Many-valley Structure Research Articles

Analysis of a long-range random field quantum antiferromagnetic Ising model

We introduce a solvable quantum antiferromagnetic model. The model, with Ising spins in a transverse field, has infinite range antiferromagnetic interactions with random fields on each site, following an arbitrary distribution. As is well-known, frustration in the random field Ising model gives rise to a many-valley structure in the spin-configuration space. In addition, the antiferromagnetism also induces a regular frustration even for the ground state. In this paper, we investigate analytically the critical phenomena in the model, having both randomness and frustration and we report some analytical results for it.

Specific heat of the Coulomb glass

The specific heat of the Coulomb glass is studied by numerical simulations. Both the lattice model with various strengths of disorder, and the random-position model are considered for the one- to three-dimensional cases. In order to extend the investigations down to very low temperatures where the many-valley structure of the configuration space is of great importance we use a hybrid-Metropolis procedure. This algorithm bridges the gap between Metropolis simulation and analytical statistical mechanics. The analysis of the simulation results shows that the correlation length of the relevant processes is rather small, and that multi-particle processes yield an essential contribution to the specific heat in all cases except the one-dimensional random-position model.

Spin Freezing Properties of an Ising Spin Glass Fe0.05TiS2in Zero and Non-Zero Magnetic Fields

In order to understand spin freezing properties, ac-susceptibility χ ac and relaxation of thermoremanent magnetization (TRM) were measured on an Ising spin glass Fe 0.05 TiS 2 in zero and non-zero fields. Analysis of χ ac above the spin glass freezing temperature shows that a rectangular distribution of spin relaxation times is appropriate and the temperature dependence of the average spin relaxation time follows the power law. The waiting time effects on the relaxation of TRM below the freezing temperature can be interpreted by a hierarchical many-valley structure. These results are consistent with Sherrington-Kirkpatrick model's prediction that the spin glass freezing is a phase transition in both zero and non-zero fields.

Ohmic Mobility of Electrons in Silicon

Ohmic mobility of electrons in Si has been theoretically studied as a function of temperature. In our theoretical model many-valley structure of Si conduction band, intra-valley scattering of acoustic phonons and impurities and intervalley scattering of phonons and electrons were included. The Green's function approach developed by Lei and Ting was extended to evaluate the frictional forces resulting from several scattering mechanisms. The numerical results compared with experimental data and indicated the effect of intervalley electron-electron interaction on the mobility of electrons.

Ferromagnetic random-bond Ising model: Metastable states and complexity of the energy surface.

Metastable states of the ferromagnetic random-bond Ising model are produced by simulated quenches from infinite to zero temperature. Two-dimensional systems (square lattice) show many domains: the many-valley picture of the energy surface of spin glasses applies to these unfrustrated systems as well. The number of the domains, which serves as a rough measure of the complexity of the energy surface in two dimensions (but not in three), is only 30% greater for a broad bond distribution (uniform from 0 to 1) than for a narrow one (uniform from 0.499 to 0.501). We conclude that the uniform-bond model also has a complex energy surface; for a square lattice the surface has many terraces (or steps) rather than many valleys, but for the honeycomb lattice it has a many-valley structure. The large domains in our two-dimensional systems have holes on many length scales and are highly ramified, with total perimeter proportional to the number of sites; their dimensions from the capacity, information, and radius-of-gyration definitions are 1.82\ifmmode\pm\else\textpm\fi{}0.06, 1.84\ifmmode\pm\else\textpm\fi{}0.04, and 1.83\ifmmode\pm\else\textpm\fi{}0.07, respectively. In three dimensions (simple cubic lattice), the initial concentrations of both up and down spins exceed the percolation threshold and the metastable states are dominated by two large spanning domains; only a few, small, embedded domains are found.

Calculation of high-field diffusivity by a many-particle Monte Carlo simulation including a complete band structure for GaAs

The electron diffusion coefficients of GaAs in the high-field regime ranging from 10 to 500 kV/cm have been calculated by a many-particle Monte Carlo simulation. The band-structure model in the simulation, calculated using the empirical pseudopotential method, includes the lowest two conduction bands and the full many-valley structure for each band. Our calculation shows that the diffusivity decreases drastically (by a factor of ≊30) as the field increases from 10 to 250 kV/cm and extremely low diffusion coefficients, 1.1 cm2/s for D1 and 6.1 cm2/s for Dt, are obtained at ≊250 kV/cm for GaAs.

Phonon attenuation in heavily doped p-type semiconductors

The phonon relaxation rate in heavily doped n-type semiconductors with many-valley structure such as Ge and Si has been already derived in our previous papers [1, 2] where are included the effects of screening by conduction electrons and of the electrons having a finite relaxation time due to both the intravalley and the intervalley scattering by impurity atoms. In this paper we extend the method to calculate the phonon relaxation rate in heavily doped p-type semiconductors. Here the interband hole-phonon interaction is explicitly taken into account as well as the intraband one. We also report the experimental results of the low-temperature thermal conductivity κ in heavily doped p-type GaSb and Si and the analyses.

