arXiv subjects

Yu. E. Lozovik

Publications and source records attributed to Yu. E. Lozovik.

118 records · Page 7Linked to original sources

arXiv1999

Phase transitions of electron-hole and unbalanced electron systems in coupled quantum wells in high magnetic field

Superfluidity of spatially separated electrons and holes and unbalanced two-layer electron system in high magnetic field is considered. The temperature of the Kosterlitz-Thouless transition to a superfluid state is obtained as a function of magnetic field and interlayer separation. The equation of state for magnetoexciton system in quasi-classical regime is analyzed. The transition from excitonic phase to electron-hole phase is considered. Possible experimental manifestations of the predicted effects are briefly discussed.

cond-mat↗

arXiv1998

Superfluidity of indirect excitons in a quantum dot

The superfluidity and Bose-Einstein condensation of indirect excitons in two-dimensional quantum dot are studied by path-integral Monte Carlo simulations. The temperature dependence of superfluid and bose-condensed fraction are calculated at different strengths of interaction. Using the Kosterlitz-Thouless recursion relations, we also predict behavior of superfluid fraction in macroscopic large systems.

cond-mat.mes-hall↗

arXiv1998

Phase diagram of 2D array of mesoscopic granules

A lattice boson model is used to study ordering phenomena in regular 2D array of superconductive mesoscopic granules, Josephson junctions or pores filled with a superfluid helium. Phase diagram of the system, when quantum fluctuations of both the phase and local superfluid density are essential, is analyzed both analytically and by quantum Monte Carlo technique. For the system of strongly interacting bosons it is found that as the boson density $n_0$ is increased the boundary of ordered superconducting state shifts to {\it lower temperatures} and at $n_0 > 8$ approaches its limiting position corresponding to negligible relative fluctuations of moduli of the order parameter (as in an array of "macroscopic" granules). In the region of weak quantum fluctuations of phases mesoscopic phenomena manifest themselves up to $n_0 \sim 10$. The mean field theory and functional integral $1/n_0$ - expansion results are shown to agree with that of quantum Monte Carlo calculations of the boson Hubbard model and its quasiclassical limit, the quantum XY model.

cond-mat.mes-hall↗

arXiv1998

Josephson array of mesoscopic objects. Modulation of system properties through the chemical potential

The phase diagram of a two-dimensional Josephson array of mesoscopic objects is examined. Quantum fluctuations in both the modulus and phase of the superconducting order parameter are taken into account within a lattice boson Hubbard model. Modulating the average occupation number $n_0$ of the sites in the system leads to changes in the state of the array, and the character of these changes depends significantly on the region of the phase diagram being examined. In the region where there are large quantum fluctuations in the phase of the superconducting order parameter, variation of the chemical potential causes oscillations with alternating superconducting (superfluid) and normal states of the array. On the other hand, in the region where the bosons interact weakly, the properties of the system depend monotonically on $n_0$. Lowering the temperature and increasing the particle interaction force lead to a reduction in the width of the region of variation in $n_0$ within which the system properties depend weakly on the average occupation number. The phase diagram of the array is obtained by mapping this quantum system onto a classical two-dimensional XY model with a renormalized Josephson coupling constant and is consistent with our quantum Path-Integral Monte Carlo calculations.

cond-mat.mes-hall↗

arXiv1998

Mesoscopic and microscopic dipole clusters: Structure and phase transitions

Two dimensional (2D) classical system of dipole particles confined by a quadratic potential is studied. For clusters of N < 81 particles ground state configurations and appropriate eigenfrequencies and eigenvectors for the normal modes are found. Monte Carlo and molecular dynamic methods are used to study in detail the order - disorder transition (the "melting" of clusters). In mesoscopic clusters (N < 37) there is a hierarchy of transitions: at lower temperatures an intershell orientational disordering of pairs of shells takes place; at higher temperatures the intershell diffusion sets in and the shell structure disappears. In "macroscopic" clusters (N > 37) an orientational "melting" of only the outer shell is possible. The most stable clusters (having both maximal lowest nonzero eigenfrequencies and maximal temperatures of total melting) are that of completed crystal shells which are concentric groups of nodes of 2D hexagonal lattice with a number of nodes placed in the center of them. The study of different quantities shows that the melting temperature is a nonmonotonic function of the number of particles in the system. The dynamical equilibrium between "solidlike" and "orientationally disordered" forms of clusters is considered.

cond-mat.mes-hall↗

arXiv1998

Theory, Simulation and Nanotechnological Applications of Adsorption on a Surface with Defects

