Electron Conduction Processes in Tantalum-Germanium Multilayers
An explosion of both theoretical and experimental research into structurally disordered materials in the late 1970s has greatly increased our understanding of these complex systems. A number of facets of the conduction processes remain unexplained, however, particularly in the area of non-simple metals. Multilayers of disordered tantalum and amorphous germanium with individual layer thicknesses of between 4 & 120A [Angstrom] and 13 & 220A [Angstrom]respectively have been prepared by vapour deposition and the in-plane resistance measured from 1.5 to 300K. Results for samples with germanium layers of sufficient thickness to prevent tunnelling between the conducting tantalum layers can be interpreted in terms of conduction in the tantalum layers alone. In these samples the behaviour of the resistance as a function of temperature and the tantalum layer thickness can be explained in terms of the interplay between quantum interference effects and disorder enhanced electron-electron interaction effects. At high temperatures the negative temperature coefficient of resistance arises from the destruction of coherent interference in the backscattered direction by phonons. From the data, the electron-phonon scattering rate is found to be comparable in magnitude to that expected for scattering in either the "clean" or "dirty" limits while the temperature dependence of the scattering rate lies between that expected for each of these limits. At lower temperatures a turn over to a positive temperature coefficient of resistance is seen as spin-orbit scattering and superconducting fluctuations become important. At still lower temperatures the resistance is dominated by electron-electron interaction effects and we have observed a transition from three-dimensional to two-dimensional behaviour as the tantalum layer thickness is reduced. Evidence for the onset of superconductivity is seen for samples with a low temperature sheet resistance of less than 3000 Omega/whitesquare. We have also investigated samples with thin germanium layers (<40A [Angstrom]) in which coupling between the layers causes an increase in the superconducting transition temperature. We present some preliminary measurements which suggest that the transition from isolated to coupled tantalum layers, as the germanium layer thickness is reduced, can be followed in the form of the fluctuation conductivity.