Tunneling Spectra and Superconducting Gap in Bi2Sr2CaCu2O8+d and Tl2Ba2CuO6+d

TL;DR

This study reports tunneling spectra across various doping levels in high-temperature superconductors Bi-2212 and Tl-2201, revealing a monotonic increase in the energy gap with underdoping and common spectral features indicative of strong coupling effects.

Contribution

It provides comprehensive tunneling spectra over a wide doping range and demonstrates that dip and hump features are universal in high-Tc superconductors, with detailed analysis of the doping dependence of the energy gap.

Findings

Energy gap increases monotonically with underdoping.

Spectral features such as dip and hump are consistent across doping levels.

Large strong coupling ratios suggest unconventional pairing mechanisms.

Abstract

Tunneling spectra are reported for Bi_{2}Sr_{2}CaCu_{2}O_{8+\delta} (Bi-2212) over a wide doping range using superconductor-insulator-superconductor (SIS) break junctions. The energy gap inferred from the tunneling data displays a remarkable monotonic dependence on doping, increasing to very large values in the underdoped region even as T_c decreases. This leads to unphysically large values of the strong coupling ratio ($\sim$20). The tunneling spectra are qualitatively similar over the entire doping range even though the gap parameter, $\Delta$, changes from 12 meV to 60 meV. Each spectrum exhibits dip and hump features at high bias with characteristic energies that scale with the superconducting gap. Tunneling spectra of near optimally-doped Tl_{2}Ba_{2}CuO_{6+\delta} (Tl-2201) also display a weak dip feature in superconductor-insulator-normal metal (SIN) junctions. Generated SIS spectra of Tl-2201 are compared with measured spectra on Bi-2212 and it is concluded that the dip and hump features are generic to high temperature superconductors.