G.J. Morgan

The hall effect in amorphous metals

Abstract We first derive a new linear response formula for the Hall effect (R H ) showing that R H only depends on electrons at the Fermi energy. We then calculate R H using the random phase model for ‘amorphous’ Ni and show that although d-electrons may contribute substantially to the conductivity they do not dominate in the Hall effect and the Hall coefficient is negative for this model. We also show how the Hall effect may be simulated by computer experiments on electron diffusion in a magnetic field.

The electronic structure and conductivity of large models of amorphous silicon

Calculations of the electronic properties of very large models of amorphous silicon are presented using Chadis tight-binding model and the equation-motion method. The electronic density of states and the conductivity are calculated for structures containing up to 13824 atoms generated using molecular dynamics techniques. The structures contain defects, but the importance of this work is that one can make comparisons with earlier calculations using pseudopotentials, work with larger structures, use longer run times and lay the basis for calculations on hydrogenated a-Si and calculations of more complex properties, such as the Hall coefficient which is a longstanding problem.

The double-sign anomaly of the Hall coefficient in amorphous silicon: Verification by computer simulations

Abstract We present the first computer simulations of the Hall effect for a realistic model of a solid, namely amorphous silicon, and succeed in verifying the anomalous change in sign of the Hall coefficient as the Fermi energy is moved from the valence band to the conduction band in accordance with the observations of LeComber, Jones and Spear in 1977. A model for this behaviour is proposed based on previous computations of the spectral function and a theory of gap formation.

Induced anisotropy of conductivity and electronic structure of glassy ZrNi alloys

Abstract A change in resistivity, due to uniaxial stress in the elastic region has been measured for several amorphous Zr 1− x Ni x alloys. The strain gauge factor ( K ) decreases linearly with x and is independent of temperature (80 K T 1− x Ni x alloys γ decreases faster with x than in Zr 1− x Cu x , and reaches zero for x = 0.65. This variation of γ with x compares well with the observed decrease in the electronic density of states at the Fermi level with x and lends further support to the relation between γ and the d-electron contribution to conductivity.

New applications of the equation-of-motion method: Optical properties

The equation-of-motion method offers advantages in the calculation of properties of large structural models, of the kind recently developed for a-Si (104 or more atoms). Originally developed for calculating densities of states, it has been shown to be adaptable to a wide variety of other purposes, including the study of localization, conductivity and the Hall effect. Recently we have shown how it may be used to calculate linear optical properties as a function of frequency. Here we present new calculations for a-Si and a-Si:H, using various structural models. The method may be further extended to evaluate nonlinear optical coefficients χ(2)(ω) and χ(3)(ω); we are applying it to the study of χ(3)(ω) for crystalline and amorphous silicon. A conventional approach to this problem, such as calculating eigenstates, would be totally impractical.

Magnetization processes in amorphous ribbons influenced by electrical current in the ribbon

Abstract Measurements of hysteresis loops and derivatives of magnetization with time versus applied field have been performed on amorphous Fe- and Co-based ribbons with a current flowing along the ribbon. Experiments have also been performed with aluminum foils glued onto surfaces of amorphous ribbons, with current passing through them.

The electronic structure and diffusivity for a fully bonded model of amorphous Si at T = 0K

Abstract The electronic conductivity, diffusion coefficient and the self-energy in the vicinity of the energy gap of amorphous Si at T=0K have been computed as a function of energy for a fully bonded model of Si. We have used the equation-of-motion method in κ-space to show, for the first time, the probable position and existence of mobility edges. The conductivity rises rapidly away from the mobility edges to take values typical of those found in liquid metals. The behaviour of the self-energy indicates that a resonance phenomenon is responsible for the formation of a gap.

The hall coefficients of CuTi and CuHf amorphous metal alloys

Abstract We present results for the measured Hall coefficient of a wide range of compositions of Cuti and CuHf amorphous metallic alloys over the temperature range 4.2−270K. The Hall coefficients are found to be independent of temperature and to change sign as a function of composition, form negative to positive, as the transition metal concentration is increased. This change of sign is discussed as a result of s-d hybridisation leading to negative group velocities at the Fermi energy.

The density of states in amorphous Si

Abstract We have used the equation of motion method in k -space to compute the spectral function and density of states for two different models of amorphous Si containing 216 atoms. The method has considerable potential for self consistent calculations and the calculation of transport properties.

The electronic spectral function for an amorphous metal

Abstract We have used the equation of motion method to calculate the spectral function for s and d-electrons using a hard sphere model of an amorphous metal. The effects of hybridisation are presented for a range of band parameters. A simple model of photoemission is then used to calculate the photoelectric current for two excitation energies. The results are in reasonable agreement with the random phase model.