the effective
scattering length and the layer separation. Resulting phases include an
interlayer s-wave pairing, an intralayer p-wave pairing, a dimer Bose-Einstein
condensation, and a Fermi gas of stable Efimov-like trimers. Our system
provides a widely applicable scheme to induce long-range interlayer
correlations in ultracold atoms.Comment: 5 pages, 5 figures; (v2) stability of trimer is emphasized; (v3)
published versio

### Quasiequilibrium supersolid phase of a two-dimensional dipolar crystal

We have studied the possible existence of a supersolid phase of a
two-dimensional dipolar crystal using quantum Monte Carlo methods at zero
temperature. Our results show that the commensurate solid is not a supersolid
in the thermodynamic limit. The presence of vacancies or interstitials turns
the solid into a supersolid phase even when a tiny fraction of them are present
in a macroscopic system. The effective interaction between vacancies is
repulsive making a quasiequilibrium dipolar supersolid possible.Comment: 5 pages, 4 figure

### 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 $B$ 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 $M_{\rm eff}$ increases with
magnetic field as $B^{1/2}$. The BEC critical temperature $T_{c}^{(0)}$
decreases as $B^{-1/4}$ 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 $B^{-1/4}$, 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

### Quantum phase transition in a two-dimensional system of dipoles

The ground-state phase diagram of a two-dimensional Bose system with
dipole-dipole interactions is studied by means of quantum Monte Carlo
technique. Our calculation predicts a quantum phase transition from gas to
solid phase when the density increases. In the gas phase the condensate
fraction is calculated as a function of the density. Using Feynman
approximation, the collective excitation branch is studied and appearance of a
roton minimum is observed. Results of the static structure factor at both sides
of the gas-solid phase are also presented. The Lindeman ratio at the transition
point comes to be $\gamma = 0.230(6)$. The condensate fraction in the gas phase
is estimated as a function of the density.Comment: 4 figures v.3 One citation added, updated Fig.4. Minor changes
following referee's and editor's comment

### Quantum orientational melting of mesoscopic clusters

By path integral Monte Carlo simulations we study the phase diagram of two -
dimensional mesoscopic clusters formed by electrons in a semiconductor quantum
dot or by indirect magnetoexcitons in double quantum dots. At zero (or
sufficiently small) temperature, as quantum fluctuations of particles increase,
two types of quantum disordering phenomena take place: first, at small values
of quantum de Boer parameter q < 0.01 one can observe a transition from a
completely ordered state to that in which different shells of the cluster,
being internally ordered, are orientationally disordered relative to each
other. At much greater strengths of quantum fluctuations, at q=0.1, the
transition to a disordered (superfluid for the boson system) state takes place.Comment: 4 pages, 6 Postscript figure

### Excitonic condensate and quasiparticle transport in electron-hole bilayer systems

Bilayer electron-hole systems undergo excitonic condensation when the
distance d between the layers is smaller than the typical distance between
particles within a layer. All excitons in this condensate have a fixed dipole
moment which points perpendicular to the layers, and therefore this condensate
of dipoles couples to external electromagnetic fields. We study the transport
properties of this dipolar condensate system based on a phenomenological model
which takes into account contributions from the condensate and quasiparticles.
We discuss, in particular, the drag and counterflow transport, in-plane
Josephson effect, and noise in the in-plane currents in the condensate state
which provides a direct measure of the superfluid collective-mode velocity.Comment: 7 pages, 3 figure

### Superfluidity of "dirty" indirect excitons and magnetoexcitons in two-dimensional trap

The superfluid phase transition of bosons in a two-dimensional (2D) system
with disorder and an external parabolic potential is studied. The theory is
applied to experiments on indirect excitons in coupled quantum wells. The
random field is allowed to be large compared to the dipole-dipole repulsion
between excitons. The slope of the external parabolic trap is assumed to change
slowly enough to apply the local density approximation (LDA) for the superfluid
density, which allows us to calculate the Kosterlitz-Thouless temperature
$T_{c}(n(r))$ at each local point $r$ of the trap. The superfluid phase occurs
around the center of the trap ($\mathbf{r}=0$) with the normal phase outside
this area. As temperature increases, the superfluid area shrinks and disappears
at temperature $T_{c}(n(r=0))$. Disorder acts to deplete the condensate; the
minimal total number of excitons for which superfluidity exists increases with
disorder at fixed temperature. If the disorder is large enough, it can destroy
the superfluid entirely. The effect of magnetic field is also calculated for
the case of indirect excitons. In a strong magnetic field $H$, the superfluid
component decreases, primarily due to the change of the exciton effective mass.Comment: 13 pages, 3 figure

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