We demonstrate that Coulomb drag between spatially separated quasi-two-dimensional electron and hole gases is strongly enhanced by Coulombic correlations.The correlations modify the carrier concentration dependence and the temperature dependence of transresistance and remove the persistent order of magnitude disagreement between the experimental data and the theories based on the mean field (random phase) approximation. Disorder scattering is shown to influence the results, particularly strongly at low concentrations.A great deal of attention has been recently devoted to double layer systems in which two quasi-two-dimensional subsystems (electron or hole gases) are placed in parallel planes separated by a potential barrier thick enough to prevent particles from tunneling across it but allowing for the interactions between the particles on both its sides. In such systems many body correlations due to Coulomb in-

We have investigated coupled layers of electron and hole liquids in semiconductor heterostructures in zero magnetic field for densities r s Շ20 using the Singwi-Tosi-Land-Sjölander self-consistent formalism generalized for layers of unequal density. We calculate susceptibilities, local fields, pair correlation functions, and the dispersion of the collective modes for a range of layer spacings. We include cases where the densities in the two layers are not equal. We find generally that static correlations acting between layers do not have a large effect on the correlations within the layers. For coupled electron-hole layers we find that as the spacing between the layers decreases there is a divergence in the static susceptibility of the liquid that signals an instability towards a charge-density-wave ground state. When the layer spacing approaches the effective Bohr radius the electron-hole correlation function starts to diverge at small interparticle separations. This effect is a precursor to the onset of excitonic bound states but this is preempted by the charge-density-wave instability. The acoustic plasmon exhibits a crossover in behavior from a coupled mode to a mode that is confined to a single layer. Correlations sometimes push the acoustic plasmon dispersion curve completely into the singleparticle excitation spectrum. For layers with different densities the Landau damping within the single-particle excitation region is sometimes so weak that the acoustic plasmon can exist inside the region as a sharp resonance. We find for the electron-hole case that proximity to the charge-density-wave instability has an unusual effect on the dispersion of the optical plasmon mode.

We show that the density for Wigner crystallization to occur for the two-dimensional electron liquid in zero magnetic field can be increased in suitable multiple-quantum-well structures. Data from Monte Carlo calculations are used to determine the properties of each layer in isolation, and the layers interact through Coulomb forces. With this mechanism the maximum solidification density can be raised by as much as a factor of 3. At higher densities, charge-density-wave ground states can occur

Measurements of magnetization, magnetic susceptibility, specific heat, and Hall effect of the Pbl ySn"Mny Te semimagnetic semiconductor (x =0. 72, y =0.03 and x =0. 64, y =0.03) were performed in samples with different concentration of carriers (p =8X10" to 1.4X10" crn '). A low-temperature ferromagnetic phase was established in the samples with carrier concentration larger than a threshold value p, =3X10 crn '. Samples with lower carrier concentrations are paramagnetic in the whole temperature range studied (T =1.5-300 K). The carrier-concentration dependence of the Curie temperature has an unusual thresholdlike character. The explanation of this effect is based on the Ruderman-Kittel-Kasuya-Yosida (RKKY) indirect exchange interaction between Mn spins and a two-valence-band model of the band structure of Pbl "~Sn,MnyTe.Theoretical calculations based on the experimentally determined parameters of the band structure are presented. The results show that the most important role in the RKKY interaction is played by heavy holes from the X band located 185 meV below the top of the L, band of light holes.

No abstract