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.

Additional lines in quantum wells excitonic spectra connected with QW asymmetry caused by technology

The calculations of intensity ratio of both the main and additional lines, the energy differences between which are fulfilled for quantum well (QW) with asymmetrical potential profile, are presented here. It is grounded on the basis of this calculation that additional line in exciton spectrum of QW can be explained by transitions between the confined states of valence and conductivity electrons with different parity, which ceases to be forbidden in the presence of asymmetry of QW potential profile caused by technology of growth. It is shown that e1-hh2 additional exciton line is more intensive in most of the actual cases. In particular, it is shown that the additional exciton line, which was observed in the laser ablation grown structures with QW, may be explained as e1-hh2 transition. The calculations show the substantial sensitivity of the results not only to the parameter of widening of the interface, but to the detailed type of the interface profile function. It is concluded that the laser ablation method of heterostructure growth leads to a larger asymmetry of QW potential profile caused by technology than MBE potential profile.Проведенi розрахунки вiдношення iнтенсивностей основної i додаткової лiнiй, енергетичної вiдстанi мiж ними для квантових ям (КЯ) з асиметричним потенцiальним профiлем. Обгрунтовано, що додаткова лiнiя у екситонному спектрi КЯ може бути пояснена переходами мiж утримуваними в КЯ станами валентних електронiв i електронiв провiдностi з рiзною парнiстю, якi перестають бути забороненими у присутностi асиметрiї потенцiального профiлю, спричиненої технологiєю вирощування КЯ. Показано, що додаткова екситонна лiнiя типу e1-hh2 є бiльш iнтенсивною в бiльшiй частинi актуальних випадкiв. Зокрема, показано, що додаткова екситонна лiнiя в структурах вирощених методом лазерної абляцiї, може бути пояснена як e1-hh2 перехiд. Розрахунки показують суттєву чутливiсть не лише до параметру розширення iнтерфейсу, але й до функцiї його профiлю. Зроблено висновок, що метод лазерної абляцiї призводить до бiльшої асиметрiї КЯ, нiж метод молекулярно-пучкової епiтаксiї

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

Monte Carlo simulations of an impurity band model for III-V diluted magnetic semiconductors

We report the results of a Monte Carlo study of a model of (III,Mn)V diluted magnetic semiconductors which uses an impurity band description of carriers coupled to localized Mn spins and is applicable for carrier densities below and around the metal-insulator transition. In agreement with mean field studies, we find a transition to a ferromagnetic phase at low temperatures. We compare our results for the magnetic properties with the mean field approximation, as well as with experiments, and find favorable qualitative agreement with the latter. The local Mn magnetization below the Curie temperature is found to be spatially inhomogeneous, and strongly correlated with the local carrier charge density at the Mn sites. The model contains fermions and classical spins and hence we introduce a perturbative Monte Carlo scheme to increase the speed of our simulations.Comment: 17 pages, 24 figures, 2 table

On the theory of carrier-induced ferromagnetism in diluted magnetic semiconductors

Two different approaches (presented in the literature as alternative approximations) to the problem of carrier-induced ferromagnetism in the system of disordered magnetic ions of a diluted magnetic semiconductor are analyzed. They are based on a self-consistent procedure for the mean exchange fields and the RKKY interaction. Calculations in the framework of exactly solvable model are carried out, and it shows that these approaches stem from two different contributions to the magnetic susceptibility. One stems from the diagonal part of the carrier-ion exchange interaction and corresponds to mean field approximation. The other one stems from the off-diagonal part of the same interaction and describes the indirect interaction between localized spins via free carriers. These two contributions can be responsible for the different magnetic properties. Thus, the aforementioned contributions are complementary but not alternative to each other. A general approach is proposed and compared with different approximations to the problem under consideration

Influence of Magnetic Field on Blue Anti-Stokes Luminescence of ZnSe:Cr Crystals

The results of photoluminescence and magneto-luminescence studies of chromium doped ZnSe crystals are presented for blue colour shallow donor-shallow acceptor pair photoluminescence, with a zero phonon line at 2.692 eV. This donor-acceptor pair photoluminescence is observed under photo-excitation with photon energies smaller than the emission energy (anti-Stokes luminescence) and dominates in the photoluminescence spectrum of ZnSe:Cr at the excitation energy about 2.41 eV and for temperature T<20 K. A quantum efficiency of this anti-Stokes luminescence is relatively large. It is of about 10$\text{}^{-3}$ at 0 T and increases with increasing magnetic field up to 7 T at temperature T=2 K. We relate the intensity of the anti-Stokes luminescence to a non-equilibrium concentration of photo-excited Cr$\text{}^{+}$ ions and propose that the rate of spin-dependent recombination of Cr$\text{}^{+}$ ions with free holes (decreases with increasing magnetic field) is responsible for the observed increase in the anti-Stokes luminescence intensity at higher magnetic fields