Phase Separation, Competition, and Volume Fraction Control in NaFe$_{1-x}$Co$_x$As

TL;DR

This study uses NMR to map the phase diagram of NaFe$_{1-x}$Co$_x$As, revealing phase separation, competition between antiferromagnetism and superconductivity, and the effects of doping and magnetic fields on these phases.

Contribution

It provides a detailed phase diagram showing phase separation and competition between AFM and superconductivity in NaFe$_{1-x}$Co$_x$As, highlighting the role of two-dimensionality.

Findings

AFM order coexists with weak AFM phase in underdoped regime

Phase separation between AFM and paramagnetic phases near optimal doping

Superconductivity suppresses AFM order and fluctuations at higher dopings

Abstract

We report a detailed nuclear magnetic resonance (NMR) study by combined $^{23}$Na and $^{75}$As measurements over a broad range of doping to map the phase diagram of NaFe$_{1-x}$Co$_x$As. In the underdoped regime ($x \le$ 0.017), we find a magnetic phase with robust antiferromagnetic (AFM) order, which we denote the {\it s}-AFM phase, cohabiting with a phase of weak and possibly proximity-induced AFM order ({\it w}-AFM) whose volume fraction $V \simeq 8$\% is approximately constant. Near optimal doping, at $x = 0.0175$, we observe a phase separation between static antiferromagnetism related to the {\it s}-AFM phase and a paramagnetic (PM) phase related to {\it w}-AFM. The volume fraction of AFM phase increases upon cooling, but both the N{\'e}el temperature and the volume fraction can be suppressed systematically by applying a $c$-axis magnetic field. On cooling below $T_c$, superconductivity occupies the PM region and its volume fraction grows at the expense of the AFM phase, demonstrating a phase separation of the two types of order based on volume exclusion. At higher dopings, static antiferromagnetism and even critical AFM fluctuations are completely suppressed by superconductivity. Thus the phase diagram we establish contains two distinct types of phase separation and reflects a strong competition between AFM and superconducting phases both in real space and in momentum space. We suggest that both this strict mutual exclusion and the robustness of superconductivity against magnetism are consequences of the extreme two-dimensionality of NaFeAs.