Tunneling and theoretical studies have suggested that Andreev bound states form at certain surfaces of unconventional superconductors. Through studies of the temperature and field dependence of the in-plane magnetic penetration depth ${\lambda }_{\mathrm{ab}}$ at low temperature, we have found strong evidence for the presence of these states in clean single crystal YBCO and BSCCO. Crystals cut to expose (110) surfaces show a strong upturn in ${\lambda }_{\mathrm{ab}}$ at around 7 K, when the field is oriented along the $c$ axis. In YBCO this upturn is completely suppressed by a field of $\sim 0.1\mathrm{T}$.

• Received 14 August 2000

DOI:https://doi.org/10.1103/PhysRevLett.86.1074