Mu SR investigation of anisotropy and temperature dependence of magnetic penetration lengths in crystals of YBa₂Cu₃O_7-d

Pulsed mu SR measurements on an aligned mosaic of single crystals of YBCO are interpreted in terms of an anisotropic magnetic penetration length arising from currents flowing parallel and perpendicular to the CuO₂ planes. At low temperatures a mean penetration depth ( lambda ₁ lambda ₃)^1/2 of 502 nm is reduced from measurements in fields parallel to the ab plane. Relaxation rate measurements for fields at angles up to 24 degrees out of the plane exhibit a marked angular anisotropy. The observed anisotropy implies a ratio of effective masses perpendicular and parallel to the CuO₂ planes of at least 30. The values obtained are compared with those obtained by other authors and techniques. A different temperature dependence of the relaxation rate is observed for fields parallel and at 24 degrees to the CuO₂ planes.