Influence of free carriers on exciton ground states in quantum wells

The influence of free carriers on the ground state of the exciton at zero magnetic field in a quasi-two-dimensional quantum well doped with electrons is considered in the framework of the random phase approximation. The effects of the exciton-charge-density interaction and the inelastic scattering processes due to the Hartree-Fock electron-electron exchange interaction are taken into account. The effect of phase-space filling is considered using an approximate approach. The results of the calculation are compared with the experimental data available.Comment: 8 pages, 5 fugure

Diffusivities and kinetics of short-range and long-range orderings in Ni-Fe permalloys

The microscopic model of atomic diffusion is considered to describe the short-range order relaxation kinetics within the f.c.c.-Ni-Fe Permalloys. The model takes into account both the discrete and anisotropic characters of atomic jumps within the long-range field of concentration heterogeneities of the interacting atoms. The diffusion coefficients and activation energies for the disordered Ni-Fe permalloy are estimated with the evaluated probabilities of atomic jumps. As shown, the increasing of a temperature with a fixed composition influences on the 'potential' field of interatomic interaction ambiguously: the field 'potential' increases for defined coordination shells and decreases for some of other ones. Although the temperature increasing promotes the increasing of any atomic-probabilities jumps generally, but decreasing of the action of 'potential' field generated by the atoms of defined element and caused by its concentration heterogeneities onto the distant sites results in increasing of the atomic-jumps' probabilities of just this element, first of all, into the sites, which are more distant from the 'source' of heterogeneity. Within the framework of the static concentration waves' method along with the self-consistent field approximation, the Onsager-type kinetics equation is obtained to describe the long-range order relaxation by the L12-type superstructure. To calculate diffusivities for the ordered Ni3Fe permalloy, the independent, diffraction experimental data of the long-range order parameter relaxation are used. Theoretical curves of the long-range order time evolution for the non-stoichiometric f.c.c.-Ni-Fe permalloys are plotted. Decreasing of the concentration of alloying element results in decelerating of the long-range order parameter change and in increasing of its relaxation time

Femtosecond study of the interplay between excitons, trions, and carriers in (Cd,Mn)Te quantum wells

We present an absorption study of the neutral and positively charged exciton (trion) under the influence of a femtosecond, circularly polarized, resonant pump pulse. Three populations are involved: free holes, excitons, and trions, all exhibiting transient spin polarization. In particular, a polarization of the hole gas is created by the formation of trions. The evolution of these populations is studied, including the spin flip and trion formation processes. The contributions of several mechanisms to intensity changes are evaluated, including phase space filling and spin-dependent screening. We propose a new explanation of the oscillator strength stealing phenomena observed in p-doped quantum wells, based on the screening of neutral excitons by charge carriers. We have also found that binding heavy holes into charged excitons excludes them from the interaction with the rest of the system, so that oscillator strength stealing is partially blockedComment: 4 pages, 4 figure

p-Type doping of II-VI heterostructures from surface states: application to ferromagnetic Cd1−x_{1-x}Mnx_xTe quantum wells

We present a study of p-type doping of CdTe and Cd$_{1-x}$Mn$_x$Te quantum wells from surface states. We show that this method is as efficient as usual modulation doping with nitrogen acceptors, and leads to hole densities exceeding $2 \times 10^{11}$ cm$^{-2}$. Surface doping was successfully applied to obtain carrier-induced ferromagnetism in a Cd$_{1-x}$Mn$_x$Te quantum well. The observed temperature dependence of photoluminescence spectra, and the critical temperature, correspond well to those previously reported for ferromagnetic quantum wells doped with nitrogen.Comment: 4 figure

Microphotoluminescence study of disorder in ferromagnetic (Cd,Mn)Te quantum well

Microphotoluminescence mapping experiments were performed on a modulation doped (Cd,Mn)Te quantum well exhibiting carrier induced ferromagnetism. The zero field splitting that reveals the presence of a spontaneous magnetization in the low-temperature phase, is measured locally; its fluctuations are compared to those of the spin content and of the carrier density, also measured spectroscopically in the same run. We show that the fluctuations of the carrier density are the main mechanism responsible for the fluctuations of the spontaneous magnetization in the ferromagnetic phase, while those of the Mn spin density have no detectable effect at this scale of observation.Comment: 4 pages, 3 figure

Carrier-induced ferromagnetism in p-Zn1-xMnxTe

We present a systematic study of the ferromagnetic transition induced by the holes in nitrogen doped Zn1-xMnxTe epitaxial layers, with particular emphasis on the values of the Curie-Weiss temperature as a function of the carrier and spin concentrations. The data are obtained from thorough analyses of the results of magnetization, magnetoresistance and spin-dependent Hall effect measurements. The experimental findings compare favorably, without adjustable parameters, with the prediction of the Rudermann-Kittel-Kasuya-Yosida (RKKY) model or its continuous-medium limit, that is, the Zener model, provided that the presence of the competing antiferromagnetic spin-spin superexchange interaction is taken into account, and the complex structure of the valence band is properly incorporated into the calculation of the spin susceptibility of the hole liquid. In general terms, the findings demonstrate how the interplay between the ferromagnetic RKKY interaction, carrier localization, and intrinsic antiferromagnetic superexchange affects the ordering temperature and the saturation value of magnetization in magnetically and electrostatically disordered systems.Comment: 14 pages, 10 figure

Subnanosecond spectral diffusion of a single quantum dot in a nanowire

We have studied spectral diffusion of the photoluminescence of a single CdSe quantum dot inserted in a ZnSe nanowire. We have measured the characteristic diffusion time as a function of pumping power and temperature using a recently developed technique [G. Sallen et al, Nature Photon. \textbf{4}, 696 (2010)] that offers subnanosecond resolution. These data are consistent with a model where only a \emph{single} carrier wanders around in traps located in the vicinity of the quantum dot