Phase transition to the conducting state in a system of charge-transfer excitons at a donor-acceptor interface

We discuss the phase transition to the conducting state in a system of 2D charge-transfer excitons (CTEs) at a donor-acceptor interface. The phase transition arises due to strong dipole-dipole repulsion between CTEs which stimulates the population of free carriers in higher energy states even at low temperature. We use the computer simulations with the random distribution of excitons, with finite lifetime explicitly taken into account. The critical concentration of CTEs and their energy distribution are calculated. We also discuss the possibility of observing the predicted phenomena. Fiz. Tverd. Tela (St. Petersburg) 41, 781–784 (May 1999) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Molecular dynamics of the intramolecular 1, 3-dipolar ene reaction of a nitrile oxide and an alkene: non-statistical behavior of a reaction involving a diradical intermediate*

ABSTRACT

Potential energy surfaces and molecular dynamics of the intramolecular 1, 3-dipolar cycloaddition and ene reaction of a nitrile oxide with an alkene were performed in the gas phase and in dichloromethane with density functional theory. One hundred trajectories were propagated in the gas phase and in dichloromethane, respectively. Twenty percent of the trajectories in the gas phase involve bicyclic intermediate and the mean time gap is 472fs. A dynamically stepwise reaction is observed. In dichloromethane, more reactive trajectories were obtained and the time gap is larger than that in the gas phase.     

Structural phase transition in FeBO under pressure

Using the density functional theory the structural and magnetic properties of iron borate under high pressure have been studied. At about P = 22.7 GPa a first order phase transition to the phase described by the same space group Rc has been found. The phase transition is accompanied by a 9% volume change of the unit cell, a four times decrease of the magnetic moment on Fe, an increase of the charge density at Fe, and a disappearance of the energy gap in the electronic density of states. Received 21 September 2001 and Received in final form 6 January 2002 Published online 6 June 2002     

Coulomb effects in a ballistic one-channel S-S-S device

We develop a theory of Coulomb oscillations in superconducting devices in the limit of small charging energy E _C ≪Δ. We consider a small superconducting grain with finite capacitance connected to two superconducting leads by nearly ballistic single-channel quantum point contacts. The temperature is assumed to be very low, so there are no single-particle excitations on the grain. Then the behavior of the system can be described in terms of the quantum mechanics of the superconducting phase on the island. The Josephson energy as a function of this phase has two minima that become degenerate when the phase difference on the leads equals to π, the tunneling amplitude between them being controlled by the gate voltage on the grain. We find the Josephson current and its low-frequency fluctuations, and predict their periodic dependence with period 2e on the induced charge Q _x =CV _g . Zh. éksp. Teor. Fiz. 114, 640–653 (August 1998) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor     

Heat release accompanying current injection into a composite superconductor in a stable superconducting state

An investigation is made of the Joule dissipation of energy in the superconducting phase of a composite superconductor as the current in it increases. It is shown that at high rates of current injection the dissipative processes are characterized by an appreciable release of heat, which substantially influences the critical energies for the loss of superconductivity and the velocity of the normal zone. An analytical expression is given for calculating the power of the thermal losses during current injection. Zh. Tekh. Fiz. 69, 125–128 (April 1999)     

Magnetic phase diagram of perovskite Mn oxides at T = 0

Minimizing total free energy by numerical calculations, we obtain the magnetic phase diagram of perovskite Mn oxides, such as with , Ca, Sr, etc. in the whole doping region from x =0 to x =1 at temperature T =0. It is discovered that a spiral state is stable in a low concentration of X ions while a canted state is stable in a high concentration of X ions, and a ferromagnetic phase can exist in the intermediate concentrations when the antiferromagnetic interaction is weak. The energy difference between spiral and canted states is found to be small when the Hund coupling is large. Magnetic field induced spiral/canted phase transition is considered as a possible mechanism of the colossal magnetoresistance (CMR) in the Mn oxides. Received: 11 July 1996 / Revised: 7 December 1996 / Accepted: 24 July 1997     

Anisotropy of the energy gap in the insulating phase of the U-t-t' Hubbard model

We apply a diagrammatic expansion method around the atomic limit () for the U-t-t ' Hubbard model at half filling and finite temperature by means of a continued fraction representation of the one-particle Green's function. From the analysis of the spectral function we find an energy dispersion relation with a modulation of the energy gap in the insulating phase. This anisotropy is compared with experimental ARPES results on insulating cuprates. Received 18 May 2000 and Received in final form 9 August 2000     

Factory-roof-shaped phase front at the paraelectric-ferroelectric transition in KD PO : an incoherent interface

The phase front during the 218 K transition in KD₂PO₄ crystals under a thermal gradient perpendicular to the c ferroelectric axis is observed to have a factory-roof shape. This shape is studied versus the magnitude of G_e in samples cut with faces in (100), (010), (001) planes or in ( 0), (110), (001) ones. A geometric approach as well as the calculation of the elastic-strain energy caused by lattice misfits along the phase front demonstrate the incoherent interface nature of the phase front. Furthemore, the results and their interpretation allow to predict the sign of the lattice deformation u _xx ( > 0). Received 25 April 2002 Published online 29 November 2002     

