Evolution of normal and superconducting properties of single crystals of Na1-δFeAs upon interaction with environment

Iron-arsenide superconductor Na1-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 to an effective stoichiometry change. Here we use this effect to study the doping evolution of normal and superconducting properties of the same single crystals. Controlled reaction with air increases the superconducting transition temperature T-c from the initial value of 12 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 approximate to 60 K) and antiferromagnetic (T-m = 45 K) transitions and superconductivity with onset T-c,T-ons = 16 K and offset T-c,T-off = 12 K. The exposed samples show T-linear variation of rho(a)(T) above T-c,T-ons = 30 K (T-c,T-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 similar to 200 K suggesting the existence of a characteristic energy scale that terminates the T-linear regime, which could be identified with a pseudogap.