Yu.V. Kopaev

Toroidal order in crystals

Abstract A new ordered state characterized by non-zero toroidal moment density is described. A review of its electrical and magnetic properties is presented. A second order phase transition to the toroidal state is accompanied by Curie-Weiss behavior of a new type of response function, the toroidal susceptibility, which describes the response to an external current. The toroidal state can be either of spin or of orbital nature. Orbital toroidal magnetics can possess large diamagnetism.

Enhancement of superconducting critical temperature due to metal-semiconductor transition

Abstract It is pointed out that there exists a qualitative agreement between the occurrence of high-temperature superconductivity in oxide systems (perovskite and layered perovskite structures) and the authors earlier theoretical predictions of superconductivity in the model of metal-insulator transition (partly contained in the book: High-temperature superconductivity). Here a brief account of the published results is given and attention is paid to some properties of such superconductors.

On the kinetic theory of the Quantum Cascade Laser

The exact solution is found of the system of coupled kinetic equations for the distribution functions of electrons and optical phonons in Quantum Cascade Laser. It is shown that under definite conditions the processes of optical phonon accumulation and reabsorption take place. The threshold current can be substantially reduced in such a situation due to the increase of the upper subband electrons lifetime. The effect is proportional to the difference in effective masses in upper and lower subbands which results from the nonparabolicity of bare energy spectrum.

Magnetoelectric phenomena in nanoelectronics

Abstract Two-well quantum structures are considered with broken space inversion and time reversal symmetries. The structures exhibit a number of magnetoelectric phenomena which on microscopic level result from energy spectrum asymmetry in quasimomentum space and from the existence of spontaneous local currents.

On the theory of phase transitions in vanadium oxides VnO2n-1 (magneli phases)

Abstract The phenomenological theory of the phase transitions in Magnelis vanadium-oxide series is proposed. The phase diagram is derived with allowance for the pecularities of these compounds. The sequence of the phase transitions and their basic features (i.e. dependence of critical temperatures of the metal-dielectric and antiferromagnetic transitions upon the index of n in formula VnO2n-1) are explained.

The interplay between structural or antiferromagnetic phase transition and superconductivity

Abstract The properties of high T s superconductors are investigated in a model in which superconductivity is enhanced by the singularity in the density of states generated as a result of structural or antiferromagnetic phase transition. The absence of the isotope effect and the possibility of large scale variation of the ratio 2Δ s (0)/ T s are shown to take place in this model. A strong interaction version of the model is presented.

Single-gated mobility modulation transistor

We have investigated an n-type field effect transistor based on a GaAs/AlGaAs structure with a channel which consists of two tunnel-coupled quantum wells. Modulation of channel conductivity is achieved through a change in electron mobility under the effect of anticrossing of quantization levels in wells while the concentration of carriers remains practically constant. With proper choice of the structural layout and doping profile, the transistor can be effectively controlled by the front gate only. The possibility of obtaining the highest amplitude of conductivity modulation by electron mobility suppression in one of the quantum wells and by varying tunnel coupling between the wells is investigated by numerical simulations. It is shown that, with a gradual decrease of electron mobility in one of the quantum wells, the conductivity modulation amplitude first increases, comes to a maximum, and then decreases due to suppression of tunnel coupling between wells by scattering of longitudinal momentum. The drain current dependence on the gate voltage of the device under investigation shows two maxima; this feature allows the same device to be used as both an n- and p-type transistor.

Wannier excitons in asymmetrical quantum well structures

Abstract Binding energies of excitons in asymmetrical quantum well systems are calculated by a perturbative approach based on the effective-mass approximation in the envelope function scheme. We studied the effects of a composition, a longitudinal uniform electric field and geometrical parameters of Al x Ga 1−x As/GaAs quantum well structures on the exciton binding energies. It is shown that, in specially selected structures, certain excitions may be absent inside rather wide domains of a composition or an electric field.

Toroidal current ordering in crystals

Abstract A new type of magnetic ordering-toroidal current state (TCS) is discussed. The state is associated with ordering of toroidal dipoles. On a microscopic level TCS arises in the model of an excitonic insulator with an imaginary singlet order parameter. Anomalously large diamagnetic susceptibility can be expected in inhomogeneous phase of TCS.

Dimensionality transformation of the exciton state in quantum wells with asymmetric barriers

Under the conditions of 2-3D transformation of one-particle states, the dependence of exciton binding energy Eex on the well width h has been theoretically studied for quantum structures with barriers of asymmetric height. It is shown that the contribution of one-particle continuous spectrum states to the exciton becomes considerable when the inverse exciton Bohr radius 1/ab is comparable to the value of wavevector kc critical for 2-3D transformation. The lack of coincidence between electron and hole kc values leads to a complicated shape of the Eex(h) dependence with a minimum at intermediate values of h. The minimum value of Eex appears to be less than that of the 3D exciton binding energy. The behaviour of the exciton PLE peak position is investigated for an asymmetric structure under the influence of an external electric field.