Yu. G. Semenov

Effect of an external magnetic field on electron-spin dephasing induced by hyperfine interaction in quantum dots

We investigate the influence of an external magnetic field on spin-phase relaxation of single electrons in semiconductor quantum dots induced by the hyperfine interaction. The basic decay mechanism is attributed to the dispersion of local effective nuclear fields over the ensemble of quantum dots. The characteristics of electron-spin dephasing are analyzed by taking an average over the nuclear-spin distribution. We find that the dephasing rate can be estimated as a spin-precession frequency caused primarily by the mean value of the local nuclear magnetic field. Furthermore, it is shown that the hyperfine interaction does not fully depolarize electron spin. The loss of initial spin polarization during the dephasing process depends strongly on the external magnetic field, leading to the possibility of effective suppression of this mechanism.

Effective scattering cross section of an assembly of small ellipsoidal particles

A model of light scattering from an assembly of small shape-distributed ellipsoidal particles is considered. The three principal assumptions used are the neglect of multiple scattering, the dipole polarizability, and the equiprobable distribution for the ellipsoid depolarization factors. These assumptions enabled us to find analytically the effective cross section for light scattering. {That for light absorption was found in a similar way by Goncharenko [J. Opt. Soc. Am. B13, 2392 (1996)]}. The solution obtained is analyzed for some special cases, in particular for low and no absorption.

Combination of Hartree and Ritz approaches for problem of excitons in semiconductor quantum wells. Additional exciton states

Abstract A combination of Hartree and Ritz approaches is applied to the problem of exciton confined in a semiconductor quantum well (QW). The multiplicative representation of exciton wave function is shown to describe satisfactorily the exciton states in the QWs of arbitrary width if electron and hole confinement components are found from the Hartree-like self-consistent equations. The method is illustrated by calculations of ground exciton states in single-band approximation. The validity of other approximations widely applied in the present literature is also discussed. The maximal error of exciton binding energy determination in the model is obtained. The method is applied for the analysis of additional exciton states that are absent without electron–hole Coulomb interaction. Asymmetry of the QW potential can be responsible for the manifestation of optical transitions associated with these additional states with quantum numbers of different parities. The relatively simple procedure developed here can be applied to systems with weak or even absent carrier localization by quantum-well potential (e.g. excitons in type II heterostructures).

Optical polarization anisotropy of quantum wells induced by a cubic anisotropy of the host material

The optical polarization anisotropy of quantum wells (QW) in structures fabricated from materials with cubic symmetry due to the anisotropic character of the Luttinger Hamiltonian for the valence band is studied. The matrix elements of interband optical transitions are shown to be different for different orientations of the linear polarization of light, oriented in the plane of a structure, provided that the structure growth axis does not coincide with any of the main crystallographic directions, i.e., with either [100],[010],[001] or [111]. The degree of this in-plane anisotropy of the polarization is calculated theoretically within an appropriate model for arbitrary structure growth orientation. The polarization anisotropy is experimentally detected for the fundamental 1hh–1e transition by studying the reflectivity from [120]-oriented Cd0.8Mn0.2Te/CdTe/Cd0.8Mn0.2Te QWs. At the same time the effect is shown to be absent in the case of QW grown along [100] direction. The measured polarization anisotropy in the former case shows a predicted π-periodicity as the polarization direction is rotated within the plane of the QW. The magnitude of the effect is also in qualitative agreement with the model. Quantitatively, the measured values tend to be some what larger than those predicted by the model. Surprisingly, the polarization anisotropy of the reflection by the barrier excitons is also detected. For the [120]-oriented structures this anisotropy has, approximately, the same magnitude as the one of the exciton transitions in the QWs. In the structure grown along [100], the anisotropy of the reflectivity of the barrier exciton transitions is also observed, although it is much smaller in magnitude. Some hypothetical explanations of these observations are put forward.

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 an exactly solvable model are carried out, and it is shown 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 the 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 give rise to different magnetic properties. Thus the aforementioned contributions are complementary and not alternative to each other. A general approach is proposed and compared with different approximations to the problem under consideration.

Anomalous Mn Spin Resonance Detected by Time-Resolved Kerr Effect in CdMnTe Quantum Wells

We report on a new spin resonance with a g-factor close to 1.5 observed in n- and p-doped CdMnTe quantum wells with nonmagnetic barriers. The resonance was detected by Fourier transformation of the time-resolved Kerr and Faraday effects measured with 100 fs pulses, in a pump-probe configuration, as a function of magnetic field, temperature, excitation density, and magnetic field orientation in the case of the p-doped sample. The properties of this spin resonance are discussed and possible interpretations are proposed.

Interface local spin states in a CdTe/(Cd, Mn)Te quantum well

Magneto-photoluminescence spectra obtained experimentally for a 1.9 nm thick CdTe quantum well with semi-magnetic barriers of Cd1−xMnx Te( x =0 :24) in the range of a magnetic 6eld B = 0–50 Tand applied pressure up to 1 :5 GPa have been analyzed through the interface � -potential model. In the model, the interface potential is expressed as A�� (z ±Lw=2), where � is the spin projection ±1=2 and Lw is the quantum well width. Our results show that the amplitude of the interface potential A is saturated at 15 Tand is not changed up to 50 T . T his behavior is described well by the modi6ed Brillouin function, which suggests that the magnetic ions in the interface region are isolated rather than clustered, in contrast to those in the barrier layers. The potential amplitude A is found to be enhanced by the pressure. c

Exactly solvable model for carrier-induced paramagnetic-ferromagnetic phase transition in diluted magnetic semiconductors

Abstract We present a model that permits to obtain and exactly describe the paramagnetic-ferromagnetic phase transition induced by carrier-ion exchange interaction and assess the different theoretical approaches to this problem. To modeling the ferromagnetic phase transition, we consider the cluster consisted of Nm magnetic ions and Ne electrons. Carrier-ion exchange interaction allows the transitions between two electronic levels. Exact energy spectrum of the model under consideration is found. Then thermodynamic parameters are calculated with straightforward calculations of partition function. The final expression for the magnetic susceptibility χ0 we obtain in thermodynamic limit for cluster volume V→∞. The model describes two different contributions to the inverse magnetic susceptibility. One term TMF stems from the diagonal part of carrier-ion exchange interaction that corresponds to mean field approximation. Next term TLeL stems from the off-diagonal part of interaction and describes the indirect interaction between localized spins via free carriers. Thus, the critical temperature of ferromagnetic ordering is determined by two aforementioned contributions. A general approach is proposed and compared with different approximations to the problem under consideration.

Investigation of the structure of the edge of the valence band of Cd1−xMnxS crystals on the basis of magnetooptical measurements

The results of magnetooptical studies are reported for Cd1−xMnxS crystals with concentrations of the magnetic component x1=0.0056, and x2=0.0013, x1=0.0002 at T=2 K in a magnetic field H⩽3.5 T for H⊥c and H∥c, where c is the hexagonal axis of the crystal. The first reliable confirmation of spin splitting of the exciton reflection spectra of the B exciton transition is successfully obtained as a result of working in an H⊥c experimental geometry. The values of the crystal-field and spin-orbit interaction parameters Δ1, Δ2, Δ3 and the exchange constants Je and Jh of Cd1−xMnxS crystals for achieving the best description of the experimental results in the mean exchange field approximation are found on the basis of the data obtained for H⊥c and for H∥c. An analysis shows that the main qualitative features of the observed reflection spectra for the entire series of investigated Cd1−xMnxS crystals can be described only if the condition (Δ1−Δ2)<0 is satisfied.