P. A. Wolff

Bound magnetic polaron interactions in insulating doped diluted magnetic semiconductors

The magnetic behavior of insulating doped diluted magnetic semiconductors (DMSs) is characterized by the interaction of large collective spins known as bound magnetic polarons. Experimental measurements of the susceptibility of these materials have suggested that the polaron-polaron interaction is ferromagnetic, in contrast to the antiferromagnetic carrier-carrier interactions that are characteristic of nonmagnetic semiconductors. To explain this behavior, a model has heen developed in which polarons interact via both the standard direct carrier-carrier exchange interaction (due to virtual carrier hopping) and an indirect carrier-ion-carrier exchange interaction (due to the interactions of polarons with magnetic ions in an interstitial region). Using a variational procedure, the optimal values of the model parameters were determined as a function of temperature. At temperatures of interest, the parameters describing polaron-polaron interactions were found to be nearly temperature-independent. For reasonable values of these constant parameters, we find that indirect ferromagnetic interactions can dominate the direct antiferromagnetic interactions and cause the polarons to align. This result supports the experimental evidence for ferromagnetism in insulating doped DMSs.

Polaron‐polaron interactions in diluted magnetic semiconductors

The bound magnetic polaron (BMP) is the characteristic collective state of diluted magnetic semiconductors. Isolated BMP are well understood, but their interactions are only beginning to be explored. Recent polaron magnetization experiments on p‐ZnMnTe suggest a ferromagnetic polaron‐polaron interaction, in contrast to the invariably antiferromagnetic impurity exchange interaction in conventional semiconductors. To investigate this question theoretically, we have developed a simplified model of polaron pairs whose central feature is competition between the usual, antiferromagnetic, virtual‐hopping interaction, and the loss of carrier‐magnetic ion exchange energy, by intermediate ions, when the polaron moments are antiferromagnetically aligned. The model is sufficiently simple that its partition function can be calculated in detail. With reasonable parameters, it predicts a ferromagnetic polaron‐polaron interaction at low temperatures.

Stimulated Compton scattering as a radiation source—theoretical limitations

Several authors have considered the possibility of using stimulated Compton scattering for obtaining laser action. To date, all gain calculations for these devices have assumed an infinite interaction region. The purpose of this paper is to consider the effect of a finite geometry on the gain. We find that in most experimental situations the gain is significantly reduced, to the point where stimulated Compton scattering seems unlikely.

Nonlinear optical studies of picosecond relaxation times of electrons in n‐GaAs and n‐GaSb

The third order nonlinear optical susceptibility of free carriers has been studied in n‐GaAs and n‐GaSb as a function of the frequency difference δω between two Q‐switched CO2 laser beams for different lattice temperatures. The nonlinear response of the electron gas results from the modulation of the carrier mass as the kinetic energy and electron distribution are driven at frequency δω. This modulation arises both from the nonparabolicity of the light mass Γ minimum and, at high temperatures, from the transfer of electrons between the Γ and the heavier mass L minima. The frequency dependence of the response yields the intervalley and energy relaxation times.

Negative absolute mobility of holes in n‐doped GaAs quantum wells

We observe the effect of negative absolute mobility for minority holes in n‐modulation‐doped GaAs quantum wells at low temperatures. Photoinjected minority holes drift towards the positive electrode, which is shown experimentally using a photoluminescence imaging technique. The effect is attributed to ‘‘carrier drag’’ by the high‐mobility electron plasma via electron‐hole scattering. From a quantitative analysis we are able to determine electron‐hole momentum relaxation times.

Narrowing and fine structure of the magnetization steps of dilute magnetic semiconductors in pulsed magnetic fields

Abstract Magnetization steps due to nearest-neighbor Mn2+ pairs in Cd0.977Mn0.023S and Zn0.97Mn0.03S are much narrower in pulsed fields than expected from thermal broadening, and show a fine structure. These narrow steps imply a pair spin-lattice relaxation time much longer than 0.5 ms. An interpretation of the narrow steps, based on the Dzyaloshinski-Moriya interaction, is proposed. The nearest-neighbor exchange constant JNN for (Zn,Mn)S is JNN kb = -16.9 0.6 K . The effects of polytypism of JNN in (Zn,Mn)S are discussed. For (Cd,Mn)S, JNN kB = -9.65 0.2 K and -11.0 0.2 K , corresponding to two inequivalent NNs in the wurtzite structure.

Bound magnetic polarons in dilute magnetic semiconductors (invited)

Optical experiments have been used to study bound magnetic polarons (BMP) in (Cd,Mn)S, (Cd,Mn)Se and (Cd,Mn)Te. This work is surveyed, and the theory of these complexes is discussed. BMP exhibit a remarkable, continuous transition from fluctuation dominated behavior at high temperatures, to a cooperative regime at low temperatures.

Optical nonlinearities due to subband structures in Al0.08In0.92Sb/InSb superlattices

We report the first observation of optical nonlinearities due to subband structures in the growth direction of an n‐type superlattice by four‐wave mixing in an end‐firing configuration. We observe a nearly eightfold increase in the third‐order optical susceptibility when the electric fields of the CO2 laser are changed from a parallel to a perpendicular orientation to the plane of an Al0.08In0.92Sb/InSb superlattice. No change is observed in a control InSb epilayer under similar conditions.

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.

Hole induced four wave mixing and intervalence band relaxation times in p-GaAs and p-Ge

Abstract Hole-induced optical nonlinearities are studied via CO 2 laser, four-wave mixing experiments in p-GaAs and p-Ge. The measured χ( 3 )s agree with those implied by the intervalence-band population modulation mechanism. Dispersion of the nonlinear coefficient determines intervalence band relaxation times in the picosecond range; they agree with those calculated for optic phonon emission.