Nucleation on a sphere: the roles of curvature,confinement and ensemble

ABSTRACT

By combining Monte Carlo simulations and analytical models, we demonstrate and explain how the gas-to-liquid phase transition of colloidal systems confined to a spherical surface depends on the curvature and size of the surface, and on the choice of thermodynamic ensemble. We find that the geometry of the surface affects the shape of the free energy profile and the size of the critical nucleus by altering the perimeter–area ratio of isotropic clusters. Confinement to a smaller spherical surface results in both a lower nucleation barrier and a smaller critical nucleus size. Furthermore, the liquid domain does not grow indefinitely on a sphere. Saturation of the liquid density in the grand canonical ensemble and the depletion of the gas phase in the canonical ensemble lead to a minimum in the free energy profile, with a sharp increase in free energy for additional growth beyond this minimum.     

Three-dimensional phase field microelasticity theory of a multivoid multicrack system in an elastically anisotropic body: Model and computer simulations

The phase field microelasticity theory of a three-dimensional, elastically anisotropic system of voids and cracks is proposed. The theory is based on the equation for the strain energy of the continuous elastically homogeneous body presented as a functional of the phase field, which is the effective stress-free strain. It is proved that the stress-free strain minimizing the strain energy of this homogeneous modulus body fully determines the elastic strain and displacement of the body with voids and/or cracks. The proposed phase field integral equation describing the elasticity of an arbitrary system of voids and cracks is exact. The geometry and evolution of multiple voids and/or cracks are described by the phase field, which is the solution of the time-dependent Ginzburg-Landau equation. Other defects, such as dislocations and precipitates, are trivially integrated into this theory. The proposed model does not impose a priori constraints on possible void and crack configurations or their evolution paths. Examples of computations of elastic equilibrium of systems with voids and/or cracks and the evolution of cracks under applied stress are considered.     

Helical instability of a straight Abrikosov vortex in an anisotropic superconductor

Because of attraction of the parallel currents forming an Abrikosov vortex, the vortex energy per unit length decreases, under bending of the vortex, by a quantity proportional to the square of the curvature. Solving the London equation in an approximation allowing for this effect makes it possible to calculate the energy of an Abrikosov vortex in the form of a helix whose length and pitch are much larger than the correlation length, whose curvature is small compared to the reciprocal London length, and whose slope in relation to an axis coinciding with the direction in which the vortex energy is the highest is also small. When the anisotropy is large, which is characteristic of high-T _c superconductors, the energy of such an Abrikosov vortex is lower than that of a straight Abrikosov vortex. Certain consequences of the fact that the Abrikosov vortices in a high-T _c superconductor are helical are discussed. Among these is a phase transition that breaks the symmetry between Abrikosov vortices shaped like right-and left-hand helixes in relation to the magnetic field. Zh. éksp. Teor. Fiz. 111, 1869–1878 (May 1997)     

Symmetry aspect of the phase transitions in boracites

The phase transitions in boracites are analysed by using the group-theoretical formulation of the Landau theory of phase transitions. It is shown that the orthorhombic, monoclinic and trigonal phase transitions could be induced by the same irreducible representation of the space groupT _d ⁵ with the star determined by the wave vectork=1/2(b ₁+b ₂). The corresponding free energy function is constructed and the symmetry of normal modes is discussed.The authors thank Dr. V. Janovec of the Institute of Physics for valuable remarks to this paper.     

Kinetics of nonsteady-state energy transfer in a medium with an amplitude-phase pattern of the nonlinear response

A solution to coupled wave equations, which describes the kinetics of nonsteady-state energy transfer between two light pulses in a bulk medium with a local nonlinear response, has been obtained in the preset pump-field approximation. It is shown that in a general case the energy transfer is an alternating function of the time delay between pulses, which is dominated by the relative magnitude of the amplitude and phase components of the nonlinear response. The solution obtained was fitted to the results of model experiments on Si. Agreement is reached with the fitting parameters determined from Drude theory for plasma-induced variation of dielectric permittivity. Fiz. Tverd. Tela (St. Petersburg) 39, 1980–1984 (November 1997)     

Collective excitations of a periodic Bose condensate in the Wannier representation

We study the dispersion relation of the excitations of a dilute Bose-Einstein condensate confined in a periodic optical potential and its Bloch oscillations in an accelerated frame. The problem is reduced to one-dimensionality through a renormalization of the s-wave scattering length and the solution of the Bogolubov-de Gennes equations is formulated in terms of the appropriate Wannier functions. Some exact properties of a periodic one-dimensional condensate are easily demonstrated: (i) the lowest band at positive energy refers to phase modulations of the condensate and has a linear dispersion relation near the Brillouin zone centre; (ii) the higher bands arise from the superposition of localized excitations with definite phase relationships; and (iii) the wavenumber-dependent current under a constant force in the semiclassical transport regime vanishes at the zone boundaries. Early results by Slater [Phys. Rev. 87, 807 (1952)] on a soluble problem in electron energy bands are used to specify the conditions under which the Wannier functions may be approximated by on-site tight-binding orbitals of harmonic-oscillator form. In this approximation the connections between the low-lying excitations in a lattice and those in a harmonic well are easily visualized. Analytic results are obtained in the tight-binding scheme and are illustrated with simple numerical calculations for the dispersion relation and semiclassical transport in the lowest energy band, at values of the system parameters which are relevant to experiment. Received 3 December 1999 and Received in final form 22 March 2000     

