Nodal structure of unconventional superconductors probed by angle resolved thermal transport measurements

Over the past two decades, unconventional superconductivity with gap symmetry other than s wave has been found in several classes of materials, including heavy fermion, high T-c, and organic superconductors. Unconventional superconductivity is characterized by anisotropic superconducting gap functions, which may have zeros ( nodes) along certain directions in the Brillouin zone. The nodal structure is closely related to the pairing interaction, and it is widely believed that the presence of nodes is a signature of magnetic or some other exotic, rather than conventional phonon mediated, pairing mechanism. Therefore experimental determination of the gap function is of fundamental importance. However, the detailed gap structure, especially the direction of the nodes, is an unresolved issue for most unconventional superconductors. Recently it has been demonstrated that thermal conductivity and specific heat measurements under a magnetic field rotated relative to the crystal axes provide a powerful method for determining the shape of the gap and the nodal directions in the bulk. Here we review the theoretical underpinnings of the method and the results for the nodal structure of several unconventional superconductors, including borocarbide YNi2B2C, heavy fermions UPd2Al3, CeCoIn5, and PrOs4Sb12, organic superconductor kappa-(BEDT-TTF)(2)Cu(NCS)(2), and ruthenate Sr2RuO4, determined through angular variation of the thermal conductivity and heat capacity.