Highly anisotropic gap function in a nonmagnetic superconductor Y ₅Rh₆Sn₁₈

The low temperature specific heat of R₅Rh₆Sn ₁₈ (R = Y, Lu) was measured under various magnetic fields (H) up to 8 T. In a zero field, electronic specific heat of Y₅Rh ₆Sn₁₈ shows a T³ dependence, indicating an anisotropic superconducting gap, although that of Lu₅Rh ₆Sn₁₈ shows an exponential dependence. In addition, the coefficient of the T-linear term γ(H) of Y₅Rh ₆Sn₁₈ in specific heat in the mixed state is found to obey a square root dependence. This is another evidence that Y₅Rh ₆Sn₁₈ has the anisotropic superconducting gap. On the other hand, the γ(H) of Lu₅Rh₆Sn₁₈ shows linear dependence, which means the isotropic superconducting gap.