Determination of the magnetic penetration depth of the high-Tc superconductor YBa2Cu3O7–x by polarized neutron reflection

Abstract

The recent discovery of high-temperature superconductivity in La1.85Ba0.15CuO41 and YBa2Cu3O7–x2, with critical temperatures Tc 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 A has been derived3 for YBa2Cu3O7–x, and values of ∼ 2,000 A were measured by muon spin rotation4,5 for La1.85Ba0.15CuO4. Felcher and co-workers6,7, 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 YBa2Cu3O7–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 A, which is small compared with penetration depths in conventional superconductors8, and with the recently quoted values for YBa2Cu3O7–x3 and La1.85Ba0.15CuO44,5.