Studies of the Pressure-Dependent Superconducting States of HoNi ₂B₂C and DyNi₂B₂C

We have investigated the pressure dependence of the superconducting states of high-quality single crystals of HoNi₂B₂C and DyNi ₂B₂C through resistivity and magnetization measurements. When a pressure is applied, the superconducting transition temperature, T _c, of HoNi₂B₂C is suppressed while the Néel temperature, T_N, increases with pressure. At the same time, a reentrance behavior found in the temperature dependence of the upper critical field, H_c2(T), of HoNi₂B₂C decreases with increasing pressure and eventually disappears completely above 8.3 kbar. This observation is in sharp contrast with the weak pressure dependence found in H_c2(T) of DyNi₂B₂C. Our experimental findings can be understood in terms of a Ginzburg-Landau type analysis.