Determination of magnetic penetration depth from saddle-point field analysis in Tl2Ba2Ca2Cu3O10+'

Tl205 NMR spectra and spin-lattice relaxation in the normal and superconducting states were measured on an aligned sample of Tl2Ba2Ca2Cu3O10+'. The spin-lattice relaxation time, T1, was found to be isotropic and, in the normal state, 1/T1T to be temperature dependent, indicating the presence of spin fluctuations in the thallium-oxygen plane similar to that found in the copper-oxygen plane of YBa2Cu3O7. $ sup 205"Tl NMR spectra at 5 K were studied and the angular dependence of the field shift at the saddle points of the magnetic-field distribution was found to be consistent with the anisotropic London theory. The average penetration depth was inferred to be 7040350. Our approach to determine the penetration depth, the saddle-point field analysis, is insensitive to distortion of the flux lattice associated with pinning. This is in contrast to the more commonly used second-moment method.