Paramagnetic Meissner effect analyzed by second harmonics of the magnetic susceptibility: Consistency with a ground state carrying spontaneous currents.

Abstract

It has been reported recently by Braunisch et al. [Phys. Rev. Lett. 68, 1908 (1992)] that some ceramic high-${\mathit{T}}_{\mathit{c}}$ superconductors (HTSC's) exhibit a paramagnetic signal if cooled through the superconducting transition temperature in small external magnetic dc fields (${\mathit{H}}_{\mathrm{dc}}$1 Oe). In the present work, this paramagnetic Meissner effect (PME) is experimentally confirmed and further experimental details are reported on this phenomenon. This is accomplished by applying a recently developed compensation technique based on the measurement of the second harmonic of the magnetic ac susceptibility in external dc fields. This technique allows one to detect sensitively the dc magnetization of a HTSC sample and makes it possible to observe the PME under conditions where its dc superconducting-quantum-interference-device signal is totally masked by the dominating diamagnetic behavior of most of the sample. The experiments were performed on melt processed ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8\mathrm{\ensuremath{-}}\mathit{x}}$ samples, some of which did not exhibit the PME. Thus, a comparison was possible between samples with and without the PME, subjected to different field-cooling procedures. The experimental results are consistent with the occurrence of a reversible transition into a PME state carrying spontaneous orbital currents resulting in corresponding paramagnetic moments, which can be reversibly aligned by small external dc fields. This PME state is attributed to loops containing relatively strong weak links, which are able to remain superconducting close to the bulk ${\mathit{T}}_{\mathit{c}}$ of the grains (e.g., at T/${\mathit{T}}_{\mathit{c}}$=0.9) and in external dc fields of at least 30 Oe.