Modeling Study of the Dip/Hump Feature in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ Tunneling Spectroscopy

TL;DR

This study models tunneling spectra in Bi-2212 high-temperature superconductors to determine if the dip-hump features are intrinsic to superconductivity or due to normal state effects, finding they are likely intrinsic.

Contribution

The paper introduces a modeling approach that distinguishes between normal state effects and intrinsic superconducting features in tunneling spectra of Bi-2212.

Findings

Dip-hump features are not caused by normal state effects.

Strong-coupling effects can produce dip features similar to experimental data.

Negative dI/dV features are intrinsic to the superconducting state.

Abstract

The tunneling spectra of high temperature superconductors on Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ (Bi-2212) reproducibly show a high bias structure in the form of a dip-hump at voltages higher than the gap voltage. Of central concern is whether this feature originates from the normal state background or is intrinsic to the superconducting mechanism. We address this issue by generating a set of model conductance curves - a ''normal state'' conductance that takes into account effects such as the band structure and a possible pseudogap, and a pure superconducting state conductance. When combined, the result shows that the dip-hump feature present in the experimental conductance curves cannot be naively attributed to a normal state effect. In particular, strong dip features found in superconductor-insulator-superconductor data on optimally-doped Bi-2212, including negative dI/dV, cannot be a consequence of an extrinsic pseudogap. However, such features can easily arise from states-conserving deviations in the superconducting density of states, e.g., from strong-coupling effects.