Local fields in YBa2Cu3O7-delta measured on Yb-170(3+) Mossbauer probes. I. Volume and surface penetration lengths

We describe a method for measuring the local fields arising from the vortex lattice in YBa2Cu3O7 - delta which is based on the field dependence of the electro-nuclear energy levels of non-perturbing Yb-170(3+) Mossbauer probe s. From 4.2 K measurements of the second moment of the field distribution a fter a field-cooled (FC) treatment, we obtain the bulk penetration lengths (lambda(ab)) for under-, optimum- and overdoped sintered samples and we est imate the evolution of the condensate density with doping. The overdoped sa mples were obtained by increasing the oxygen content or by adding 20% Ca. W e find the presence of Ca increases the condensate density above the level obtained by adding oxygen to fill the chains and the condensate density of the planes increases at the same time that T-c decreases. For an optimally doped grain-oriented sample of YBa2Cu3O7 - delta, we obtain the 4.2 K bulk penetration lengths (lambda(ab) and lambda(c)) and we also obtain estimates for the surface penetration lengths by measuring the induction appearing i n the surface layers of the grains when fields below H-c1 are applied after a zero-field cooling treatment. The surface penetration lengths are found to be not representative of the lengths obtained from within the bulk. (C) 1999 Elsevier Science B.V. All rights reserved.