Bipolarons in anisotropic crystals and low dimensional structures

We consider a simple bipolaron approach to description of anisotropic crystals in the strongcoupling limit. We have taken into account anisotropy of effective band masses and dielectric constants of crystals. The calculations of energy for two - electron system was fulfiled using wave functions chosen as a linear combination of gaussian orbitals, what enables us to easily estimate the electron correlation effects which can play a key role. Bipolaron formation should be quite possible in crystals with hightemperature superconductivity. We have considered an example of La₂CuO₄,, in which there is a great anisotropy of effective masses and dielectric permittivities, where in the limiting case of the maximum anisotropy of effective masses mxy << mz the bipolaron binding energy is 25 % of the polaron energy

The enhancement of ferromagnetism in uniaxially stressed diluted magnetic semiconductors

We predict a new mechanism of enhancement of ferromagnetic phase transition temperature $T_c$ in uniaxially stressed diluted magnetic semiconductors (DMS) of p-type. Our prediction is based on comparative studies of both Heisenberg (inherent to undistorted DMS with cubic lattice) and Ising (which can be applied to strongly enough stressed DMS) models in a random field approximation permitting to take into account the spatial inhomogeneity of spin-spin interaction. Our calculations of phase diagrams show that area of parameters for existence of DMS-ferromagnetism in Ising model is much larger than that in Heisenberg model.Comment: Accepted for publication in Phys. Rev.

Suppression of carrier induced ferromagnetism by composition and spin fluctuations in diluted magnetic semiconductors

We suggest an approach to account for spatial (composition) and thermal fluctuations in "disordered" magnetic models (e.g. Heisenberg, Ising) with given spatial dependence of magnetic spin-spin interaction. Our approach is based on introduction of fluctuating molecular field (rather than mean field) acting between the spins. The distribution function of the above field is derived self-consistently. In general case this function is not Gaussian, latter asymptotics occurs only at sufficiently large spins (magnetic ions) concentrations $n_i$. Our approach permits to derive the equation for a critical temperature $T_c$ of ferromagnetic phase transition with respect to the above fluctuations. We apply our theory to the analysis of influence of composition fluctuations on $T_c$ in diluted magnetic semiconductors (DMS) with RKKY indirect spin-spin interaction.Comment: 6 pages, 2 figure