Possible Nodeless Superconducting Gaps in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ and YBa$_2$Cu$_3$O$_{7-x}$ Revealed by Cross-Sectional Scanning Tunneling Spectroscopy

TL;DR

This study uses cross-sectional scanning tunneling spectroscopy to reveal spatial variations in the superconducting gap structure of cuprate high-temperature superconductors, suggesting the presence of nodeless gaps in these materials.

Contribution

It demonstrates the existence of nodeless superconducting gaps in Bi2212 and YBCO using a novel cross-sectional tunneling approach, providing new insights into their pairing mechanisms.

Findings

Identification of U-shaped gaps indicating nodeless superconductivity

Observation of V-shaped gaps likely induced by proximity effects

Detection of spatial inhomogeneity in the superconducting gap structure

Abstract

Pairing in the cuprate high-temperature superconductors and its origin remain among the most enduring mysteries in condensed matter physics. With cross-sectional scanning tunneling microscopy/ spectroscopy, we clearly reveal the spatial-dependence or inhomogeneity of the superconducting gap structure of Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ (Bi2212) and YBa$_2$Cu$_3$O$_{7-x}$ (YBCO) along their $c$-axes on a scale shorter than the interlayer spacing. By tunneling into the (100) plane of a Bi2212 single crystal and a YBCO film, we observe both U-shaped tunneling spectra with extended flat zero-conductance bottoms, and V-shaped gap structures, in different regions of each sample. On the YBCO film, tunneling into a (110) surface only reveals a U-shaped gap without any zero-bias peak. Our analysis suggests that the U-shaped gap is likely a nodeless superconducting gap. The V-shaped gap has a very small amplitude, and is likely proximity-induced by regions having the larger U-shaped gap.