Superconducting state in a crystal of low symmetry: Impurity effect in an extendeds-wave superconductor with nodes in the excitation gap

It is argued that an extended s-wave superconducting state having order parameter that changes its sign on the Fermi surface is likely to occur in crystals of low symmetry such as organic conductors, if the pairing interaction is of electronic origin. Impurity effects in those superconducting states are then studied. Special attention is paid to the nuclear magnetic relaxation rate 1/T₁. It is found that impurities give rise to peculiar behavior of 1/T₁ such as a coherence peak just below the transition temperature and T-linear variation, caused by impurity-induced finite density of states at the chemical potential, at low temperatures, at moderate impurity concentration.