Magnetic Field Dependence of the Low-Temperature Specific Heat of the Borocarbide Superconductor LuNi₂B₂C

The low temperature specific heat of the borocarbide superconductor LuNi₂B₂C was measured under various magnetic fields (H) up to 8 T. The coefficient of the T-linear term in specific heat in the mixed state shows marked deviations from the H-linear dependence expected for conventional superconductors and is found to obey a √H dependence. A good description of the data can be obtained by assuming the presence of a zero-gap region along lines on the Fermi surface, indicating the possibility of unconventional pairing in the superconducting state of the borocarbides.