Pressure effect on superconductivity of $A_{x}$Fe$_2$Se$_2$ ($A$ = K and Cs)

TL;DR

This study investigates how applying hydrostatic pressure up to 1.7 GPa affects the superconducting transition temperature ($T_c$) of $A_{x}$Fe$_2$Se$_2$ ($A$ = K, Cs) single crystals, revealing dome-like $T_c$ behavior and optimal doping conditions.

Contribution

First systematic pressure-dependent resistivity measurements on $A_{x}$Fe$_2$Se$_2$ superconductors, identifying optimal pressures for maximum $T_c$ and analyzing resistivity hump behavior.

Findings

$T_c$ shows dome-like dependence on pressure with a maximum at specific pressures.

Optimal doping pressure increases $T_c$ to 32.7 K for K-based and 31.1 K for Cs-based samples.

Resistivity hump shifts to higher temperature with increasing pressure, possibly due to Fe or Se vacancies.

Abstract

We performed the high hydrostatic pressure resistivity measurements (up to 1.7 GPa) on the newly discovered superconductors $A_{x}$Fe$_2$Se$_2$ ($A$ = K and Cs) single crystals. Two batches of single crystals $K_xFe_2Se_2$ with different transition temperatures ($T_c$) were used to study the effect of pressure. The $T_c$ of the first one gradually decreases with increasing pressure from 32.6 K at ambient pressure. While a dome-like behavior was observed for the crystal with $T_c=31.1$ K, and $T_c$ reaches its maximum value of 32.7 K at the pressure of 0.48 GPa. It indicates that there exists a optimal doping with maximum $T_c$ of 32.7K in $K_xFe_2Se_2$ system. The behavior of $T_c$ vs. pressure for $Cs_xFe_2Se_2$ also shows a dome-like behavior, and $T_c$ reaches its maximum value of 31.1 K at the pressure of 0.82 GPa. The hump observed in temperature dependence of resistivity for all the samples tends to shift to high temperature with increasing pressure. The resistivity hump could arise from the vacancy of Fe or Se.