Search for broken time-reversal symmetry near the surface of YBCO films   using Beta-NMR
  Weak spontaneous magnetic fields are observed near the surface of YBCO films using Beta-detected Nuclear Magnetic Resonance. Below Tc, the magnetic field distribution in a silver film evaporated onto the superconductor shows additional line broadening, indicating the appearance of small disordered magnetic fields. The line broadening increases linearly with a weak external magnetic field applied parallel to the surface, and is depth-independent up to 45 nm from the Ag/YBCO interface. The magnitude of the line broadening at 10 K extrapolated to zero applied field is less than 0.2 G, and is close to nuclear dipolar broadening in the Ag. This indicates that any fields due to broken time-reversal symmetry are less than 0.2 G. 
H. Saadaoui, G. D. Morris, Z. Salman, Q. Song, K. H. Chow, M. D.   Hossain, C. D. P. Levy, T. J. Parolin, M. R. Pearson, M. ...
cond-mat.supr-con