Pseudogap-like phase in Ca(Fe$_{1-x}$Co$_x$)$_2$As$_2$ revealed by $^{75}$As NQR

TL;DR

This study uses $^{75}$As NQR measurements to identify a pseudogap-like phase in Ca(Fe$_{1-x}$Co$_x$)$_2$As$_2$, revealing its evolution with doping and its relation to superconductivity.

Contribution

It provides the first detailed phase diagram showing the pseudogap-like phase and its suppression near optimal doping in Ca(Fe$_{1-x}$Co$_x$)$_2$As$_2$ using NQR techniques.

Findings

Identification of a pseudogap-like phase below $T^*$ in under- and optimally-doped samples.

$T^*$ intersects $T_c$ and rapidly decreases near optimal doping.

Suppression of spin-density-wave order correlates with the pseudogap behavior.

Abstract

We report $^{75}$As NQR measurements on single crystalline Ca(Fe$_{1-x}$Co$_x$)$_2$As$_2$ ($0\leq x \leq 0.09$). The nuclear spin-lattice relaxation rate $T_1^{-1}$ as a function of temperature $T$ and Co dopant concentration $x$ reveals a normal-state pseudogap-like phase below a crossover temperature $T^*$ in the under- and optimally-doped region. The resulting $x$-$T$ phase diagram shows that, after suppression of the spin-density-wave order, $T^*$ intersects $T_c$ falling to zero rapidly near the optimal doping regime. Possible origins of the pseudogap behavior are discussed.