Nondegenerate Band Research Articles

Spin-polarization instability in a tilted magnetic field of a two-dimensional electron gas with filled Landau levels.

We have investigated the stability of a two-dimensional electron gas with two filled Landau levels (of opposite spin) in the high-field limit. The Zeeman energy can be increased by adding a component of the magnetic field parallel to the surface. The lowest-lying excitations can be described in terms of singlet and triplet excitons. Taking interaction effects into account, we have found that at a critical Zeeman energy (smaller than the cyclotron energy), there is a first-order transition to a fully spin-polarized state in which two Landau levels of equal spin are filled. Exotic intermediate states of the spin-density-wave type have been found not to occur in the simple case of nondegenerate bands.

Weak localization in tellurium: Two spin-nondegenerate valleys

The weak localization correction to the conductivity due to holes of the spin-orbit split nondegenerate uppermost Te band is considered. It is shown that in spite of strong spin-orbit interaction the quantum magnetoresistance may be negative as well as positive depending on the Fermi level position and impurity potential properties.

Spontaneous symmetry breaking of optimum fluctuations in semiconductors

The optimum fluctuation method (OFM) has been applied to the tails of the density of states, arising near the edges of the spherically symmetric degenerate bands. In this case the optimum fluctuations (OF) have been shown to undergo a qualitative change, as compared to the case of nondegenerate bands, they lose the spherical symmetry and become elongated or flattened. This means that spontaneous breaking of symmetry takes place. In addition to the usual mechanism of tailing due to the potential of impurities, another mechanism connected with the field of random deformations, arising due to difference in the size of the guest and host atoms, has been also considered. The method used for treating this problem is intimately related to the techniques of the theory of self-trapping. The density of states in the tails, and in some cases the shape of OFs, have been found for all the cases under consideration.

Cohesive energy in strongly correlated electronic systems in the non-degenerate band approximation

The electronic contribution to the cohesive energy is calculated in narrow band systems where the splitting of bands due to intra-atomic correlation is larger than the band width by solving the effective Hamiltonian for the lower Hubbard subband in the non-degenerate band approximation on the basis of different decouplings of the quasielectron Green function. Using a rectangular form of the density of states for non-interacting electrons analytical expressions for the ground state energies for different magnetic phases are obtained. The importance of the kinetic exchange on the stability of the magnetic phases is indicated for different fillings of the band. The significance of these results for the understanding of the cohesive energies of strongly correlated systems is discussed.

Bound Exciton and Hole: An Exactly Solvable Three-Body Model in Any Number of Dimensions

A three-body problem, concerning two holes in a nondegenerate valence band and a single electron in a conduction band, with strong short-range interactions, is solved exactly in any number of dimensions. The binding depends nontrivially on the ratio of the valence to conduction bandwidths (i.e., on the inverse ratio of their effective masses) and always vanishes when this ratio exceeds 1. The effective mass of the "trion" bound state also depends sensitively on this ratio and on dimensionality.

Transition metal surface kink electronic structure

In the tight binding approximation applied to a non-degenerate band, we show that the kink local density of states does neither depend on the surface nor on the step type, but only on the bulk crystal structure. This result is not valid for the kink local density of states of each atomic orbital of a degenerate band like the transition metal d band. Nevertheless, the total local densities of states for different kinks are very close to each other and also only depend on the crystal structure.

Transition metal surface kink electronic structure

Abstract In the tight binding approximation applied to a non-degenerate band, we show that the kink local density of states does neither depend on the surface nor on the step type, but only on the bulk crystal structure. This result is not valid for the kink local density of states of each atomic orbital of a degenerate band like the transition metal d band. Nevertheless, the total local densities of states for different kinks are very close to each other and also only depend on the crystal structure.

The Hubbard sub-band structure and the cohesive energy of narrow band systems

The Hubbard sub-band structure in a narrow band has been obtained in an analytical form in the third Hubbard approximation, using the Lorentzian form for the density of states for non-interacting electrons. The imaginary part of the self-energy signifying the lifetime of the interacting electrons is analysed. The contribution to the cohesive energy from electrons in a narrow non-degenerate band is then obtained analytically. Certain aspects of the band and the atomic contributions to the cohesive energy are clarified. The results are compared with those obtained earlier. Possible origins of the asymmetry of the experimental data on the heat of formation of transition-metal compounds for particles and holes are discussed.

Excitation of degenerate vibrations in non-degenerate electronic bands

The vibrational rule structuic of the 2A' 1 band in the photoelectron spectrum of BF 3 is analyzed Using ab initio calculated coupling constants, It is shown that some lines of this spectrum represent excitation of single quanta of the degenerate stretching vibration They borrow their intensity from the adjacent 2E' state via a pseudo-Jahn-Teller interaction

Properties of excitons bound to neutral donors

The binding energy of excitons to neutral donors in the approximation of spherical, nondegenerate energy bands, is calculated by a variational method for relevant range of electron-to-hole mass ratio. Estimated relative intensities of LO-phonon replicas of radiative recombination of the exciton-neutral-donor complex are in fair agreement with measured intensities of fluorescent lines.

