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‐Tc superconductor YBa2Cu3O7-\delta as a function of temperature T and magnetic field H. The magnetic field distribution in high quality single crystals of YBa2Cu3O6.95 was fit assuming a triangular vortex lattice with an average in‐plane penetration depth \lambdaab=1155(3) Å, extrapolated to zero temperature and magnetic field. We find that \lambdaab(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_3O6.60. The lineshapes for YBa2Cu3O6.60 clearly show the high‐field cutoff due to the finite coherence length \xiab. A preliminary analysis indicates that \xiab(T,H) increases linearly as a function of temperature and decreases with increasing magnetic field.