Evolution of normal and superconducting properties of single crystals of Na$_{1-\delta}$FeAs upon interaction with environment

TL;DR

This study investigates how environmental exposure alters the normal and superconducting properties of Na$_{1-delta}$FeAs single crystals, revealing doping evolution, increased $T_c$, and potential quantum criticality.

Contribution

It demonstrates controlled environmental reaction as a method to tune doping and superconductivity in Na$_{1-delta}$FeAs crystals, providing insights into their phase diagram and electronic properties.

Findings

Superconducting transition temperature increases from 12 K to 27 K with exposure.

Resistivity shows a transition from features at structural and magnetic transitions to T-linear behavior.

Evidence suggests a quantum critical point at optimal doping.

Abstract

Iron-arsenide superconductor Na$_{1-\delta}$FeAs is highly reactive with the environment. Due to the high mobility of Na ions, this reaction affects the entire bulk of the crystals and leads an to effective stoichiometry change. Here we use this effect to study the doping evolution of normal and superconducting properties of \emph{the same} single crystals. Controlled reaction with air increases the superconducting transition temperature, $T_c$, from the initial value of 12 K to 27 K as probed by transport and magnetic measurements. Similar effects are observed in samples reacted with Apiezon N-grease, which slows down the reaction rate and results in more homogeneous samples. In both cases the temperature dependent resistivity, $\rho_a(T)$, shows a dramatic change with exposure time. In freshly prepared samples, $\rho_a(T)$ reveals clear features at the tetragonal-to-orthorhombic ($T_s$ $\approx$ 60 K) and antiferromagnetic ($T_m$=45 K) transitions and superconductivity with onset $T_{c,ons}$=16 K and offset $T_{c,off}$=12 K. The exposed samples show $T-$linear variation of $\rho_a(T)$ above $T_c,ons$=30 K ($T_{c,off}$=26 K), suggesting bulk character of the observed doping evolution and implying the existence of a quantum critical point at the optimal doping. The resistivity for different doping levels is affected below $\sim$200 K suggesting the existence of a characteristic energy scale that terminates the $T-$linear regime, which could be identified with a pseudogap.