We report on pronounced structures in the current-voltage characteristics of intrinsic Josephson junctions in the high- $T_c$ superconductors ${\mathrm{Tl}}_2{\mathrm{Ba}}_2{\mathrm{Ca}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{10+\delta }$ and ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$. The structures appear well below the superconducting gap, independent of sample geometry, magnetic field, and temperature up to ${0.5T}_c$. We discuss possible explanations and show that the structures could be explained by a single peak in the quasiparticle current, possibly arising from a subgap structure in the quasiparticle density of states. For superconducting superlattices a subgap peak has been predicted and could explain our results.

• Received 6 February 1996

DOI:https://doi.org/10.1103/PhysRevLett.76.4943