The photoexcited spin-aligned state of high-density exciton magnetic polarons and the effect of magnetic field in semimagnetic semiconductor Cd(0.8)Mn(0.2)Te

In Cd(0.8)Mn(0.2)Te, nonlinear photoluminescence (PL) appears only when localized excitons are selectively excited to high-density states. Here, the effect of a magnetic field is compared between nonlinear PL and PL due to localized magnetic polarons. Nonlinear PL shows a shift towards lower energy under an applied magnetic field, whereas PL of a localized magnetic polaron band shows a slight shift towards higher energy. The experimental results support the hypothesis that the origin of the nonlinear PL is a spin-aligned state of high-density exciton magnetic polarons. In the spin-aligned state, most spins of electrons (holes) in many magnetic polarons point in the same direction. In this new high-density photoexcited state, the s, p-d exchange interaction between photoexcited electrons (holes) and magnetic ions plays an important role.     

Temperature behavior of bound magnetic polarons in antiferromagnetic chain

The behavior of a one-dimensional antiferromagnetic (AF) chain doped by non-magnetic donor impurities is analyzed, in the limit of low impurity density n  . The doping leads to the formation of ferromagnetically correlated regions localized near impurities (bound magnetic polarons or ferrons). The temperature evolution of the chain is calculated using an approximate variational method, and a Monte Carlo simulation. Both these methods give the similar results. The analysis of correlation functions for neighboring local spins demonstrates that the ferromagnetic correlations inside a ferron are significant even at high temperatures. The AF correlations in the rest part of the chain decay much faster with temperature. So, the ferron is a stable object that does not disappear even above the Néel temperature _TN$T_{\mathrm{N}}$. At rather small values of the electron–impurity coupling energy V$V$ (for V$V$ lower then the electron hopping integral t  ), the bound ferron depins from impurity retaining its magnetic structure. Such a depinning occurs at T∼V$T\sim V$.     

Mid gap photoluminescence from GaN:Mn,a magnetic semiconductor

The defect and morphology of GaN monocrystals with Mn content 10¹⁹ cm⁻³ were examined by fluorescence confocal microscopy and spectroscopy. The fluorescence spectral investigation was carried out in a region very close to the defect centers. Contrary to earlier results, we did observe a characteristic fluorescence line of Mn corresponding to the ⁴T₁→⁶A₁ and ⁴T₂→⁶A₁ transitions, suggesting the predominant presence of Mn²⁺ (d⁵). In addition, strong emission lines were observed at 1.60 and at 1.85 eV when the sample was excited with light of 436 and 365 nm, respectively. An energy scheme is proposed to explain the observed data coherently.     

PHOTOCONDUCTIVITY OF Cd0.7Mn0.3Se

The photoconductivity spectra of semimagnetic semiconductor Cd0.7Mn0.3Se have been measured at 50-300 K and in the wavelength range of 5500 ? to 8000 ?. It is found that the energy gap of Cd0.7Mn0.3Se changes with the temperature linearly, and the temperature coefficient (dEg/dT) is about -7×10-4 eV/K. A photoconductivity peak which is related to Mn2+ is also found. This peak is located around 1.85 eV, nearly unshifted with variation of the temperature. The possible transition mechanisms in Cd1-xMnxSe have been discussed in the light of group theory and crystal field theory, and the results are in good agreement with experiments.     

Optically detected magnetic resonance of Mn-related excitations in (Cd,Mn)Te quantum wells

Optically detected magnetic resonance (ODMR) was applied to reveal the exchange interaction effects between Mn²⁺ ions and confined holes in (Cd,Mn)Te quantum wells with 2D hole gas. Two anisotropic ODMR signals with different angular variations were found and ascribed to isolated manganese ions and to exchange-coupled complexes consisting of manganese and holes. It is shown that calculations on the basis of spin Hamiltonian for these systems are in agreement with the experimental data.     

Pressure induced resistivity changes in diluted magnetic semiconductors of (Hg,Fe)Se and (Hg,Mn)Te

Abstract

Electrical resistance measurements have been performed on samples of Hg_1-xFe_xSe and Hg_1-xMn_xTe for × ≤ 0.30 as a function of pressure to 7.0 GPa. Both materials crystallize in the B3 structure at atmospheric pressure and room temperature and belong to the class of materials known as diluted magnetic semiconductors. At elevated pressures, HgTe undergoes a phase change from the B3 to B9 phase which is accompanied by an increase in the electrical resistivity of several orders of magnitude. The results reported here are that increasing × in both materials serves to increase the B3-B9 transition pressure. The implication being that partial replacement of the Hg-bonds by Fe-Se and Mn-Te, respectively, increases the stability range of the B3 phase     

Ground state energies of bound polarons in semiconductors in a strong magnetic field

Effect of the electron-lattice coupling on the ground state energies of shallow donors in a strong magnetic field is investigated for a few polar semiconductors. Extending the theory of Bajaj for bound polarons in the weak coupling limit, and using the ground state wave function of Yafet et al. for a shallow donor in a magnetic field, it is found that the coupling enhances the ground state energies. While this effect is negligible for most of the semiconductors, the results indicate softening of the coupling in a magnetic field.     

