BALLISTIC AND NONBALLISTIC REGIMES IN POINT-CONTACT SPECTROSCOPY

Current carrying states in point contacts provide information concerning quasiparticle scattering mechanisms in solids. If both the elastic, l(imp), and the inelastic, l(epsilon), mean free paths of electrons are larger than the contact diameter, then the non-Ohmic component of the contact current is related to the spectral electron-phonon interaction function g(omega) as d2I/dV2 = - C(l(imp)/d)<Kg(omega)>\(omega)=eV, where K is a kinematic factor. In the diffusive regime similar relation holds with C approximately d/l(imp). The opposite regime of the small mean free path resultes in specific thermal PC spectroscopy, since the hot spot regime sets in the contact with the temperature controlled by the bias according to the relation eV = 3.63 kT. Modulation in V results in modulation of T and therefore in nonlinearity of the CVC. Elastic scattering of electrons by localized excitations as e.g., two-level systems, magnetic excitations, etc., results in population spectroscopy because the effective energy of excitation in a contact crucially depends upon eV. Similarly, if the scattering is elastic, but energy-dependent (as e.g. in the Kondo effect), there appears a nonlinear conponent in the CVC resulting in energy-dependent elastic PC spectroscopy. Trapping of phonons in the contact influences that part of the PC spectrum which lies above the phonon energies. This part can be removed by increasing the modulation frequency up to 10(10) - 10(11) Hz. The frequency dependence of the CVC represents in itself a new kind of PC spectroscopy, namely that of phonon relaxation mechanisms in metals.