Strong coupling superconductivity and prominent superconducting fluctuations in the new superconductor Bi4O4S3

TL;DR

This study investigates the superconducting properties of Bi4O4S3, revealing strong coupling superconductivity and significant fluctuations that persist under high magnetic fields, supported by transport and tunneling measurements.

Contribution

It provides evidence of strong coupling superconductivity and prominent fluctuations in Bi4O4S3, extending understanding of its superconducting behavior beyond conventional BCS theory.

Findings

Superconducting gap of about 3 meV with a high ratio of 16.6

Superconducting fluctuations persist at high magnetic fields

A kink in resistivity indicates strong fluctuations

Abstract

Electric transport and scanning tunneling spectrum (STS) have been investigated on polycrystalline samples of the new superconductor Bi4O4S3. A weak insulating behavior in the resistive curve has been induced in the normal state when the superconductivity is suppressed by applying a magnetic field. Interestingly, a kink appears on the temperature dependence of resistivity near 4 K at all high magnetic fields above 1 T when the bulk superconductivity is completely suppressed. This kink associated with the upper critical field as well as the wide range of excess conductance at low field and high temperature are explained as the possible evidence of strong superconducting fluctuation. From the tunneling spectra, a superconducting gap of about 3 meV is frequently observed yielding a ratio of 2\Delta/(kB*Tc) ~ 16.6. This value is much larger than the one predicted by the BCS theory in the weak coupling regime (2\Delta/(kB*Tc) ~ 3.53), which suggests the strong coupling superconductivity in the present system. Furthermore, the gapped feature persists on the spectra until 14 K in the STS measurement, which suggests a prominent fluctuation region of superconductivity. Such superconducting fluctuation can survive at very high magnetic fields, which are far beyond the critical fields for bulk superconductivity as inferred both from electric transport and tunneling measurements.