Point-Contact Spectroscopy on Conventional and Unconventional Superconductors

Abstract

In the past decades it has become evident that a growing number of metals exhibit exotic types of superconductivity as manifested by their thermodynamic and transport properties. Multi-band superconductivity has attracted renewed interest triggered by experimental results on MgB₂ and Nb₃Sn. Unconventional superconductivity is observed in many U- and Ce-based heavy-fermion superconductors as well as in the oxide superconductors. Among other methods, point-contact spectroscopy is an important tool to study the symmetry and the nodal structure of the energy gap Δ. Andreev reflection of charge carriers at the normal metal/superconductor interface leads to minima at V≈ ± Δ /e in the differential resistance dV/dI as a function of applied bias V and thus allows determination of the gap size. In addition, Andreev reflection causes an excess current through metallic (low-barrier) point contacts and the temperature dependence of the excess current can be analyzed with respect to the order-parameter symmetry. Here, recent developments in this field will be reviewed with a particular focus on investigations of the heavy-fermion superconductor CeCoIn₅, a possible d-wave superconductor, and the oxide superconductor Sr₂RuO₄, a candidate for p-wave superconductivity.