Electronic states of a negative donor ion in silicon and germanium

We present a review of recent theoretical developments on the electronic structures of an isolated negative donor ion (D − ion) in silicon and germanium in a magnetic field and in zero field. In the case of zero field particular emphasis is placed on the effects of many-valley structure in a conduction band and of anisotropy of conduction band effective masses on the binding energy of a D − ion. The binding energy calculated in a variational method is compared with recent experimental results by long-wavelength photoconductivity measurements. On the other hand, in the case of high magnetic fields the features of D − states are discussed on the basis of theoretical prediction made for the case of an isotropic effective mass in a single valley.

Electron drift velocity in silicon

Experimental results for electrons obtained with the time-of-flight technique are presented for temperatures between 8 and 300\ifmmode^\circ\else\textdegree\fi{}K and fields ranging between 1.5 and 5 \ifmmode\times\else\texttimes\fi{} ${10}^{4}$ V ${\mathrm{cm}}^{\ensuremath{-}1}$ oriented along $〈111〉$, $〈110〉$, and $〈100〉$ crystallographic directions. At 8\ifmmode^\circ\else\textdegree\fi{}K the dependence of the transit time upon sample thickness has allowed a measurement of the valley repopulation time when the electric field is $〈100〉$ oriented. These experimental results have been interpreted with Monte Carlo calculations in the same ranges of temperature and field. The theoretical model includes the many-valley structure of the Si conduction band, acoustic intravalley scattering with correct momentum and energy relaxation and correct equilibrium phonon population, several intervalley scatterings, and ionized impurity scattering.

Theory of Two-Dimensional Electron-Hole Liquids –Application to Layer-Type Semiconductors–

The ground state energy of two-dimensional electron-hole metallic liquid is calculated with use of the generalized random phase approximation of Hubbard. The following two cases are treated to investigate effects of dimensionality and many-valley structure: (i) the system with a single extremum is conduction and valence bands and (ii) the system with the many-valley structure in the conduction band. It is shown that the ground state energy of the two-dimensional system is four times larger than that of corresponding three-dimensional system in the case (i) while it becomes more than four times larger in the case (ii) where the effective mass of electrons is lighter than that of holes. A possibility of producing the electron-hole pancake in highly excited layer-type semiconductors is pointed out. The dispersion relations of two-dimensional plasmons and acoustic modes are also derived.

A detailed analysis of the metal-semiconductor contact

A general quantum and electronic theory able to explain the electric and photoelectric experimental properties of the metal-semiconductor contacts is proposed. The theory consists firstly in calculating the electric space charge due to the quantum mechanical tunneling of the electrons from the metal into the semiconductor, and vice-versa, and to the metal and semiconductor bands bending. Then the electric charge so obtained is utilised to solve in an appropriate and complete way the Poisson equation so as to determine the electric field and potential as functions of the abscissa x. The electric field F( x) is employed to obtain a new expression for the junction capacitance C, holding in the general case of a non-uniform charge, whereas the electric potential ν i( x) is used to calculate general expressions for the thermionic and photoelectric currents i and i ph , respectively, taking into account in this both the tunneling probability through the energy barrier and the many-valley structure of the semiconductor energy bands. Finally, from ν i( x), C, i and i ph four new expressions of the energy barrier height of the contact are deduced. The theoretical results relative to the barrier height so determined (which hold for both n-and p-type semiconductors) are compared with published experimental values obtained, by means of capacitance and photocurrent measurements: (a) on contacts between n-type CdS and Au, Cu, Ag and Pt; (b) on contacts between n-type GaAs and Au, Ag, Cu, Sn, Al and Pt and; (c) on contacts between p-type GaAs and Au and Al. The agreement between the theoretical and experimental values is very good.

Galvanomagnetic properties and band structure of SrTiO3between 4.2 and 300 K

Abstract The magnetoresistance of Nb-doped n-type SrTiO3 was measured between 4.2 and 120 K. The data are discussed in light of the band structure calculations of Kahn and Leyendecker as well as Mattheiss. They support rather a warped constant energy surface with (100) fingers of the conduction band than a many-valley structure in the cubic phase.

Longitudinal Magnetoresistance inn-Type Germanium: Experimental

Measurements of the longitudinal magnetoresistance of single crystals of pure and doped $n$-type germamanium oriented in the $〈100〉$, $〈110〉$, and $〈111〉$ directions have been made in magnetic fields up to 300 kgauss over the temperature interval 20\ifmmode^\circ\else\textdegree\fi{}-300\ifmmode^\circ\else\textdegree\fi{}K. The magnetoresistance ratio $\frac{\ensuremath{\rho}(H)}{\ensuremath{\rho}(0)}$ was found to vary linearly with magnetic field strength in the quantum limit. Magnetoresistance ratios less than one were observed and explained on the basis of the many-valley structure of the conduction band. The saturation of magnetoresistance predicted by classical transport theory was observed at the higher end of the temperature range and used to demonstrate a temperature variation of the anisotropy parameter $K$.