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

Orientational melting of two-shell carbon nanoparticles: molecular dynamics study

The energetic characteristics of two-shell carbon nanoparticles ("onions") with different shapes of second shell are calculated. The barriers of relative rotation of shells are found to be surprisingly small; therefore, free relative rotation of shells can take place at room temperature. The intershell orientational melting of the nanoparticle $C_{60}@C_{240}$ is studied by molecular dynamics. The parameters of Arrhenius formula for jump rotational intershell diffusion are calculated. The definition of orientational melting temperature is proposed as the temperature when the transition probability over barrier between equivalent potential minima is equal to 1/2. The temperature of orientational melting of the nanoparticle $C_{60}@C_{240}$ is about 60 K.Comment: 9 pages, 10 figures, some new simulation results and formulations introduce

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

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

Pseudo-magnetoexcitons in strained graphene bilayers without external magnetic fields

The structural and electronic properties of graphene leads its charge carriers to behave like relativistic particles, which is described by a Dirac-like Hamiltonian. Since graphene is a monolayer of carbon atoms, the strain due to elastic deformations will give rise to so-called `pseudomagnetic fields (PMF)' in graphene sheet, and that has been realized experimentally in strained graphene sample. Here we propose a realistic strained graphene bilayer (SGB) device to detect the pseudo-magnetoexcitons (PME) in the absence of external magnetic field. The carriers in each graphene layer suffer different strong PMFs due to strain engineering, which give rise to Landau quantization. The pseudo-Landau levels (PLLs) of electron-hole pair under inhomogeneous PMFs in SGB are analytically obtained in the absence of Coulomb interactions. Based on the general analytical optical absorption selection rule for PME, we show that the optical absorption spectrums can interpret the corresponding formation of Dirac-type PME. We also predict that in the presence of inhomogeneous PMFs, the superfluidity-normal phase transition temperature of PME is greater than that under homogeneous PMFs.}Comment: 16 pages, 6 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

"Shaking" of an atom in a non-stationary cavity

We consider an atom interacting with a quantized electromagnetic field inside a cavity with variable parameters. The atom in the ground state located in the initially empty cavity can be excited by variation of cavity parameters. We have discovered two mechanisms of atomic excitation. The first arises due to the interaction of the atom with the non-stationary electromagnetic field created by modulation of cavity parameters. If the characteristic time of variation of cavity parameters is of the order of the atomic transition time, the processes of photon creation and atomic excitation are going on simultaneously and hence excitation of the atom cannot be reduced to trivial absorption of the photons produced by the dynamical Casimir effect. The second mechanism is "shaking" of the atom due to fast modulation of its ground state Lamb shift which takes place as a result of fast variation of cavity arameters. The last mechanism has no connection with the vacuum dynamical Casimir effect. Moreover, it opens a new channel of photon creation in the non-stationary cavity. Nevertheless, the process of photon creation is altered by the presence of the atom in the cavity, even if one disregards the existence of the new channel. In particular, it removes the restriction for creation of only even number of photons and also changes the expectation value for the number of created photons. Our consideration is based on a simple model of a two-level atom interacting with a single mode of the cavity field. Qualitatively our results are valid for a real atom in a physical cavity.Comment: 12 pages,4 *.eps figures, this version is identical to the one to be published in Physics Letters A (in print

Properties of two - dimensional dusty plasma clusters

Two-dimensional classical cluster of particles interacting through a screened Coulomb potential is studied. This system can be used as a model for "dusty particles" in high-frequency discharge plasma. For systems consisting of N = 2 - 40 particles and confined by a harmonic potential we find ground-state configurations, eigenfrequencies and eigenvectors for the normal modes as a function of the Debye screening length R_D in plasma. Variations in R_D cause changes in the ground-state structure of clusters, each structural rearrangement can be considered as a phase transition of first or second order (with respect to parameter R_D). Monte Carlo and molecular dynamics are used to study in detail the melting of the clusters as the temperature is increased. By varying the density and the temperature of plasma, to which the particles are immersed, one can modulate thermodynamical properties of the system, transforming it in a controllable way to an ordered (crystal-like), orientationaly disordered or totally disordered (liquid-like) states. The possibility of dynamical coexistence phenomena in small clusters is discussed.Comment: 5 pages, 6 Postscript figures; to appear in Phys.Lett.