Unconventional superconductivity in the BiS$_2$-based layered superconductor NdO$_{0.71}$F$_{0.29}$BiS$_2$

TL;DR

This study reveals that NdO$_{0.71}$F$_{0.29}$BiS$_2$, a BiS$_2$-based superconductor with a relatively low critical temperature, exhibits strongly anisotropic superconducting gaps, indicating an unconventional pairing mechanism despite expectations of conventional behavior.

Contribution

The paper provides the first evidence of anisotropic superconducting gaps in NdO$_{0.71}$F$_{0.29}$BiS$_2$, challenging the assumption of conventional pairing mechanisms in BiS$_2$-based superconductors.

Findings

Observation of strongly anisotropic superconducting gap

Evidence suggesting unconventional pairing mechanism

Implication of multiple pairing interactions

Abstract

We investigate the superconducting-gap anisotropy in one of the recently discovered BiS$_2$-based superconductors, NdO$_{0.71}$F$_{0.29}$BiS$_2$ ($T_c$ $\sim$ 5 K), using laser-based angle-resolved photoemission spectroscopy. Whereas the previously discovered high-$T_c$ superconductors such as copper oxides and iron-based superconductors, which are believed to have unconventional superconducting mechanisms, have $3d$ electrons in their conduction bands, the conduction band of BiS$_2$-based superconductors mainly consists of Bi 6$p$ electrons, and hence the conventional superconducting mechanism might be expected. Contrary to this expectation, we observe a strongly anisotropic superconducting gap. This result strongly suggests that the pairing mechanism for NdO$_{0.71}$F$_{0.29}$BiS$_2$ is unconventional one and we attribute the observed anisotropy to competitive or cooperative multiple paring interactions.

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