Theory of adsorption on a surface with nanolocal defects is proposed. Two efficacy parameters of surface modification for nanotechnological purposes are introduced, where the modification is a creation of nanolocal artificial defects. The first parameter corresponds to applications where it is necessary to increase the concentration of certain particles on the modified surface. And the second one corresponds to the pattern transfer with the help of particle self-organization on the modified surface. The analytical expressions for both parameters are derived with the help of the thermodynamic and the kinetic approaches for two cases: jump diffusion and free motion of adsorbed particles over the surface. The possibility of selective adsorption of molecules is shown with the help of simulation of the adsorption of acetylene and benzene molecules in the pits on the graphite surface. The process of particle adsorption from the surface into the pit is theoretically studied by molecular dynamic technique. Some possible nanotechnological applications of adsorption on the surface with artificial defects are considered: fabrication of sensors for trace molecule detection, separation of isomers, and pattern transfer.

physics.chem-ph↗

arXiv1998

Transport of magnetoexcitons in single and coupled quantum wells

The transport relaxation time $τ(P)$ 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 $τ(P)$ 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 $λ(P)$ 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 $λ(P)$ increases with $H$ for types (i,iii) of scattering processes and decreases in the case (ii).

cond-mat.mes-hall↗

arXiv1998

New model for system of mesoscopic Josephson contacts

Quantum fluctuations of the phases of the order parameter in 2D arrays of mesoscopic Josephson junctions and their effect on the destruction of superconductivity in the system are investigated by means of a quantum-cosine model that is free of the incorrect application of the phase operator. The proposed model employs trigonometric phase operators and makes it possible to study arrays of small superconducting granules, pores filled with superfluid helium, or Josephson junctions in which the average number of particles $n_0$ (effective bosons, He atoms, and so on) is small, and the standard approach employing the phase operator and the particle number operator as conjugate ones is inapplicable. There is a large difference in the phase diagrams between arrays of macroscopic and mesoscopic objects for $n_0 < 5$ and $U<J$ ($U$ is the characteristic interaction energy of the particle per granule and $J$ is the Josephson coupling constant). Reentrant superconductivity phenomena are discussed.

cond-mat.mes-hall↗

arXiv1998

Manifestation of exciton Bose condensation in induced two-phonon emission and Raman scattering

The unusual two-photon emission by Bose-condensed excitons caused by simultaneous recombination of two excitons with opposite momenta leaving the occupation numbers of excitonic states with momenta $p nonequal to 0$ unchanged (below coherent two-exciton recombination) is investigated. Raman scattering accompanied by the analogous two-exciton recombination (or creation) is also analyzed. The excess momentum equal to the change of the electromagnetic field momentum in these processes can be transferred to phonons or impurities. The processes under consideration take place if there is Bose condensation in the interacting exciton system, and, therefore, can be used as a new method to reveal exciton Bose condensation. If the frequency of the incident light $ω< 2Ω$ ($Ω$ is the frequency corresponding to the recombination of an exciton with p=0), the coherent two-exciton recombination with the excess momentum elastically transferred to impurities leads to the appearance of the spectral line $2Ω-ω$ corresponding to the induced two-photon emission. In this case the anti-Stokes line on frequency $ω+2Ω$ also appears in the Raman spectrum. If $ω>2Ω$, there are both Stokes and anti-Stokes lines on frequencies $ω\pm2Ω$ in the Raman spectrum. The induced two-photon emission is impossible in this case. The spectral lines mentioned above have phonon replicas on frequencies $|ω\pm (2Ω-nω^s_0)|$ corresponding to the transmission of the excess momentum (partially or as a whole) to optical phonons of frequency $ω^s_0$ ($n$ is an integer number).

cond-mat.stat-mech↗

arXiv1998

Topological phase transition in 2D quantum Josephson array

Path Integral Quantum Monte Carlo simulation is used to study thermodynamic properties and a phase diagram of 2D quantum Josephson array, described by 2+1 XY model. The helicity and vorticity moduli, correlation function of phases and other characteristics of the system as functions of quantum parameter $q$ and temperature $T$ are studied ($q=2e/\sqrt{JC_0}$, $J$ is the Josephson coupling constant, $C_0$ is the intragrain capacitance). Quantum fluctuation induced superconductor - normal phase transition is studied in detail through the use of behavior of above-mentioned quantities. No discontinuous or reentrant phase transition in $q-T$ plane is found. Analysis of the vorticity and the renormalized coupling constant leads to the conclusion that the whole line of phase transition is of Kosterlitz - Thouless type.

cond-mat.supr-con↗