D. Ridgley

Picosecond time-resolved photoluminescence studies of exciton-magnetic polaron complexes in Cd1−xMnxSe

Abstract The evolution of magnetic polarons in Cd 1−x Mn x Se was examined using time-resolved photoluminescence spectra of excitons at cryogenic temperatures. Sample with manganese concentration x = 0.05, 0.10 and 0.20 were studied using a streak camera, with a system resolution of 80 psec. The formation of magnetic polarons was observed to have a large effect on the exciton lifetime, and to be strongly dependent on temperature and manganese concentration.

Optical orientation of excitons in (Cd,Mn)Se and (Cd,Mn)Te

Abstract This paper reports 2 K optical pumping studies of luminescence by localized excitons in Cd1−xMnxSe and Cd1−xMnxTe. The luminescence is weakly polarized when the crystals are excited with circularly polarized light at or above the free exciton energy, implying free carrier spin relaxation times in the picosecond range. The signal abruptly increases when the pump laser frequency is tuned below the free exciton. Peak polarizations of 50 and 15% are observed in (Cd,Mn)Se and (Cd,Mn)Te. With low energy pumping, excitons can only be created in regions of magnetic fluctuation whose large internal fields make spin flip of the hole energetically unfavorable; the effect is enhanced by BMP formation. Exciton spins “trapped” in this way have relaxation times comparable to the exciton lifetime.

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

Abstract Luminescence from Cd 1- x Mn x Se , 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-Mn 2+ 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.

Magnetoresistance near the metal-insulator transition of Cd0.99Mn0.01Se

Abstract At 0.5 ≤ T ≤ 4.2 K samples of Cd 0.99 Mn 0.01 Se which are near the metal-insulator transition exhibit a large positive magnetoresistance (MR) in fields H ∽ 10 k0e. The effect is accompanied by an increase in the Hall coefficient and a decrease in the Hall mobility. At the lowest temperatures and for fields well above 10 k0e a small negative MR is observed. The positive MR is tentatively attributed to an increase in the screening radius r s , which leads to enhanced Coulomb-potential fluctuations.

Single crystal growth of cadmium manganese selenides with controlled carrier concentration

Single crystal boules of Cd1−xMnxSe with x = 0.01, 0.05, and 0.10 have been grown by a modified Bridgeman method using 5% excess selenium to lower the melting point. A technique has been developed for the growth of high quality, single grain boules with demonstrated uniform manganese distribution. Free carriers have been introduced in a controlled fashion by adding gallium to the melt and annealing crystal slices in cadmium vapor. Reproducible carrier concentrations from 2 × 1017 to 2 × 1018cm−3 have been achieved.

Magnetoresistance of Cd1−xMnxSe near the Mott transition

At 0.5≤T≤4.2 K and for fields of order 10 kOe, a large positive magnetoresistance is observed in Cd0.95Mn0.05Se and Cd0.99Mn0.01Se samples which are near the Mott transition. The effect is attributed to an increase in the screening radius, caused by the splitting of the conduction band. At higher fields, but below ∼80 kOe, samples with 5% Mn show negative magnetoresistance which is attributed to a decrease in the separation between the Fermi level and the mobility edge.

OPTICAL PUMPING AND POLARIZED PHOTOLUMINESCENCE IN (Cd,Mn)Se

Optical pumping is observed for exciton luminescence in (Cd, Mn)Te and (Cd, Mn)Se. For band edge pumping the respective spin relaxations are <0.5 and ~5 psec. The polarization of the exciton luminescence increases by an order of magnitude when excited slightly below the free exciton energy.