Green's function approach in the theory of interaction of exciton-polaritons in semiconductors with direct band gaps

The renormalization procedure of perturbation theory is applied to study polariton effects in the optical processes involving photons with energies near that of an exciton: resonant Raman scattering, resonant electronic Raman scattering, and resonant high — order optical harmonic generation. To demonstrate the general method and the computation technique we consider in this work a simple model with the following properties: (a) The semiconductor has a direct band gap with allowed electrical dipole transition and with nondegenerate valence and conduction bands. (b) We consider only the photons in a definite polarization state; they are in resonance with one bound state of an electron-hole pair, and we take into account only the transition between this exciton state and the photon in the given polarization state. We work in the framework of the Green's function approach and we shall see that this approach provides for the inclusion of damping effects in a relatively rigorous manner.

Wave-vector-nonconserving optical transitions in a model dilute alloy

We explore the theory of the imaginary part ${\ensuremath{\epsilon}}_{2}(\ensuremath{\omega})$ of the dielectric constant for a simple model alloy, with emphasis on the possibility of studying critical points in the host band structure through wave-vector-nonconserving transitions induced by impurities. We find Van Hove singularities in the host band structure produce discontinuities in slope of $\ensuremath{\partial}\frac{{\ensuremath{\epsilon}}_{2}(\ensuremath{\omega})}{\ensuremath{\partial}\ensuremath{\omega}}$ at photon energies equal to the absolute magnitude of the difference between the Fermi energy and those of the critical points. The paper presents a study of ${\ensuremath{\epsilon}}_{2}(\ensuremath{\omega})$ and its frequency derivative for a single impurity placed in a host with nondegenerate conduction band of tight-binding form.

Analytic properties of band functions

We consider the analytic properties of Bloch Hamiltonians and their energy levels as functions of quasi-momentum k. Among other results are: (1) The only entire band is the trivial free electron parabola. (2) The only isolated singularities allowed are branch points. (3) In one dimension, no natural boundaries or logarithmic branch points occur. (4) The periodic attractive screened Coulomb lattice has a nondegenerate lowest band (i.e., the “direct gap” is strictly positive for all k).

Selfconsistent model calculations of electronic states at narrow-band-metal surfaces

For a nondegenerate narrow energy band spanned by a semiinfinite chain of three-dimensional atoms, the electronic potential and the electron density of states are calculated selfconsistently in the vicinity of the chain end. The electron-electron interaction is treated in the Hartree-Fock approximation, using the Green function method. The results for the potential and the density of states are discussed in terms of the parameters which determine the bulk electronic structure, such as the Fermi energy E F and the intra- and interatomic Coulomb repulsion k 0 and K 1. Futhermore, the self consistent method is extended to an impurity atom at the chain end. The existence of bonding and antibonding surface states is found to depend on both the bulk and impurity parameters, such as the intraatomic Coulomb repulsion U α and the nearest neighbour hopping element T.

Angle resolved UV photoemission spectra from a Bi(0001) surface

Angle resolved photoemission spectra taken from a Bi(0001) surface for hv = 16.9 and 21.2 eV exhibit (1) broad features which appear to be associated with one dimensional density of states, rather than band like features, and (2) a weak dependence upon the direction of incident light These effects can be explained by a large number of nondegenerate final state bands available for optical emission and by a uniform mixing of atomic orbitals in the initial states.

Electronic structure of bulk and surface disordered alloys

Electronic densities of states of bulk and cleaved binary alloys are determined through a tight-binding model, using a continued fraction technique and a computer simulation on clusters of several thousands of atoms. Results are presented for nondegenerate bands and simple and face centred cubic lattices. They are compared with the coherent potential approximation (CPA) and Bethe-Peierls approximation (BPA). The method can deal with cleaved binary alloys and also non-homogeneous alloys, and an example of a cleaved alloy with surface segregation is given. The surface density of states is shown to be strongly sensitive to the composition of the first layer and this could be an important factor in studying surface segregation.

Absence of magnetization in a nondegenerate half-filled band of electrons: Weak coupling

Absence of magnetization in a nondegenerate half-filled band of electrons: Weak coupling

Electron and hole conductivity in CuInS 2

Single crystals of CuInS 2 have been grown from the melt and annealed in In or S to produce good n- or p-type conductivity, respectively. Two donor levels, one shallow and one deep (0.35 eV), and one acceptor level at 0.15 eV are identified. The hole-mobility data are best fitted with an effective mass m p ∗≅1.3m e , which can be explained by simple, two band k . p theory if the valence band has appreciable d character. Above 300°K, the hole mobility falls rapidly, evidently due to multiband conduction and/or interband scattering between the nondegenerate and degenerate valence bands. The conduction band mobility appears to be dominated, in many samples, by large concentrations ( >10 18cm −3) of native donors and acceptors, which are closely compensated.

Van Hove singularities

Two methods of determining Van Hove singularities of non-degenerate electronic energy bands in non-magnetic crystals are compared. The equivalence of the two methods is shown.

Peierls transition in alternant lattices

It is shown that, in alternant lattices with a half-filled nondegenerate tight-binding band, the static zero temperature dielectric function diverges logarithmically for a wavevector Q=( pi , pi , pi ) in the reciprocal lattice. This implies that such three-dimensional systems may distort in a way which results in a non-conducting ground state.