A.P. van Gelder

Point-contact spectroscopy in metals

Point contacts between normal metals at low temperatures show very interesting non-linear phenomena in the current-voltage characteristics. The observed deviations from Ohms law in metallic constrictions can be used for an energy-resolved spectral analysis of the interaction mechanisms of the conduction electrons with elementary excitations in a metal. A review is given on the theoretical and experimental work dealing with this spectroscopic method. A theoretical analysis, involving an iterative solution of the full non-linear Boltzmann equation for the transport problem of a current through a point-contact geometry, is able to show that the observed second-derivative signal is proportional to Eliashbergs form of the electron-phonon interaction alpha 2F( omega ), with a slight modification due to a transport efficiency function. Non-equilibrium effects of the phonon system are discussed in relation to the observed background signal on which the alpha 2F signal is thought to be superimposed. The theory can easily be extended to other scattering mechanisms than the electron-phonon interaction, and some experiments show that the interactions of conduction electrons with magnons and paramagnetic impurities can be detected using point contacts between ferromagnetic metals and between magnetically dilute alloys.

A theory for the α2F-dependence of the electrical impedance of a point contact between metals

Abstract The current-voltage characteristic of a metal-metal point contact has been calculated via a solution of the semi-classical Boltzmann transport equation. Electron-phonon interaction is contained in this procedure. The point contact current-voltage characteristic appears to be mainly due to processes involving the spontaneous creation of a single phonon. The resulting signal is proportional with α2F, but it also depends on the tunneling probability T of the contact. Double-phonon creation processes, and electron-hole pair production ones are considered as contributions to the background signal.

Application of point-contact spectroscopy in metals to the Kondo problem

A general relationship between the characteristic structure in the electrical resistance of a point contact between two metals and the dynamics of the scatterers is used to discuss new experimental results on Kondo systems. The method allows one to measure the energy dependence of the relaxation time of the conduction electrons directly.

Localized polariton modes of small cubic crystals

Localized polariton modes of small cubic crystals are studied. The eigenfrequencies of these modes are calculated for small cubic crystals in the continuum approximation, using a variational approach. An attempt was made to observe these modes by measuring the far infrared transmission of small cubic AgBr crystals.

On the structure of the d2J/dV2 characteristics of point contacts between metals

Abstract A generalized relation between the main structure of d 2 J/dV 2 characteristics of point contacts with α 2 (ω)F(ω) and with spontaneous emission backflow processes is given. The background structure is explained in terms of stimulated emission and absorption backflow processes.

A new mechanism for transverse linear magnetoresistance in simple metals

Abstract A quantum mechanical theory of electrons bound magnetically to internal surfaces or dislocations, similar to the more familiar externally bound magnetic surface states, is shown to yield a linear magnetoresistance in parameter ranges consistent with experimental realities.

A new method for measuring the conductivity of arbitrarily shaped two-dimensional samples

Abstract A new method is proposed to determine the assumed homogeneous conductivity of a thin sample with uniform thickness and arbitrary shape. Positions of the current and voltage contacts are likewise arbitrary and need not be restricted upon the samples edge (periphery of its domain).

Edge effect in near‐intrinsic germanium observed with a novel method for resistivity measurements

We report the observation of edge effects in the magnetoconductance of macroscopic samples of near‐intrinsic germanium near room temperature as a model system for an ambipolar conductor. We used a novel method for resistivity measurements, which allows for a check of the consistency of a description with a single resistivity tensor for the whole sample. Such a description was found to fail near the magnetic field strengths with a vanishing Hall coefficient. The inconsistency is attributed to the spatial variation of the density of charge carriers, which is caused by the unbalance of the influxes of carriers to the sample’s edge and their recombination/generation via traps in the edge region.

3-D magneto-ballistic transport

Abstract Magnetoresistance measurements of Al/Al point contacts are a delicate probe for the boundary condition of wave functions at the insulating interface. Vanishing of the normal derivative is necessary to explain the order of magnitude of observed signals.