Landau effective Hamiltonian and its application to magnetic systems

The Landau definition of the effective Hamiltonian (of the nonequilibrium free energy) is realized in a microscopic theory. According to Landau remark, the consideration is based on classical statistical mechanics. In his approach nonequilibrium states coinciding with equilibrium fluctuations are taken into account (the Onsager principle). The definition leads to the exact fulfillment of the Boltzmann principle written in the form with the complete free energy. The considered system is assumed to consist of two subsystems. The first subsystem is an equilibrium one. The second subsystem is a nonequilibrium one and its state is described by quantities that are considered as order parameters. The effective Hamiltonian is calculated near equilibrium in the form of a series in powers of deviations of the order parameters from their equilibrium values. The coefficients of the series are expressed through equilibrium correlation functions of the order parameters. In the final approximation correlations of six and more order parameters are neglected and correlations of four parameters are assumed to be small that leads to the corresponding perturbation theory. The developed theory is compared with the phenomenological Landau theory of phase transitions of the second kind. The obtained results are concretized for paramagnetic-ferromagnetic system. The consideration is restricted by paramagnetic phase.

О неоднородных поляризованных состояниях вблизи точки фазового перехода в тонкой сегнетоэлектрической пленке

It is shown in terms of the phenomenological Landau theory of phase transitions that a phase transition to an inhomogeneous polar phase preceding in temperature a phase transition to a homogeneous polar state is possible. As a result of solving a boundary eigenvalue problem for the polarization equilibrium equation and electrostatics equations, wave vector k _⊥ characterizing the inhomogeneous phase has been determined and the temperature boundaries of its existence in the dependence on the film thickness and its surface properties have been found.

Spin-Lattice Coupling and Superconductivity in Fe Pnictides

We consider strong spin-lattice and spin-phonon coupling in iron pnictides and discuss its implications on superconductivity. Strong magneto-volume effect in iron compounds has long been known as the Invar effect. Fe pnictides also exhibit this effect, reflected in particular on the dependence of the magnetic moment on the atomic volume of Fe defined by the positions of the nearest neighbor atoms. Through the phenomenological Landau theory, developed on the basis of the calculations by the density functional theory (DFT) and the experimental results, we quantify the strength of the spin-lattice interaction as it relates to the Stoner criterion for the onset of magnetism. We suggest that the coupling between electrons and phonons through the spin channel may be sufficiently strong to be an important part of the superconductivity mechanism in Fe pnictides.

Method for Calculation of Dielectric Properties of Modulated Semiconducting Structures

A new method for calculation the effects of composition modulation upon the dielectric properties of semiconductors is proposed. Specific consideration is carried out for ultrathin [001] oriented (AIIICV)m(BIIICV)n superlattices (SL). These SL are formally considered to result from complete A(B) atoms orderiig over a-sublattice in a hypothetical initial structure –bulk mixed crystal (AxB1−x)C with zincblende lattice and x = m/(m + n). From this point of view a variation in dielectric constants tensor (DCT) induced by the ordering can be calculated nearly in a spirit of the phenomenological Landau theory of second-order phase transitions. It is shown that the changes in spatial dispersion of the DCT are expressed through the coefficients at invariants in the expansion of SL thermodynamic potential in a power series of the order parameter components. The way is described for numerical estimation of the thermodynamic potential coefficients from microscopic model of the initial alloy which take account of spatial correlations in A(B) atoms distribution.

Elastic Modulus of the (GaA.s)m(A.l A.s)n Superlattices

ABSTRACTThe ultrathin (GaAs)m (AlAs)n superlattices ( SL ) grown along [001] are formally considered as a result, of ordering-type phase transition from an initial hypothetical mixed crystal (Gax Al1-x)As with x = m/(m+n) . From this point of view changes of elastic modulus tensor (EMT) induced by ordering can he calculated nearly in a spirit of phenomenological Landau theory of second-order phase transitions. It .is shown that a change of EMT component. S66 is completely determined by symmetry properties of two translational (Dzyaloshinskji) invariants of m+n power .in the expansion of SL thermodynamic potential in a power series .in order parameter components. The changes of the other EMT components depend on the second power of order parameter .The expressions are found out to connect. EMT of a definite ordered phase (SL - family with m+n = 3) with that, of the initial one.