Heavy and light hole subbands exchange induced mixing in CdTe-(Cd,Mn)Te magnetic superlattices

We present the first calculation of the electronic structure of hole subbands in a semimagnetic superlattice in the presence of an applied magnetic field, taking into account the existence of a preferential axis of quantization for the hole spin and the fourfold degeneracy of the top of the valence band. We show that, for fields applied parallel to the layers, there is a strong exchange induced mixing of heavy and light holes subbands for any value of k, the Bloch wave vector parallel to the layers. This mixing dominates over the usual k·p induced mixing and should be observable in resonant tunneling and magnetotransport measurements on p-type samples.     

Formation of magnetic polarons bound to interfaces in quantum wells with semimagnetic barriers

It is shown that the hole state centred in the middle of the well can become unstable during magnetic polaron formation resulting in a polaron state connected with an interface. The criterion of formation of interface-trapped polarons in a quantum well with semimagnetic barriers is found. We have shown that in the case of the experimentally observed two-dimensional exciton magnetic polaron, interface trapping is prevented mainly due to the electron-hole Coulomb attraction. The effect of an external magnetic field on the stability criterion is also considered.     

Determination of the band structure parameters of Cd0.95Mn0.05Se from magnetoabsorption measurements

Band parameters and the exchange integrals of Cd_0.95Mn_0.05Se are evaluated using magnetoabsorption measurement results. The anisotropy of the exchange interaction for valence band electrons and the magnetic field dependent wave function mixing coefficients are obtained. The use of a quasi-cubic model for describing a CdMnSe crystal in the presence of magnetic field is shown to be unjustified.     

Mechanoluminescence and high pressure photoluminescence of (Zn,Cd) S phosphors

The mechanoluminescence spectra of (Zn, Cd)S phosphors shift towards shorter wavelength side as compared to the photoluminescence spectra, however, the photoluminescence spectra shift towards shorter wavelength side with increasing pressure with less pressure coefficient. This finding eliminates the thermal population mechanism and suggests the electrical excitation mechanism for the mechanoluminescence excitation. The decay of mechanoluminescence after the deformation (Zn, Cd)S phosphors may be controlled by the recombination rate of holes and electrons, i.e. by the finite times required for the liberation of the electrons from the traps and for the electron transport, and consequently the decay of mechanoluminescence may be similar to the decay of photoluminescence.     

Photo-induced magnetic polarons in semiconductors

Exciton magnetic polarons observed in dilute magnetic semiconductors were investigated by steady-state and pico-second time-resolved photoluminescence measurements and have shown characteristic behavior of exciton localization processes in bulk-Cd_1-x Mn _x Te and also in the quantum structures composed of the dilute magnetic semiconductors. For the quantum structures spin-dependent coherent polarizations associated with excitons and biexcitons were studied by degenerate four-wave mixing experiment. Also investigated for different chalcogenide spinel ferromagnetic semiconductors was photo-induced enhancement of exchange interaction between magnetic ions by direct magnetic flux detection in the vicinity of the Curie temperatures.     

Saturation of the Mn2+ ionic subsystem in Cd0.5Mn0.5Te and Zn0.91Mn0.09Se bulk samples and in Cd0.6Mn0.4Te/Cd0.5Mg0.5Te superlattices

It is experimentally shown that the Mn²⁺ ionic subsystem in the three-and two-dimensional structures of diluted magnetic semiconductors of the II–VI groups is saturated upon pulsed excitation by high-power laser radiation with a wavelength of 532.1 nm at temperatures of 77 and 4 K. The direct excitation under these conditions leads to saturation of the inhomogeneously broadened ⁴ T ₁ level of a part of Mn²⁺ ions, which is confirmed by the estimation of the fraction of excited ions. Processing of the integral kinetic curves of the intracenter luminescence band in the region of 2 eV shows that an increase in the excitation intensity gives rise to a fast component with a decreasing lifetime. Based on the data obtained, it is assumed that, along with the previously studied cooperative effect, redistribution occurs of excitation from localized to delocalized states as a result of more effective saturation of the former.