Fluctuational internal Josephson effect in topological insulator film

Tunneling between opposite surfaces of topological insulator thin film populated by electrons and holes is considered. We predict considerable enhancement of tunneling conductivity by Cooper electron-hole pair fluctuations that are precursor of their Cooper pairing. Cooper pair fluctuations lead to the critical behavior of tunneling conductivity in vicinity of critical temperature with critical index \nu=2. If the pairing is suppressed by disorder the behavior of tunneling conductivity in vicinity of quantum phase transition is also critical with the index \mu=2. The effect can be interpreted as fluctuational internal Josephson effect and it is general phenomenon for electron-hole bilayers. The peculiarities of the effect in other realizations of electron-hole bilayer are discussed.Comment: 8 pages and 7 figures. Extended version accepted to Phys. Rev.

Dielectric response and novel electromagnetic modes in three-dimensional Dirac semimetal films

Using the Kubo formalism we have calculated the local dynamic conductivity of a bulk, i.e., three-dimensional (3D), Dirac semimetal (BDS). We obtain that at frequencies lower than Fermi energy the metallic response in a BDS film manifests in the existence of surface-plasmon polaritons, but at higher frequencies the dielectric response is dominated and it occurs that a BDS film behaves as a dielectric waveguide. At this dielectric regime we predict the existence inside a BDS film of novel electromagnetic modes, a 3D analog of the transverse electric waves in graphene. We also find that the dielectric response manifests as the wide-angle passband in the mid-infrared (IR) transmission spectrum of light incident on a BDS film, which can be used for the interferenceless omnidirectional mid-IR filtering. The tuning of the Fermi level of the system allows us to switch between the metallic and the dielectric regimes and to change the frequency range of the predicted modes. This makes BDSs promising materials for photonics and plasmonics.Comment: 12 pages, 8 figure

Edge magnetoplasmons in graphene: Effects of gate screening and dissipation

Magnetoplasmons on graphene edge in quantizing magnetic field are investigated at different Landau level filling factors. To find the mode frequency, the optical conductivity tensor of disordered graphene in magnetic field is calculated in the self-consistent Born approximation, and the nonlocal electromagnetic problem is solved using the Wiener-Hopf method. Magnetoplasmon dispersion relations, velocities and attenuation lengths are studied numerically and analytically with taking into account the screening by metallic gate and the energy dissipation in graphene. The magnetoplasmon velocity decreases in the presence of nearby gate and oscillates as a function of the filling factor because of the dissipation induced frequency suppression occurring when the Fermi level is located near the centers of Landau levels, in agreement with the recent experiments.Comment: 9 pages, 5 figures; Appendices A and B discussing the problems of conductivity calculations were adde

Many-body filling-factor dependent renormalization of Fermi velocity in graphene in strong magnetic field

We present the theory of many-body corrections to cyclotron transition energies in graphene in strong magnetic field due to Coulomb interaction, considered in terms of the renormalized Fermi velocity. A particular emphasis is made on the recent experiments where detailed dependencies of this velocity on the Landau level filling factor for individual transitions were measured. Taking into account the many-body exchange, excitonic corrections and interaction screening in the static random-phase approximation, we successfully explained the main features of the experimental data, in particular that the Fermi velocities have plateaus when the 0th Landau level is partially filled and rapidly decrease at higher carrier densities due to enhancement of the screening. We also explained the features of the nonmonotonous filling-factor dependence of the Fermi velocity observed in the earlier cyclotron resonance experiment with disordered graphene by taking into account the disorder-induced Landau level broadening.Comment: 14 pages, 8 figures; additional calculation data (in particular, Fig. 4) were included in this new versio

Graphene nanoribbon based spaser

A novel type of spaser with the net amplification of surface plasmons (SPs) in doped graphene nanoribbon is proposed. The plasmons in THz region can be generated in a dopped graphene nanoribbon due to nonradiative excitation by emitters like two level quantum dots located along a graphene nanoribbon. The minimal population inversion per unit area, needed for the net amplification of SPs in a doped graphene nanoribbon is obtained. The dependence of the minimal population inversion on the surface plasmon wavevector, graphene nanoribbon width, doping and damping parameters necessary for the amplification of surface plasmons in the armchair graphene nanoribbon is studied.Comment: 10 pages, 5 figure

