Damping Effect on Spin-Lattice Relaxation Rate in Organic Superconductors

The electrical resistivity in organic superconductors show the T 2 -dependence at low temperatures above T c . This fact suggests that the electron-electron interaction determines the damping of quasi-particle in the Fermi liquid state. In this paper, we calculate selfconsistently the temperature dependence of damping rate due to the electron-electron interaction. Then, using this damping rate, we calculate the temperature dependence of NMR relaxation rate, for the two cases with s and d -pairing states, respectively. Our result shows that, when the damping rate at T c is of the same order as T c , no coherence peak appears in T -1 1 even if the superconducting state possesses s -wave symmetry. However, we cannot eliminate a hump structure appearing at low temperatures, at which the s -wave gap begins to open against the damping effect. On the other hand, the relaxation rate for d -wave decreases smoothly. This is the important difference in determining the pairing symmetry. We discuss the comparison of our result with experimental data for several systems.