Coexistence of superconductivity and spin density wave orderings in the organic superconductor (TMTSF) 2 PF 6

The phase diagram of the organic superconductor (TMTSF)₂PF₆has been revisited using transport measurements with an improved control of the applied pressure. We have found a 0.8 kbar wide pressure domain below the critical point (9.43 kbar, 1.2 K) for the stabilisation of the superconducting ground state featuring a coexistence regime between spin density wave (SDW) and superconductivity (SC). The inhomogeneous character of the said pressure domain is supported by the analysis of the resistivity between T _SDW and T _SC and the superconducting critical current. The onset temperature T _SC is practically constant ( 1.20±0.01 K) in this region where only the SC/SDW domain proportion below T _SC is increasing under pressure. An homogeneous superconducting state is recovered above the critical pressure with T _SC falling at increasing pressure. We propose a model comparing the free energy of a phase exhibiting a segregation between SDW and SC domains and the free energy of homogeneous phases which explains fairly well our experimental findings. Received 3 September 2001 and Received in final form 9 November 2001     

Effect of the anticrossing of excitonic states on their phase relaxation time in a GaAs/AlGaAs symmetric double quantum well

The change in the properties of excitonic states near anticrossing is investigated experimentally. It is shown that the phase relaxation time of light excitons in a GaAs/AlGaAs symmetric double quantum well increases by a factor of five as a result of mixing of the energy states of light and heavy excitons. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 6, 426–430 (25 March 1999)     

Statistical Physics on the Space (x, v) for Dissipative Systems and Study of an Ensemble of Harmonic Oscillators in a Weak Linear Dissipative Medium

We use the phase space position-velocity (x, v) to deal with the statistical properties of velocity dependent dynamical systems, like dissipative ones. Within this approach, we study the statistical properties of an ensemble of harmonic oscillators in a linear weak dissipative media. Using the Debye model of a crystal, we calculate at first order in the dissipative parameter the entropy, free energy, internal energy, equation of state and specific heat using the classical and quantum approaches. For the classical approach we found that the entropy, the equation of state, and the free energy depend on the dissipative parameter, but the internal energy and specific heat do not depend of it. For the quantum case, we found that all the thermodynamical quantities depend on this parameter. PACS: 05.20.Gg, 05.30.Ch, 05.20.-y, 05.30.-d     

Lattice dynamics of a Rb2KScF6 crystal in unstable cubic and tetragonal phases and in a stable monoclinic phase

The results of a nonempirical calculation of the static and dynamic properties of a Rb₂KScF₆ crystal with elpasolite structure in cubic, tetragonal, and monoclinic phases are presented. The calculation is performed on the basis of a microscopic model of an ionic crystal that takes account of the deformability and polarizability of the ions. The deformability parameters of the ions are determined from the condition that the total energy of the crystal is minimum. The computational results for the equilibrium lattice parameters are in satisfactory agreement with experimental data. Unstable vibrational modes are found in the vibrational spectrum of the lattice in the cubic and tetragonal phases. These modes occupy the phase space throughout the entire Brillouin zone. The characteristic vectors of the most unstable mode at the center of the Brillouin zone of the cubic phase are related to the displacements of the fluorine ions and correspond to rotation of ScF₆ octahedra. Condensation of this mode leads to a tetragonal distortion of the structure. In the tetragonal phase the most unstable mode belongs to the boundary point of the Brillouin zone and condensation of this mode leads to monoclinic distortion with doubling of the unit-cell volume. In the monoclinic phase unstable modes are absent in the vibrational spectrum of the lattice. Fiz. Tverd. Tela (St. Petersburg) 41, 1297–1305 (July 1999)     

Marginal Distributions of Wigner Function in a Mesoscopic L-C Circuit at Finite Temperature and Thermal Wigner Operator

Wigner function in phase space has its physical meaning as marginal probability distribution in coordinate space and momentum space respectively, here we endow the Wigner function with a new physical meaning, i.e., its marginal distributions’ statistical average for q ²/(2C) and p ²/(2L) are the energy stored in capacity and in inductance of a mesoscopic L-C circuit at finite temperature, respectively. PACS numbers: 03.65.-w, 73.21.-b     

Phase-transition kinetics with the formation of topological defects in superconductors with a multicomponent order parameter

This paper discusses the kinetics of phase transitions to superconductivity with a multicomponent order parameter in zero external field. It is shown that as it approaches equilibrium the superconductor passes through an intermediate vortex-like state containing domain walls, single-quantum, and multiquantum axially nonsymmetric vortices and antivortices. The energy and other parameters of the domain walls are derived. Rigid superconducting bubbles are discussed and criteria are established for their local stability. Zh. éksp. Teor. Fiz. 112, 1351–1373 (October 1997)