Superconducting gap anisotropy probed by ultrasonic attenuation

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Ultrasonic attenuation a of highly anisotropic g-wave superconductor YNi_2B_2C has been measured for all the symmetrically independent elastic modes, to explore the location of zero superconducting gap region on the Fermi surface. The attenuation of longitudinal mode shows pronounced anisotropy i n the superconducting state : While a shows thermally activated behavior along [110] and [001], it shows -linear dependence along [100]. These results together with those for the transverse modes demonstrate the presence of point nodes or zero-gap regions along [100] and [010] directions. This is a clear demonstration of ultrasonic attenuation as a powerful probe for the structure of anisotropic superconducting gap.