ON THE MAGNETIC STUDY OF THE PEAK EFFECT IN THE ANISOTROPIC SUPERCONDUCTOR 2H-NBSE2 - EVIDENCE FOR REENTRANT BEHAVIOR

We present the results of DC and AC magnetization measurements of the peak effect (PE) at low fields in several samples of an anisotropic su perconductor, 2H-NbSe2. Close to the T-c(H) line, the PE makes itself manifest as a dip feature in the in-phase (chi') AC susceptibility dat a. From our temperature- and field-dependence studies we construct the loci of this dip feature for H\\c and H\\alpha in the H-T plane. We f ind that the PE temperature (T-peak) increases as H decreases from the high-field end to about 200 Oe. However, between 200 Oe to about 30 O e, T-peak decreases as H decreases from about 150 Oe to 50 Oe. Relying on the recent observations that the PE phenomenon in the 2H-NbSe2 sys tem can be associated with the melting of its soft flux-line lattice, we interpret our central result, namely, the two intersections of T-pe ak(H) curve with a fixed T line, in terms of reentrant behavior of the flux-lattice melting line at low fields. We believe that such a reent rance of the melting line, first proposed on theoretical grounds by D. R. Nelson in 1988, has not been unambiguously experimentally elucidate d in any superconducting system prior to our new data on 2H-NbSe2.