F. Baumann

Comparison of amorphous and liquid alloys by photoelectron spectroscopy

Abstract Photoemission data of liquid AuSn alloys are presented and compared with corresponding measurements on amorphous vapour-quenched films. A fairly close general similarity between the valence bands of the two phases is observed. This result supports earlier studies of the atomic structure of the amorphous films which have been interpreted in terms of a liquid-like structure with a strong tendency to compound formation within the nearest-neighbour arrangements. Distinct deviations of spectra measured on the liquid samples compared with the amorphous phase have been observed; the Au 5d band is shifted to lower binding energies and no density-of-states minimum is observed at EF. The Au d band exhibits a continuous temperature-dependent shift in the liquid state which has been observed for the first time and which can be extrapolated to −193 °C in the amorphous phase. The origin of this temperature effect is discussed in terms of changes in the electron density and structural alterations in the short-range order at the gold site.

The specific heat and upper critical field of amorphous - electronic and vibrational excitations

The specific heat C of amorphous films was investigated between 0.3 K and 7 K in the superconducting state as well as in the normal state using a modified AC method. The analysis of the data allows one to determine the electronic density of states at the Fermi energy , the density of states of low-energy excitations and the Debye temperature . The concentration dependence of shows a pronounced minimum at x = 60 at.% in comparison to the free-electron model, in agreement with specific heat and UPS measurements and theoretical calculations published recently. The specific heat of the low-energy excitations is of the same order of magnitude as data available from the literature, and shows a linear temperature dependence. increases with copper concentration and the values agree with those published recently. In addition, the critical magnetic field of the same films was measured. The analysis of these data which should be valid for strongly disordered superconductors yields values of which are about 20% higher than those determined by specific heat measurements. Arguments are given as to why values taken from critical field measurements might be too high. Finally, the specific heat of an amorphous film was measured. was found to be 30% below the free-electron model value, in contrast to results published previously by Rieger and Baumann who found to be in accordance with the free-electron model.

Experimental evidence for a structure-induced minimum of the density of states at the Fermi energy in amorphous alloys

Abstract Photoemission spectra are reported for the valence band of quench-condensed Au x Sn 100−x (0⩽x⩽100) films in the amorphous state. It is found that the valence-band spectra for the 20⩽x⩽80 region, measured just after condensation exhibit a decrease in intensity towards the Fermi energy E F which can be understood in terms of a minimum in the density of states near E F . The minimum seems to be induced by a maximum in the structure factor and is correlated with the stability of the amorphous state.

Thermoelectric power and electrical conductivity of amorphous AuxSb100-x films near the metal-insulator transition

The thermoelectric power S and the electrical conductivity sigma of amorphous AuxSb100-x films have been investigated in the temperature range between about 5 K and 350 K. A detailed experimental investigation is performed near the metal-insulator transition occurring at xc=8.1 at.%Au. For xc<x<16.8 at.% the extrapolated values of sigma (0) show a scaling behaviour with a critical exponent nu approximately=1. In the same concentration range the low-temperature slope of the diffusion thermopower SD, i.e. SD(T)/T mod T to 0, increases very rapidly for samples approaching xc. A linear correlation between SD(T)/T mod T to 0 and 1/ sigma (0) is found which is interpreted in terms of a non-interacting electron system showing an Anderson transition.

Specific heat of amorphous AuxSn100-x at low temperatures-electrons and low-energy excitations

Measurements of the specific heat of amorphous AuxSn100-x films (27<or=x<or=74) between 0.35 K and 6 K in the superconducting as well as in the normal state are reported. In addition the temperature dependence of the upper critical field of these films is determined. In the superconducting state the specific heat shows a contribution proportional to the temperature which is strongly composition dependent and largest for x=74. In the authors opinion this contribution is caused by low-energy excitations well established in amorphous solids. Taking into account this contribution the specific heat in the normal state yields the electronic density of states at the Fermi energy in agreement with the data taken from critical field measurements. Using McMillans formula the calculated bare density of states agrees with the free electron model over the whole concentration range. This result is surprising because recent Hall-effect and photoelectron-spectroscopy measurements indicate strong deviations from the free electron model, especially for samples with high Au concentrations.