Bose-Einstein condensation of polaritons in graphene in a high magnetic field

The Bose-Einstein condensation (BEC) of magnetoexcitonic polaritons in a graphene layer embedded in a optical microcavity in a high magnetic field $B$ is predicted. The essential property of this system (in contrast, e.g., to a quantum well embedded in a cavity) is stronger influence of magnetic field and weaker influence of disorder. A two-dimensional (2D) magnetoexcitonic polaritons gas is considered in a planar harmonic electric field potential applied to excitons or a parabolic shape of the optical cavity causing the trapping of microcavity photons. 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 is obtained.Comment: 7 pages, 1 figur

Drag effects in the system of electrons and microcavity polaritons

The theory of the drag effects in the system of spatially separated electrons and excitons in coupled quantum wells (QW) embedded in an optical microcavity is developed. It is shown that at low temperature an electron current induces the (normal component) polariton flow, therefore, a transport of photons along the cavity. However, the electron current dragged by the polariton flow is strongly suppressed below polariton superfluid transition temperature and hence, the strong suppression of the induced electron current indicates the superfluidity of polaritons. Therefore, the transport properties of polaritons can be investigated by measuring the current or voltage in the electron subsystem. At high temperatures we study the exciton-electron drag effects. At high temperatures regime, from one hand, the existence of the electric current in an electron QW induces the exciton flow in the other QW, from the other hand, the electron current in one QW induces the exciton flow in the other QW via the drag of excitons by the electrons. The drag coefficients for the polariton-electron systems are calculated and analyzed. We discuss the possible experimental observation of the drag effects in the system of electrons and microcavity polaritons, that also allow to observe the cavity polaritons superfluidity.Comment: 16 pages, 7 figures, Physical Review B, in press (2010

Virial theorem, boundary conditions, and pressure for massless Dirac electrons

The virial and the Hellmann--Feynman theorems for massless Dirac electrons in a solid are derived and analyzed using generalized continuity equations and scaling transformations. Boundary conditions imposed on the wave function in a finite sample are shown to break the Hermiticity of the Hamiltonian resulting in additional terms in the theorems in the forms of boundary integrals. The thermodynamic pressure of the electron gas is shown to be composed of the kinetic pressure, which is related to the boundary integral in the virial theorem and arises due to electron reflections from the boundary, and the anomalous pressure, which is specific for electrons in solids. Connections between the kinetic pressure and the properties of the wave function on the boundary are drawn. The general theorems are illustrated by examples of uniform electron gas, and electrons in rectangular and circular graphene samples. The analogous consideration for ordinary massive electrons is presented for comparison.Comment: 14 pages, 5 figures; Section IV.B and Appendices C, D were adde

Bose condensation of direct excitons in an off-resonant cavity at elevated temperatures

We propose a way to increase the lifetime of two-dimensional direct excitons and show the possibility to observe their macroscopically coherent state at high temperatures. For a single GaAs quantum well embedded in photonic layered heterostructures with subwavelength period, we predict the exciton radiative decay to be strongly suppressed. Quantum hydrodynamic approach is used to study the Berezinskii-Kosterlitz-Thouless crossover in a finite exciton system with intermediate densities. Below the estimated critical temperatures, drastic growth of the correlation length is shown to be accompanied by a manyfold increase of the photoluminescence intensity.Comment: 6 pages of the main text, and 5 pages of the Supplemental Material; 7 figure

Can we move photons?

The drag effects in the system of spatially separated electrons and excitons in coupled quantum wells (CQWs) embedded in an optical microcavity are predicted. It is shown that at low temperature an electron current induces the polariton flow, therefore, a transport of photons along the cavity. However, the superfluid polariton component does not contribute to the electron drag. The polariton-electron at the low temperatures and exciton-electron at the high temperatures drag coefficients are presented. It is shown that the drag coefficients increase when temperature increases. We discuss possible experiments for the observation of the electron-polariton drag effect.Comment: 5 pages, 2 figure