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

TL;DR

This study uses ultra-low temperature tunneling spectroscopy in high magnetic fields to investigate the quasiparticle density of states in Bi2212 high-temperature superconductor crystals, revealing complex gap structures.

Contribution

It provides new experimental insights into the quasiparticle density of states and gap structure in Bi2212, challenging simple pairing symmetry models.

Findings

Zero flat region in tunneling conductance within the energy gap.

Observation of a second energy gap at approximately 13 mV.

Data inconsistent with pure s-wave or d-wave pairing symmetry.

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.