V. Fleurov

Double-exchange mechanisms for Mn-doped III-V ferromagnetic semiconductors

A microscopic model of indirect exchange interaction between transition metal impurities in dilute magnetic semiconductors (DMS) is proposed. The hybridization of the impurity d-electrons with the heavy hole band states is mainly responsible for the exchange of electrons between the impurities, whereas the Hund rule for the electron occupation of the impurity d-shells makes it spin selective. The model is applied to such systems as n-type (Ga,Mn)N and p-type (Ga,Mn)As, p-type (Ga,Mn)P. In n-type DMS with Mn 2+/3+ impurities the exchange mechanism is rather close to the kinematic exchange proposed by Zener for mixed-valence Mn ions. In p-type DMS ferromagnetism is governed by the kinematic mechanism involving the kinetic energy gain of the heavy hole carriers caused by their hybridization with 3d electrons of Mn 2+ impurities. Using the molecular field approximation, the Curie temperatures TC are calculated for several systems as functions of the impurity and hole concentrations. Comparison with the available experimental data shows a good agreement.

Properties of the intermediately bound and -excitons in ZnO:Cu

A microscopical model is proposed, describing the origin and properties of three closely spaced zero-phonon lines observed in the green Cu band in ZnO:Cu crystals labelled and . These excitations are known to be formed by a charge-transfer reaction with hole bound states. These lines are shown to originate from an intermediately bound exciton of acceptor type, . This sort of exciton, in which both carriers are captured at intermediate-radius orbitals, results from the wurzite-type symmetry of the ZnO:Cu system. The electronic structure obtained for these three intermediately bound excitons enables us to explain their magneto-optic behaviour and to calculate their g-values. Additionally, we determined the quantum efficiency of both intracentre and exciton transitions by using time-resolved and calorimetric absorption spectroscopy. While no luminescence is observed in ZnS, the exciton states in ZnO are purely radiative only to the ground state, . The picture of an intermediately bound exciton explains the recombination channels and also makes clear the difference between copper states in the ZnS and ZnO systems.

Superexchange in dilute magnetic dielectrics : Application to (Ti,Co)O2

We extend the model of ferromagnetic superexchange in dilute magnetic semiconductors to the ferromagnetically ordered highly insulating compounds (dilute magnetic dielectrics). The intrinsic ferromagnetism without free carriers is observed in oxygen-deficient films of anatase

Fano resonances with discrete breathers

{\mathrm{TiO}}_{2}

Discrete breathers in classical spin lattices.

doped with transition-metal impurities in cation sublattice. We suppose that ferromagnetic order arises due to superexchange between complexes (oxygen vacancies

One-dimensional stochastic Lévy-Lorentz gas

+

An All-Optical Event Horizon in an Optical Analogue of a Laval Nozzle

magnetic impurities), which are stabilized by charge transfer from vacancies to impurities. The Hund rule controls the superexchange via empty vacancy related levels so that it becomes possible only for the parallel orientation of impurity magnetic moments. The percolation threshold for magnetic ordering is determined by the radius of vacancy levels, but the exchange mechanism does not require free carriers. The crucial role of the nonstoichiometry in formation of the ferromagnetism makes the Curie temperatures extremely sensitive to the methods of sample preparation.

Dynamics of fluctuations in an optical analogue of the Laval nozzle

A theoretical study of linear wave scattering by time-periodic spatially localized excitations (discrete breathers) is presented. A peculiar effect of total reflection occurs due to a Fano resonance when a localized state originating from closed channels resonates with the open channel. For the discrete nonlinear Schrödinger chain, we give an analytical result for the frequency dependence of the transmission coefficient, including the possibility of resonant reflection. We extend the analysis to chains of weakly coupled anharmonic oscillators and discuss the relevance of the effect for electronic transport spectroscopy of mesoscopic systems.

Tunneling of localized excitations: Giant enhancement due to fluctuations.

Discrete breathers ~nonlinear localized modes! have been shown to exist in various nonlinear Hamiltonian lattice systems. In the present paper, we study the dynamics of classical spins interacting via the Heisenberg exchange on spatial d-dimensional lattices ~with and without presence of single-ion anisotropy!. We show that discrete breathers exist for the cases when the continuum theory does not allow for their presence ~easy-axis ferromagnets with anisotropic exchange and easy-plane ferromagnets!. We prove the existence of localized excitations, using the implicit function theorem, and obtain necessary conditions for this existence. The most interesting case is the easy-plane one, which yields excitations with locally tilted magnetization. There is no continuum analog for such a solution and there exists an energy threshold for it, which is estimated analytically. We support our analytical results with numerical high-precision computations, including also a stability analysis for the excitations.

Classical and quantum radiation of perturbed discrete breathers

We introduce a Levy-Lorentz gas in which a light particle is scattered by static point scatterers arranged on a line. We investigate the case where the intervals between scatterers xi(i) are independent random variables identically distributed according to the probability density function &mgr;(xi) approximately xi(-(1+gamma)). We show that under certain conditions the mean square displacement of the particle obeys <x(2)(t)>>/=Ct3-gamma for 1<gamma<2. This behavior is compatible with a renewal Levy walk scheme. We discuss the importance of rare events in the proper characterization of the diffusion process.