Magnetic field dependence of the density of states in the multiband superconductor $\beta$-Bi$_2$Pd

TL;DR

This study uses low-temperature STM to investigate the superconductor $eta$-Bi$_2$Pd, revealing a single isotropic gap but multiband characteristics evidenced by vortex behavior and critical field analysis.

Contribution

It demonstrates that $eta$-Bi$_2$Pd is a single gap multiband superconductor, a novel insight into its electronic structure and superconducting properties.

Findings

Single isotropic superconducting gap identified.

Magnetic field dependence suggests multiband effects.

Vortex lattice locking to atomic lattice observed.

Abstract

We present very low temperature scanning tunneling microscopy (STM) experiments on single crystalline samples of the superconductor $\beta$-Bi$_2$Pd. We find a single fully isotropic superconducting gap. However, the magnetic field dependence of the intervortex density of states is higher than the one expected in a single gap superconductor, and the hexagonal vortex lattice is locked to the square atomic lattice. Such increase in the intervortex density of states and vortex lattice locking have been found in superconductors with multiple superconducting gaps and anisotropic Fermi surfaces. We compare the upper critical field $H_{c2}(T)$ obtained in our sample with previous measurements and explain available data within multiband supercondutivity. We propose that $\beta$-Bi$_2$Pd is a single gap multiband superconductor. We anticipate that single gap multiband superconductivity can occur in other compounds with complex Fermi surfaces.