Logarithmic ac response in the heavy-fermion superconductor URu2Si2

Ac-magnetic-susceptibility χac(T) measurements as a function of frequency f and excitation field hac have been performed on a single crystal of the heavy-fermion superconductor URu2Si2 in magnetic fields H up to 30 kOe applied along the c axis. The apparent superconducting transition temperature Tc shows an unexpected and approximately logarithmic increase (decrease) with f (hac) that is enhanced with H, which suggests the existence of an irreversibility line below the upper critical field Hc2(T) in URu2Si2. Isothermal χac vs H sweeps were used to define Hc2(T) which was independent of f and hac