Van Vleck paramagnetism in the diluted magnetic semiconductor Cd1-xFexSe; A photoluminescence study

We present a photoluminescence study of Cd_1-xFe_xSe, a new diluted magnetic semiconductor (DMS). The luminescence spectrum was recorded as a function of temperature and applied magnetic field, revealing behaviour which was significantly different from that observed in manganese based DMS. We conclude that Cd_1-xFe_xSe exhibits Van Vleck paramagnetism instead of the more usual Larmor paramagnetism.     

Magnetic polaron contribution to donor bound exciton in Cd1−xMnxSe

Photoluminescence spectra associated with the donor bound exciton has been studied in Cd_1-xMn_xSe as a function of composition (x = .10, .05, and 0.0) and temperature in zero magnetic field. In comparison with the previously studied neutral donor, the bound exciton shows a large enhancement in its binding energy and broadening, attributed to the exchange interaction with the Mn⁺⁺-ion 3d electrons. In particular, a transition from a bound polaron to a fluctuation dominated regime is in evidence for x = .10 in this multiparticle system.     

Energy of a bound polaron in a magnetic field in asymmetric polar semiconductor heterostructures

In this paper, a new modified Hamiltonian of a polaron bound to a donor impurity in asymmetric step quantum wells (QWs) in the presence of an arbitrary magnetic field is given, in which the coupling of an electron with confined bulk-like LO phonons, half-space LO phonons and interface phonon modes is included. Especially, the interaction of the impurity with all possible optical-phonon modes is also considered. The ionization energy of a bound polaron in a magnetic field for asymmetric step QWs are studied by using a modified Lee-Low-Pines (LLP) variational method. The effects of the finite electronic confinement potential and the subband nonparabolicity are also considered. The relative importance of the donor impurity located at the well and the step is analyzed. Our results show the interaction between the impurity and the phonon field in screening the Coulomb interaction has a significant influence on the binding energy of bound polaron. The influence of subband non-parabolicity is appreciable on the bound polaron effects for the narrow well. The binding energy of bound polaron given in this paper are excellent agreement with the experimental measurement.     

Anomalies in transport properties of EuO under hydrostatic pressure due to bound magnetic polaron

Resistivity vs hydrostatic pressure has been measured at room temperature for several EuO samples which span a large range of resistivity. We show that the pressure coefficient of activation energy of resistivity β = KT (d log ?/d p) varies continuously with room temperature resistivity. The samples which present an insulator-metal Transition have unusual values of β. We relate these values to the presence of bound magnetic polaron or molecular bound magnetic polaron.     

Excited states of a bound polaron

The results of the calculations of energies of states 2S and 2P of a bound electron interacting weakly with optical phonon field are presented.     

Adiabatic approximation for a bound acoustic deformation potential polaron in a strong magnetic field

On the basis of the usual adiabatic scheme applied to a bound polaron in a strong magnetic field we observe an enhancement of the binding energy due to electron-lattice coupling via the deformation potential. This enhancement, being more pronounced with increasing field intensity, is, however, negligible for most common materials in experimentally interesting magnetic fields.     

Excited states of a bound polaron

Expressions for the energies of the 3s, 3p and 3d states of a polaron, bound to a positively charged impurity ion in a polar crystal through a Coulomb potential, are obtained in analytic forms using perturbation theory. The values of these energies are calculated in several polar crystals.     

A bound polaron in a spherical quantum dot

The binding energy of a bound polaron in a spherical quantum dot has been investigated by using the variational method. The influence of LO and SO phonons have taken into consideration. Result shows that the phonon contribution to the binding energy is dependent on the size of the quantum dot as well as the position of the impurity in the quantum dot. Numerical calculation on the ZnSe quantum dot shows that such contribution is about 5% to 20% of the total binding energy. Received: 13 October 1997 / Revised: 4 March 1998 / Accepted: 26 May 1998     

Anisotropy of cubic semimagnetic Cd1−x MnxTe solid solutions and excitonic magnetic polaron energy from polarized-luminescence measurements

Polarized photoluminescence of Cd_1−x Mn_xTe crystals in a weak magnetic field has been studied in Faraday and Voigt geometries. A simple method is proposed to determine the exciton mobility edge and excitonic magnetic-polaron energy. “Forbidden” polarization components of the recombination radiation have been experimentally detected. It has been established that the moments of magnetic polarons are oriented predominantly along the {111} axes. Fiz. Tverd. Tela (St. Petersburg) 40, 894–896 (May 1998)     

Perturbation theory of a bound piezoelectric polaron

The shift of the ground state energy of a piezoelectric polaron bound in a Coulomb potential is calculated, numerically, to second order in the electron-phonon interaction for arbitrary strengths of the Coulomb potential. The results obtained are discussed so that some comments on the previous communications of other authors are drawn.     

Ground state energy of a bound piezoelectric polaron

The ground state energy of a piezoelectric polaron bound in a Coulomb potential is calculated analytically in the weak coupling limit.     

Polarized photoluminescence from bound magnetic polarons in (Cd,Mn) Se

Luminescence from Cd_1-xMn_xSe, with x～10^-1, shows strong circular polarization in the presence of a weak magnetic field. The polarization saturates at fields much smaller than does the magnetization. The carrier-Mn²⁺ exchange interaction is responsible for the field and temperature dependence of the polarization. At high temperatures, the polarization is similar to that inferred from band splittings; at lower temperatures, it is modified by bound magnetic polaron effects and a spin diffusion bottleneck.     

Light induced molecular magnetic polaron in EuO

Stoechiometric EuO samples do not present insulator metal transition below Curie temperature (T_c = 70 K). Experimental data presented in this paper show that when these samples are illuminated, it appears: an insulator metal transition below T_c, and, at room temperature, a decrease of activation energy, as well as a small increase in conductivity. Oxygen deficient samples exhibit usually this insulator metal transition. When they are illuminated room temperature activation energy remains unchanged, and the insulator metal transition is more important. These results cannot be explained by means of a free electron model. Electrons are photoexcitated from 4? levels, or donor levels, to conduction band. Then, these electrons polarise Eu²⁺ spins and make magnetic polarons. These polarons cannot be “Bound Magnetic Polarons” because samples do not contain oxygen vacancies. In addition these polarons would be stable until room temperature. Thus, they are expected to be “Molecular Magnetic Polarons”. Our results confirm then this model proposed by Kusuya.     

A study of two-dimensional magnetic polaron

By using the variational method and anneal simulation, we study in this paper the self-trapped magnetic polaron (STMP) in two-dimensional anti-ferromagnetic material and the bound magnetic polaron (BMP) in ferromagnetic material. Schwinger angular momentum theory is applied to changing the problem into a coupling problem of carriers and two types of Bosons. Our calculation shows that there are single-peak and multi-peak structures in the two-dimensional STMP. For the ferromagnetic material, the properties of the two-dimensional BMP are almost the same as that in one-dimensional case; but for the anti-ferromagnetic material, the two-dimensional STMP structure is much richer than the one-dimensional case.