ANOMALOUS ELECTRICAL-RESISTIVITY IN THE SUPERCONDUCTING STATE OF CERU2

Low frequency ac electrical resistivity p measurements were performed as a function of magnetic field H and temperature T on a polycrystalli ne sample of CeRu2 with a zero field superconducting critical temperat ure T-c of 6.1 K. These measurements revealed the development of a fin ite resistance anomaly in the superconducting state well below the upp er critical field H-c2(T). When H is increased at constant T, the anom aly takes the following form: p(H) becomes finite at a field H-1, incr eases slowly at first and then nearly linearly, drops abruptly to zero at a field H-2, and remains zero until it increases to its normal sta te value around H-c2. The transition into the state with finite resist ivity is reversible at H-1 but hysteretic at H-2; H-1, H-2, and H-c2 a pproach each other with increasing T, while the hysteresis at H-2 decr eases with increasing T and vanishes at T = 5.4 K. The anomaly is also observed in p(T) measurements at constant N. In some regions of the H -T plane near H-1(T), the resistivity exhibits transient behavior that could be due to flux jumps. The characteristic field H-2 corresponds to the field where the onset of irreversible behavior of the magnetiza tion M(H) is observed. The anomaly is very sensitive to the transport current density and appears to be due to a decrease and subsequent inc rease of fluxoid pinning as H-c2(T) is approached from within the mixe d state.