Temperature and magnetic field dependences of the London penetration depth and coherence length in the vortex state of YBa2Cu3O7-delta

We report TF‐μ⁺SR measurements of the London penetration depth in the high‐T_c superconductor YBa₂Cu₃O_7-delta as a function of temperature T and magnetic field H. The magnetic field distribution in high quality single crystals of YBa₂Cu₃O_6.95 was fit assuming a triangular vortex lattice with an average in‐plane penetration depth lambda_ab=1155(3) Å, extrapolated to zero temperature and magnetic field. We find that lambda_ab(T,H) increases linearly both as a function of magnetic field and temperature in the field range studied. This unusual behaviour is attributed to an unconventional pairing scheme in which there are line nodes in the superconducting energy gap function. Measurements have also been made on a deoxygenated sample, YBa_2Cu_3O_6.60. The lineshapes for YBa₂Cu₃O_6.60 clearly show the high‐field cutoff due to the finite coherence length xi_ab. A preliminary analysis indicates that xi_ab(T,H) increases linearly as a function of temperature and decreases with increasing magnetic field.