Quasiparticle density of states in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ single crystals probed using tunneling spectroscopy at ultra-low temperatures in high magnetic fields

Abstract

Break-junction tunneling spectroscopy at temperatures 30-50 mK in high magnetic field is used to directly probe the quasiparticle density of states within the energy gap in a single crystal Bi2212 high-$T_c$ superconductor. The measured tunneling conductances $dI/dV(V)$ in the subgap region have a zero flat region with no evidence for a linear increase of the density of states with voltage. A number of tunnel break-junctions exhibited $dI/dV(V)$ curves with a second energy gap structure at the average magnitude 2$\Delta _{p-p}/e=13$ mV. Our data cannot be explained by either a pure $s$ pairing or a pure $d_{x^2-y^2}$ pairing.