Interplay of the atomic and electronic structure in liquid and amorphous AlGe alloys

Abstract It is well known that in simple disordered metallic systems maxima of the structure factor, S ( q ), are related to minima in the electronic density of states (DOS). Photoelectron spectroscopy measurements of liquid Al 75 Ge 25 , liquid Al 65 Ge 35 , and amorphous Al 68 Ge 32 are presented, showing a lowering of the DOS at the Fermi energy in comparison to the pure liquid alloy constituents. The depression is large in the amorphous and small in the liquid state, where it vanishes with increasing temperature. This feature is explained as being induced by the atomic structure. The temperature dependence of the structure-induced depression of the DOS is a consequence of the temperature dependence of the structure factor itself.

Influence of thermal relaxation and ion bombardment on the electronic properties of amorphous SiAu films

Effects of thermal relaxation on amorphous SiAu films are investigated by means of photoelectron spectroscopy and measurements of the electrical resistivity and specific heat. Annealing of the films in the amorphous state increases the resistivity and reduces the electronic density of states at the Fermi energy. These relaxation effects can be reversed by irradiation with 5 keV argon ions at low temperature. The results are discussed in the framework of structural relaxation, clustering and a scaling theory given by McMillan (1981).

Systematics in the electronic structure of amorphous transition metal/tin alloys

Abstract Photoemission measurements were performed on in-situ prepared amorphous TM x Sn 100 − x films (TM = Fe, Co, Ni, Pd). Valence band spectroscopy and core level analysis are in contradiction to the charge transfer model, often used to explain the magnetism and binding of transition metal/metalloid alloys. A comparison of valence-band spectra with theoretical density of states (DOS) curves indicates the existence of topological and chemical short-range order. Band structure calculations explain the stability of these alloys as being due to a hybridization pseudo-gap at the Fermi energy. A similar pseudo-gap is also expected within a nearly-free-electron model.

Thermoelectric power and electrical resistance of thin, quench-condensed Sb/Au bilayers - a study of an amorphous phase at the interface

Our investigation of the electrical resistance and the thermoelectric power of thin Au films on top of amorphous Sb films as a function of increasing thickness of the Au film allows us to compare the data with those on amorphous films with increasing Au content. The comparison shows a substantial similarity of the low-temperature data as well as of the annealing behaviour. This gives additional support to the hypothesis of the formation of an amorphous phase at the interface in layered Sb/Au films. The thicknesses of those parts of the Sb film and the Au film which contribute to the formation of the amorphous interface are determined and agree well with data taken from photoelectron spectroscopy and resistance measurements during evaporation. The quantitative differences which exist in the concentration dependence of the electrical resistance and the thermoelectric power between the Sb/Au bilayers and the amorphous films are probably caused by the reduced geometry of the ultrathin interface.

Synchrotron radiation study on amorphous AuSb and AuSn

Abstract One of the most interesting electronic properties exhibited by amorphous alloys of the noble metal-polyvalent element-type is a decrease in the electronic density of states towards the Fermi level, EF. This reduction has been attributed to the enhanced backscattering by the structure of the amorphous phase of nearly free sp-electrons with Fermi momentum. On the other hand, such reduction has also been suggested to result from the hybridization between sp-electrons and noble metal d-states. In order to obtain information about the angular momentum distribution of the electronic states near EF, photoemission experiments have been performed on in situ prepared amorphous AuSn and AuSn films using synchrotron radiation in the energy range of 17–92 eV. A strong photon energy and angular momentum dependence of the photoionization cross-section has been observed. The results clearly demonstrate that (i) sp-states are dominant near EF and (ii) hybridization effects between sp- and d-electrons can be neglected to account for the minimum in the density of states at EF.