243 research outputs found
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
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
Transport of magnetoexcitons in single and coupled quantum wells
The transport relaxation time and the mean free path of
magnetoexcitons in single and coupled quantum wells are calculated ( 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 depends nonmonotonously on in the case (ii) and in the cases
(i), (iii) for smaller than some critical value ( is the interwell
separation, is the magnetic length). For the
transport relaxation time increases monotonously with . The magnetoexciton
mean free path has a maximum at in the cases (i), (iii).
It decreases with increasing . 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 increases with 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
Stability of Sarma phases in density imbalanced electron-hole bilayer systems
We study excitonic condensation in an electron-hole bilayer system with
unequal layer densities at zero temperature. Using mean-field theory we solve
the BCS gap equations numerically and investigate the effects of intra-layer
interactions. We analyze the stability of the Sarma phase with \bk,-\bk
pairing by calculating the superfluid mass density and also by checking the
compressibility matrix. We find that with bare Coulomb interactions the
superfluid density is always positive in the Sarma phase, due to a peculiar
momentum structure of the gap function originating from the singular behavior
of the Coulomb potential at zero momentum and the presence of a sharp Fermi
surface. Introducing a simple model for screening, we find that the superfluid
density becomes negative in some regions of the phase diagram, corresponding to
an instability towards a Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) type superfluid
phase. Thus, intra-layer interaction and screening together can lead to a rich
phase diagram in the BCS-BEC crossover regime in electron-hole bilayer systems
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 . 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
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
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
Dipolar superfluidity in electron-hole bilayer systems
Bilayer electron-hole systems, where the electrons and holes are created via
doping and confined to separate layers, undergo excitonic condensation when the
distance between the layers is smaller than typical distance between particles
within a layer. We argue that the excitonic condensate is a novel dipolar
superfluid in which the phase of the condensate couples to the {\it gradient}
of the vector potential. We predict the existence of dipolar supercurrent which
can be tuned by an in-plane magnetic field and detected by independent contacts
to the layers. Thus the dipolar superfluid offers an example of excitonic
condensate in which the {\it composite} nature of its constituent excitons is
manifest in the macroscopic superfluid state. We also discuss various
properties of this superfluid including the role of vortices.Comment: 5 pages, 1 figure, minor changes and added few references; final
published versio
Collective excitations in electron-hole bilayers
We report a combined analytic and Molecular Dynamics analysis of the
collective mode spectrum of an electron-hole (bipolar) bilayer in the strong
coupling quasi-classical limit. A robust, isotropic energy gap is identified in
the out-of-phase spectra, generated by the combined effect of correlations and
of the excitation of the bound dipoles; the in-phase spectra exhibit a
correlation governed acoustic dispersion for the longitudinal and transverse
modes. Strong nonlinear generation of higher harmonics of the fundamental
dipole oscillation frequency and the transfer of harmonics between different
modes is observed. The mode dispersions in the liquid state are compared with
the phonon spectrum in the crystalline solid phase, reinforcing a coherent
physical picture.Comment: 4 pages, 5 figure
Collective properties of magnetobiexcitons in quantum wells' and graphene superlattices
We propose the Bose-Einstein condensation (BEC) and superfluidity of
quasi-two-dimensional (2D) spatially indirect magnetobiexcitons in a slab of
superlattice with alternating electron and hole layers consisting from the
semiconducting quantum wells (QWs) and graphene superlattice in high magnetic
field. The two different Hamiltonians of a dilute gas of magnetoexcitons with a
dipole-dipole repulsion in superlattices consisting of both QWs and graphene
layers in the limit of high magnetic field have been reduced to one effective
Hamiltonian a dilute gas of two-dimensional excitons without magnetic field.
Moreover, for excitons we have reduced the problem of
dimensional space onto the problem of dimensional space by
integrating over the coordinates of the relative motion of an electron (e) and
a hole (h). The instability of the ground state of the system of interacting
two-dimensional indirect magnetoexcitons in a slab of superlattice with
alternating electron and hole layers in high magnetic field is established. The
stable system of indirect quasi-two-dimensional magnetobiexcitons, consisting
of pair of indirect excitons with opposite dipole moments is considered. The
density of superfluid component and the temperature of the
Kosterlitz-Thouless phase transition to the superfluid state in the system of
two-dimensional indirect magnetobiexcitons, interacting as electrical
quadrupoles, are obtained for both QW and graphene realizations.Comment: 9 pages, 3 figure
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