Phase-incoherent superconducting pairs in the normal state of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$

TL;DR

This study provides spectroscopic evidence of phase-incoherent superconducting pairs existing in the normal state of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$, observed through Andreev reflection up to 1.3 times the critical temperature.

Contribution

It demonstrates the presence of phase-incoherent superconducting pairs in the normal state of ferropnictide superconductors using point-contact spectroscopy.

Findings

Andreev reflection observed above $T_c$ up to 1.3$T_c$

Superconducting pairs with fluctuating phase exist in the normal state

Evidence supports phase-incoherent pairing in ferropnictides

Abstract

The normal state properties of the recently discovered ferropnictide superconductors might hold the key to understanding their exotic superconductivity. Using point-contact spectroscopy we show that Andreev reflection between an epitaxial thin film of Ba(Fe$_{0.92}$Co$_{0.08}$)$_2$As$_2$ and a silver tip can be seen in the normal state of the film up to temperature $T\sim1.3T_c$, where $T_c$ is the critical temperature of the superconductor. Andreev reflection far above $T_c$ can be understood only when superconducting pairs arising from strong fluctuation of the phase of the complex superconducting order parameter exist in the normal state. Our results provide spectroscopic evidence of phase-incoherent superconducting pairs in the normal state of the ferropnictide superconductors.