Point-contact spectroscopy on heavy-fermion superconductors

Hints at unconventional superconductivity are present in many heavy-fermion superconductors. Among other methods, point-contact spectroscopy is an important tool to study the symmetry and the nodal structure of the energy gap of unconventional superconductors. Andreev reflection of charge carriers at the interface between a normal metal and a superconductor is the main mechanism for a current at low bias through a nanosized constriction, leading to maxima in the differential conductance d I / d V 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. We will review recent developments in this field and focus particularly on investigations of CeCoIn 5 , a possible d-wave superconductor.