The doping dependence of superconductivity of (Ca1-xNax)Fe2As2

TL;DR

This study investigates how sodium doping in CaFe2As2 affects its structural, magnetic, and superconducting properties, revealing a wide doping range where superconductivity coexists with magnetic order and identifying high upper critical fields.

Contribution

It provides a comprehensive phase diagram of (Ca1-xNax)Fe2As2 showing the evolution of superconductivity and magnetic order with doping, including coexistence regions and high critical fields.

Findings

Superconductivity appears over a broad doping range.

Na doping suppresses the structural/magnetic transition.

High upper critical fields (~103 T) are observed.

Abstract

Single crystalline CaFe2As2 and (Ca1-xNax)Fe2As2 polycrystals (0 < x < 0.66) are synthesized and characterized using structural, magnetic, electronic transport, and heat capacity measurements. These measurements show that the structural/magnetic phase transition in CaFe2As2 at 165 K is monotonically suppressed by the Na doping and that superconductivity can be realized over a wide doping region. For 0.3 < x < 0.36, the magnetic susceptibilities indicate the possible coexistence of the spin density wave (SDW) and superconductivity. Superconducting phases corresponding to the Na doping level in (Ca1-xNax)Fe2As2 for nominal x = 0.36, 0.4, 0.5, 0.6, and 0.66, with Tc = 17 K, 19 K, 22 K, 33 K, and 33 K, respectively, are identified. The effects of the magnetic field on the superconductivity transitions for x = 0.66 samples with high upper critical fields Hc2 \approx 103 T are studied, and a phase diagram of the SDW and superconductivity as a function of the doping level is thus presented.