Yu. E. Lozovik

Wigner Crystallization in Mesoscopic 2D Electron Systems

Wigner crystallization of electrons in 2D quantum dots is reported. It proceeds in two stages: (i) via radial ordering of electrons on shells and (ii) freezing of the intershell rotation. The phase boundary of the crystal is computed in the whole temperature-density plane, and the influences of quantum effects and the particle number are analyzed.

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 a quantum Monte Carlo technique. Our calculation predicts a quantum phase transition from a gas to a solid phase when the density increases. In the gas phase, the condensate fraction is calculated as a function of the density. Using the Feynman approximation, the collective excitation branch is studied and the appearance of a roton minimum is observed. The results of the static structure factor at both sides of the gas-solid phase are also presented. The Lindemann ratio at the transition point becomes gamma=0.230(6). The condensate fraction in the gas phase is estimated as a function of the density.

Phase transitions in a system of two coupled quantum wells

It is predicted that an excitonic liquid is formed in a system of spatially separated electrons (e) and holes (h) in a system of two coupled quantum wells. The ground-state energy and the equilibrium density of the excitonic liquid are calculated as a function of the distance D between the wells. A gas-liquid quantum transition with increasing D is studied. The Berezinskii-Kosterlitz-Thouless transition temperatures at which superfluidity appears in the system are found (for different D). A quantum Mott metal-insulator transition in an anisotropic double-quantum-well structure is investigated. The region of existence of crystalline order in a system of spatially separated e and h is studied. Possible experimental manifestations of the predicted effects are discussed.

From spatially indirect excitons to momentum-space indirect excitons by an in-plane magnetic field

An in-plane magnetic field is found to change drastically the photoluminescence spectra and kinetics of interwell excitons in

On a modified Lindemann-like criterion for 2D melting

\mathrm{GaAs}/{\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}

Phase transitions in a system of spatially separated electrons and holes

coupled quantum wells. This effect is due to the in-plane magnetic-field-induced displacement of the interwell exciton dispersion in momentum space, which results in the transition from the momentum-space direct exciton ground state to the momentum-space indirect exciton ground state. An in-plane magnetic field is, therefore, an effective tool for exciton dispersion engineering.

Electron-hole pair condensation in a graphene bilayer

Abstract By means of a molecular dynamics simulation the existence of a Lindemann-like criterion for 2D melting is proved. The criterion is the constancy of the value γ M c ≡ 〈[ u ( R + a 0 )− u ( R )] 2 〉 a 2 along the melting curve; this value being practically independent of the nature of a 2D classical crystal (the Lindemann constant is infinite for a 2D crystal in the thermodynamical limit). For 2D dipole and 2D Lennard-Jones crystals γMc = 0.12. For a 2D electron crystal γMc ≈ 0.10. These values are consistent with the results of calculations in the framework of the phonon-mediated melting theory.

Superconductivity at dielectric pairing of spatially separated quasiparticles

The formation of a superfluid exciton liquid in a system of spatially separated electrons and holes in a system of two coupled quantum wells is predicted and its properties are investigated. The ground-state energy and the equilibrium density of the exciton liquid are calculated as functions of distance D between the quantum wells. The properties of a rarefied exciton gas with dipole-dipole repulsions are considered, where this gas is the metastable phase for D<1.9a* and the stable phase for D<1.9a* (a* is the radius of the two-dimensional exciton). The gas-liquid quantum transition is examined for increasing D. The Berezinskii-Kosterlitz-Thouless transition temperatures, at which superfluidity arises in the system, are found for different values of D. Possible experimental manifestations of the predicted effects are discussed.

Coulomb clusters in a trap

The pairing of electrons and holes due to their Coulomb attraction in two parallel, independently gated graphene layers separated by a barrier is considered. At a weak coupling, there exists the BCS-like pair-condensed state. Despite the fact that electrons and holes behave like massless Dirac fermions, the problem of BCS-like electron—hole pairing in the graphene bilayer turns out to be rather similar to that in usual coupled semiconductor quantum wells. The distinctions are due to the Berry phase of electronic wavefunctions and different screening properties. We estimate the values of the gap in a one-particle excitation spectrum for different interlayer distances and carrier concentrations. The influence of the disorder is discussed. At a large enough dielectric susceptibility of the surrounding medium, the weak coupling regime holds at arbitrarily small carrier concentrations. Localized electron—hole pairs are absent in graphene, thus the behavior of the system versus the coupling strength is cardinally different from usual BCS—BEC crossover.

Influence of well-width fluctuations on the binding energy of excitons, charged excitons, and biexcitons in GaAs -based quantum wells

Abstract A new mechanism of the superconductivity based upon the pairing of the spatially separated electrons and holes due to their Coulomb attraction is presented. In the systems considered the phase of the order parameter is not fixed and therefore the charges superfluid flow connected with non-dissipative electrical currents is possible. The critical temperature Tc may be high (⪆ 100°K). The critical current has been found. The diamagnetic response and the coefficient of the electromagnetic wave absorption in the systems have been calculated. The possible experiments are discussed.