C. S. Ting

TIGHT-BINDING CALCULATION OF ZNSE/GE SUPERLATTICES : ELECTRONIC STRUCTURE AND OPTICAL PROPERTY

The results of a detailed tight‐binding calculation of the electronic structure and optical properties of (ZnSe)n/(Ge2)m (110) superlattices are presented for a wide range of n,m≤20. It is found that the fundamental energy gap increases with decreasing superlattice period due to spatial quantum confinement effects. For all reasonable values (ranging from 0.2 to 2.0 eV) of the valence band discontinuity used in the calculation, no interface states are found in the thermal gap of the (ZnSe)n/(Ge2)m (110) (n,m≤20) system. An indirect–direct band gap transition is found to be driven by increasing valence band offset. The optical properties of the superlattices are investigated by calculating the absorption coefficient. The results are analyzed according to the calculated electronic structure and compared with those of the corresponding bulk materials.

Optical epilayers on silicon substrate: Electronic and optical properties of ZnS/Si superlattice

The optimal epilayers on a silicon substrate are suggested to integrate the superior properties of ZnS semiconductor with the mature technology of Si. In a semiempirical tight‐binding scheme, the band structures and optical transitions are studied for the (ZnS)n/(Si2)m (110) superlattices with a wide range of n,m≤20. Because of the quantum confinement effect caused by the large band‐gap ZnS layers, the band‐edge states are confined two dimensionally in the Si quantum wells. A single empty interface band is found lying below the conduction band. Furthermore, the influence of valence‐band discontinuity has been checked over all possible energy ranges. The optical matrix elements of the superlattices are calculated and compared with those of bulk ZnS and Si.

Quantum size effect on optical absorption edge in thin antimony films

The quantum size effect in thin antimony films grown along (111) direction on a GaSb substrate is studied based on a simple model. The critical film thickness, at which the semimetal‐semiconductor transition takes place, is determined. It is proposed that the optical experiments can be used to measure the critical thickness since the dependence of optical transition on thickness in the semimetal region is very different from that in semiconductor region.

Electronic structures of Sb/Ga(Al)Sb (111) semimetal‐semiconductor superlattices

The electronic structures of semimetal‐semiconductor (Sb2)m/(GaSb)n (111) and (Sb2)m/(AlSb)n (111) (m,n≤10) superlattices are calculated by using a tight‐binding theory including spin‐orbit interaction. It is found that a narrow gap forms in these materials due to the quantum confinement effect. This may allow strong optical nonlinearity in the infrared region. With increasing the thickness of the Sb layer, a possible semiconductor‐semimetal transition is suggested at a certain thickness. The influence of interface states on the formation of the band gap is investigated by adjusting the interface relaxation and band offsets. Our study shows that semimetal‐semiconductor Sb/Ga(Al)Sb superlattices could potentially open a new possibility in electro‐optical device manufactures.

Theory of resistivity for charged bosons in high-Tc oxides compounds

Abstract The elementary charged excitations in La2−xBaxCuO4-type high Tinc compound have recently been proposed by Kivelson et al. to be two-dimensional spinless soliton-like bosons. In this paper we show that the resistivity of such a system due to acoustic phonons is linearly T-dependent as long as T ⪢ T ∗ = mv s 2 2 , with m as the effective mass of the boson and vs as the speed of the sound. The result for the resistivity due to charged impurities is also presented and discussed.

Excitons in spatially separated electron–hole systems: A quantum Monte Carlo study

Variational and diffusion quantum Monte Carlo simulations of excitons in the type‐II semiconductor quantum well system In1−xGaxAs/GaSb1−yAsy are performed for a wide range of composition (0,0)≤(x,y)≤(0.62,0.64) and well width 25 A ≤L≤200 A. The exciton binding energy is found to increase with the decrease of the layer thickness and with the increase of the composition. The calculated results suggest that a novel semiconductor–excitonic insulator (Bose condensate)–semimetal transition should be observable in this system.

Enhancement of optical absorption induced by disorder in three‐dimensional random superlattices

The effect of disorder on the optical absorption of the realistic random superlattice has been investigated based on a three‐dimensional tight‐binding Hamiltonian. It is found that the absorption intensity close to the band edge of the random superlattice is considerably enhanced, which can be explained by optical matrix elements of the relevant eigenfunctions localized strongly over 2–4 monolayers. An energy‐level crossing behavior at the conduction‐band bottom is also obtained.

Effect of the degree of disorder on electronic and optical properties in random superlattices

A three‐dimensional tight‐binding calculation is developed and used to study disorder effects in a realistic random superlattice. With increasing disorder, a tendency of possible indirect–direct band‐gap transition is suggested. Direct evidence of mobility edges between localized and extended states in three‐dimensional random systems is given. As system disorder increases, the optical absorption intensities increase dramatically from five to forty‐five times stronger than the ordered (GaAs)1/(AlAs)1 superlattice. It is believed that the degree of disorder significantly affects electronic and optical properties of GaAs/AlAs random superlattices.

Mean field theory study of a tight-binding model for the high Tc oxides

Abstract The magnetic and electronic properties of a two dimensional extended Hubbard model for the Cu-O planes in high Tc oxides have been studied in the Hartree-Fock approximation. Its magnetic phase diagram is similar to that of single-band Hubbard model. The density of states and the charge distribution on Cu and O ions are presented. Doping affects the charge distribution on both Cu and O ions. The intersite Coulomb interaction has a strong influence on the properties of system.

MEAN-FIELD APPROACH TO CHARGE, ORBITAL, AND SPIN ORDERING IN MANGANITES

We present a mean-field theory of charge, orbital, and spin ordering in manganites at 50% and 0% dopings by considering Jahn–Teller interaction, nearest-neighbor repulsion, and no single-site double occupancy. For spinless fermions, we show that Jahn–Teller distortion and charge-orbital ordering occur simultaneously. In our two-dimensional model at 50% doping, for small nearest-neighbor repulsion the system is orbitally polarized while for larger repulsion the system undergoes CE type ordering. As for the 0% doping case, the ground state is orbitally antiferromagnetic. Upon including spin degree of freedom, at both 50% and 0% dopings the ordering remains the same at small antiferromagnetic coupling between adjacent core spins.