Magnetic Screening and Flux Distribution in the High- <i>T</i> _c Superconductor La _1.85 Sr _0.15 CuO ₄

Abstract

The magnetic screening and flux distribution within a polycrystalline sample of La1.85Sr0.15CuO4 has been measured by means of muon spin rotation spectrocopy (μSR) between 7.6 K and Tc = 34 K in applied fields of 10 Oe to 2 kOe. Above 500 Oe the transverse field relaxation rate of the μ+ spin polarization is quasi-field-independent, as expected for a superconducting material in the mixed state. At low field, a clear reduction of the relaxation rate is observed, interpreted as the disappearance of the mixed state. However, the average internal field at the muon sites, as deduced from the μ+ Larmor frequency does not reveal a sizeable Meissner effect in clear contradiction with the experimental results of the d.c. magnetization carried out on the same sample.