A.K. Bhattacharjee

Model for the Mn acceptor in GaAs

Abstract We present a model based on the Baldereschi–Lipari effective mass theory and the sp–d exchange interaction between the valence band and the Mn d electrons (3d5). It fits in with the available experimental data and yields a relationship between the exchange-induced splitting of the 1S3/2 impurity ground state and the valence band exchange constant N0β. By using the recently reported value of the splitting, deduced from infrared spectroscopy, and fitting the known ground state binding energy, we obtain N 0 β≃−0.9 eV . We also study the two-hole bound state in the Hartree self-consistent approximation and account for the absence of Mn-associated bound exciton and discuss the sign anomalies in the magneto-optical data in dilute Ga1−xMnxAs.

Photoluminescence Stokes shift and exciton fine structure in CdTe nanocrystals

The photoluminescence spectra of spherical CdTe nanocrystals with zincblende structure are studied by size-selective spectroscopic techniques. We observe a resonant Stokes shift of 15 meV when the excitation laser energy is tuned to the red side of the absorption band at 2.236 eV. The experimental data are analyzed within a symmetry-based tight-binding theory of the exciton spectrum, which is first shown to account for the size dependence of the fundamental gap reported previously in the literature. The theoretical Stokes shift presented as a function of the gap shows a good agreement with the experimental data, indicating that the measured Stokes shift indeed arises from the electron-hole exchange interaction.

Amorphous rare-earth alloys: random-anisotropy antiferromagnetism

Abstract The model of randomly oriented single-ion uniaxial anisotropy field is extended to the case of antiferromagnetic exchange interaction between rare-earth ions. A self-consistent two-spin cluster approximation is used to calculate the magnetization curves for a model system of spin 1. The results are in good qualitative agreement with experimental data on amorphous RE x Cu 1− x and RE x Ag 1− x alloys (RE = Tb, Dy, Ho). In particular, the observed increase of the “coercive” field with increasing x in RE x Cu 1− x alloys is accounted for.

Exchange interaction and acoustical phonon modes in CdTe nanocrystals

Photoluminescence of CdTe nanocrystals (NC) is excited resonantly in the lowest energy absorption peak. The spectrum shows a luminescence line shifted to a lower energy and acoustical and optical phonon replica. The Stokes shift between the luminescence and excitation lines is attributed to the electron–hole exchange energy in the nanocrystal. By tuning the laser line inside the absorption peak, we are able to measure the Stokes shift as a function of the excitation energy. Calculation of the absorption gap and the Stokes shift is done in a tight-binding theory. It allows us to determine the radius R of NC excited at a given wavelength and to compare the experimental and theoretical values of the exchange energy as a function of R. A very good agreement is obtained. The observed size dependence of the acoustical phonon mode energy provides a further confirmation of our analysis.

Exchange induced band splitting in wide-gap semimagnetic semiconductors in external field : Second order correction

Abstract The well known proportionality of the conduction and valence band splittings to magnetization is based on a first-order perturbation treatment of the e-Mn exchange Hamiltonian. We have studied the energy shift of each subband in the second order of perturbation. This leads to a substantial decrease of the band splitting obtained in the first order. Within a simple model, the correction is found to increase with the Mn concentration and the ratio of the exchange parameter to the band width. Our results can account for the anomalously large apparent decrease of the exchange parameters, reported in Zn 1−x Mn x Te and Cd 1−x Mn x Te at high x values.

Density of states of vibrations in a disordered chain

Abstract We study the local density of vibrational states of a disordered binary chain using a renormalization approach suitable for the alloy problem. We include isotopic and spring constant disorder. Short range order effects in the alloy are taken into account in the pair approximation. We present some numerical results for the case of spring constant disorder and discuss their relevance for the interpretation of neutron scattering experiments in some metallic amorphous systems.

A Statistical Mechanical Model for Acceptor Bound Magnetic Polaron in Cd1-XMnxTe

The partition function is calculated, starting from an exchange Hamiltonian, by treating the Mn spins as classical vectors. We consider a realistic arrangement of Mn over the cation sublattice and cut off the hole-Mn interaction at a finite distance from the impurity center, where it becomes coirparable in size to the small Mi-Mn interaction. The resulting magnetic binding energy as a function of temperature shows excellent agreement with experimental data.

Amorphous rare-earth alloys: neutron diffraction and correlation functions in the random anisotropy model

The authors study the longitudinal and transverse spin-correlation functions for an amorphous rare-earth alloy in the random anisotropy model within the Oguchi pair approximations. Both ferromagnetic and antiferromagnetic interactions are considered, as well as correlated and uncorrelated nearest-neighbour anisotropy axes. They relate their results to the elastic coherent spin-flip neutron scattering cross section, which should provide a test for the model.