Determination of the magnetic penetration depth of the high-T_c superconductor YBa₂Cu₃O_7–x by polarized neutron reflection

Abstract

The recent discovery of high-temperature superconductivity in La_1.85Ba_0.15CuO₄¹ and YBa₂Cu₃O_7–x², with critical temperatures T_c of ∼40 K and >90 K respectively, has resulted in great activity in the investigation of these materials. Associated with the Meissner effect is the well-known surface flux penetration; a penetration depth of ∼1,500 Å has been derived³ for YBa₂Cu₃O_7–x, and values of ∼ 2,000 Å were measured by muon spin rotation^4,5 for La_1.85Ba_0.15CuO₄. Felcher and co-workers^6,7, in their work on superconducting films of niobium, lead and lead–bismuth, have used the reflection of spin-polarized slow neutrons to obtain a direct and absolute measurement of the penetration depth. We report here the first such measurement of the penetration depth in a sample of YBa₂Cu₃O_7–x. At a temperature of 4.8 K and in an applied magnetic field of 350 oersteds we obtain a value, which represents an upper limit, of 225±75 Å, which is small compared with penetration depths in conventional superconductors⁸, and with the recently quoted values for YBa₂Cu₃O_7–x³ and La_1.85Ba_0.15CuO₄^4,5.