We study the Wigner crystallization in semiconductor quantum wires within the density-functional approach. As the density of electrons in quasi-one-dimensional structures is lowered, we find that the system favors the crystalline phase as envisioned by Wigner. The dependence of the critical density on the lateral width of the quantum wire is also investigated. In a structure consisting of two parallel quantum wires, the Wigner transition to the solid phase is enhanced similarly to the bilayer systems. ͓S0163-1829͑98͒01039-X͔ PHYSICAL REVIEW B 15 OCTOBER 1998-I VOLUME 58, NUMBER 15 PRB 58 0163-1829/98/58͑15͒/9886͑4͒/$15.00 9886

We study the ground state properties of a boson-fermion mixture interacting via a hard-core repulsive potential with an attractive tail at zero temperature. We use the self-consistent field approach to calculate the ground state partial structure factors and the effective interactions between the species. We compare our results with the experimental data on liquid 3 He-4 He mixtures to find qualitative agreement. Collective modes and dynamic structure factors for the mixture are also discussed.

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Accurate characterization of charged impurity centers is of importance for the electronic devices and materials. The role of valley-spin degeneracy on the screening of an attractive ion by the mobile carriers is assessed within a range of systems from spin-polarized single-valley to six-valley. The screening is treated using the self-consistent local-field correction of Singwi and co-workers known as STLS. The bound electron wave function is formulated in the form of an integral equation. Friedel oscillations are seen to be influential especially in two-dimensions which cannot be adequately accounted for by the hydrogenic variational approaches. Our results show appreciable differences at certain densities with respect to simplified techniques, resulting mainly in the enhancement of the impurity binding energies. The calculated Mott constants are provided where available. The main conclusion of the paper is the substantial dependence of the charged impurity binding energy on the valley-spin degeneracy in the presence of screening. 73.20.Hb, 71.45.Gm

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