Polarized-neutron study of the YBa2Cu3O7 system: Granular versus bulk superconductivity

Abstract

Neutron depolarization measurements have been carried out in order to investigate superconductivity in the $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-x}$ system on both powdered and sintered polycrystalline samples. This technique provides indirect evidence of superconductivity through magnetic-flux trapping in low fields as well as vortex-line penetration in higher fields. Powdered and sintered samples show markedly different behavior, probably connected to their granular nature: a sintered sample traps magnetic flux by developing shielding currents when a low magnetic field (<100 Oe) is removed after being applied below the superconducting temperature $T_c$, whereas a powder does not. $T_c$ can be well determined (about 90 K) whereas $H_{c1\mathrm{}}\mathrm{}\left(T\right)\mathrm{}$ can only be estimated.