Possible superconducting fluctuation and pseudogap state above $T_c$ in CsFe$_2$As$_2$

TL;DR

This study presents evidence from multiple experimental techniques indicating a possible superconducting fluctuation or pseudogap state in CsFe$_2$As$_2$ existing well above its superconducting transition temperature.

Contribution

It provides comprehensive experimental evidence for a pseudogap or fluctuation state above $T_c$ in CsFe$_2$As$_2$, expanding understanding of high-temperature precursor phenomena.

Findings

Detection of a characteristic temperature $T^* \,\sim\, 13$ K much higher than $T_c$

Observation of a diamagnetic signal extending up to $T^*$

Identification of gap-like features in tunneling spectra disappearing near 9 K

Abstract

Resistive, magnetization, torque, specific heat and scanning tunneling microscopy measurements are carried out on the hole heavily doped CsFe$_2$As$_2$ single crystals. A characteristic temperature $T^*\sim13$ K, which is several times higher than the superconducting transition temperature $T_c=2.15$ K, is observed and possibly related to the superconducting fluctuation or the pseudogap state. A diamagnetic signal detected by torque measurements starts from the superconducting state, keeps finite and vanishes gradually until a temperature near $T^*$. Temperature dependent resistivity and specific heat also show kinks near $T^*$. An asymmetric gap-like feature with the energy of 8.4 meV and a symmetric superconducting related gap of 2.2 meV on the scanning tunneling spectra are detected, and these pseudogap-related features disappear at temperatures up to at least 9 K. These observations by different experimental tools suggest the possible existence of superconducting fluctuation or pseudogap state in the temperature range up to 4 - 6 times of $T_c$ in CsFe$_2$As$_2$.