Andreev-reflection experiments on layered superconductors

Excitations of a solid can be investigated by using ballistic electrons in point contacts as a probe. In particular, point-contact spectroscopy of normal metal-superconductor contacts is an important tool to study the symmetry and the nodal structure of the energy gap of the superconductor. The main mechanism for a current at low bias through a nanosized constriction is Andreev reflection of charge carriers at the interface between a normal metal and a superconductor. Andreev reflection leads to maxima in the differential conductance dI/dV vs. V. The position of the maxima depends not only on the size of the gap along given directions, but also on the order-parameter symmetry, the microscopic details of the contact and the barrier strength. In addition, if the order parameter exhibits a sign change an Andreev surface bound state leads to a conductance anomaly at zero bias. Here, recent investigations of some layered superconductors by point-contact spectroscopy are reviewed.