Yasuhiro NAKAZAWA and Kazushi KANODA
[Phys. Rev. B 55, R8670 (1997)]
The low-temperature specific heat of single crystals of (kappa)-(BEDT-TTF)2Cu[N(CN)2]Br was studied in a temperature range between 0.11K and 4.5K and in magnetic fields up to 2T. The electronic specific heat Cel, in the superconducting state shows a quadratic temperature dependence. As is shown in Figure 1, the magnetic-field dependence of the temperature-linear term (gamma) obeys (gamma)=k(gamma)n(H/Hc2)1/2 relation, where Hc2=10T, (gamma)n=22mJ/molK2, and k=0.80. These results support the existence of line-nodes in the superconducting gap and therefore suggests that the symmetry of electron pairs in this material is unconventional with anisotropic wave function. The existence of line nodes is most reasonably attributable to the two-dimensional dx2-y2 wave model or anisotropic d+s wave model recently discussed for high-Tc cuperates.
Figure 1. Field dependence of the (gamma) term in the electronic specific heat.