Amorphization of vortex matter and indication of a reentrant peak effect in YBa₂Cu₃O_7-δ

Abstract

The peak effect (PE) has been observed in a twinned crystal of YBa₂Cu₃O_7-δfor H||c in the low-field range, close to the zero-field superconducting transition temperature [T_c(0)]. A sharp depinning transition succeeds the peak temperature T_p of the PE. The PE phenomenon broadens and its internal structure smoothens out as the field is increased or decreased beyond the interval between 250 and 1000 Oe. Moreover, the PE could not be observed above 10 kOe and below 20 Oe. The locus of the Tp(H) values is indicative of the occurrence of a reentrant characteristic with a noselike feature located at T_p(H)/T_c(0)≈0.99 and H≈100 Oe (where the flux line lattice constant a₀≈penetration depth λ). The upper part of the PE curve (0.5<H<10 kOe) can be fitted to a melting scenario with the Lindemann number c_L≈0.25. The vortex phase diagram near T_c(0) determined from the characteristic features of the PE in YBa₂Cu₃O_7-δ(H||c) bears close resemblance to that in the 2H-NbSe₂ system, in which a reentrant PE had been distinctly observed earlier.