We propose experiments to observe Bose-Einstein condensation (BEC) and superfluidity of quasitwo-dimensional (2D) spatially indirect magnetoexcitons in bilayer graphene. The magnetic field B 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 nS and the temperature of the Kosterlitz-Thouless phase transition Tc are shown to be increasing functions of the excitonic density n but decreasing functions of B and the interlayer separation D. Numerical results are presented from these calculations.PACS numbers: 71.35.Ji, 71.35.Lk, Indirect excitons in coupled quantum wells (CQWs) in the presence or absence of a magnetic field B have been the subject of recent experimental investigations [1,2,3,4]. These systems are of particular interest because of the possibility of Bose-Einstein condensation (BEC) and the superfluidity of indirect excitons formed from electron-hole pairs. These may result in persistent electrical currents in each QW or coherent optical properties and Josephson junction phenomena [5,6,7,8,9]. In high magnetic fields, two-dimensional (2D) excitons survive in a substantially wider temperature range, as the exciton binding energies increase with magnetic field [10,11,12,13,14,15,16].In this Letter we propose a new physical realization of magnetoexcitonic BEC and superfluidity in bilayer graphene with spatially separated electrons and holes in high magnetic field. Recent technological advances have allowed the production of graphene, which is a 2D honeycomb lattice of carbon atoms that form the basic planar structure in graphite [17,18] Graphene has been attracting a great deal of experimental and theoretical attention because of unusual properties in its bandstructure [19,20,21,22]. It is a gapless semiconductor with massless electrons and holes which have been described as Dirac-fermions [23]. Since there is no gap between the conduction and valence bands in graphene without magnetic field, the screening effects result in the absence of excitons in graphene in the absence of magnetic field. A strong magnetic field produces a gap since the energy spectrum becomes discrete formed by Landau levels. The gap reduces screening and leads to the formation of magnetoexcitons.We consider two parallel graphene layers separated by an insulating slab of SiO 2 . The electrons in one layer and holes in the other can be controlled as in the experiments with CQWs[1, 2, 3, 4] by laser pumping (far infrared in graphene). The spatial separation of electrons and holes in different graphene layers can be achieved by applying an external electric field. Furthermore, the spatially separated electrons and holes can be created by varying the chemical potential by using a bias voltage between two graphene layers or between two gates located near the corresponding graphene sheets. Indirect magnetoexcitons are bound states of spatially separated electrons and holes in a...
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.
The physical properties of a wide range of nonchiral single-walled carbon nanotubes (SWNT) and double-walled carbon nanotubes (DWNT) with nonchiral commensurate walls are studied. Equilibrium structures of SWNT and DWNT, as well as the interwall interaction energies of DWNT, are computed using a local density approximation within density functional theory with periodic boundary conditions and Gaussian-type orbitals. Based on ab initio structural characteristics, elastic properties of SWNT and DWNT are calculated. Relative motion of the walls of DWNT with different radii and chiralities is explored using ab initio results for the interwall interaction energies. Relative positions of nonchiral commensurate walls of DWNT which correspond to extrema of the interwall interaction energy are derived. For DWNT with incompatible rotational symmetries of the walls, the possibility of orientational melting is predicted. Ab initio values of barriers to relative rotation and sliding of the walls of DWNT are used to calculate threshold forces. For nonreversible telescopic extension of the walls, maximum overlap of the walls for which threshold forces are greater than capillary forces is estimated. A method for selecting pairs of nonchiral commensurate walls in multiwalled carbon nanotubes (MWNT) is proposed. © 2006 The American Physical Society
We calculate the dispersion equations for magnetoplasmons in a single layer, a pair of parallel layers, a graphite bilayer and a superlattice of graphene layers in a perpendicular magnetic field. We demonstrate the feasibility of a drift-induced instability of magnetoplasmons The magnetoplasmon instability in a superlattice is enhanced compared to a single graphene layer. The energies of the unstable magnetoplasmons could be in the terahertz (THz) part of the electromagnetic spectrum. The enhanced instability makes superlattice graphene a potential source of THz radiation.
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