Revisiting the phase diagram of LaFe$_{1-x}$Co$_x$AsO on single crystals by thermodynamic methods

TL;DR

This study maps the phase diagram of Co-doped LaFeAsO single crystals, revealing how magnetic, nematic, and superconducting phases evolve with doping using thermodynamic measurements, highlighting the suppression of magnetic and nematic orders and the emergence of superconductivity.

Contribution

The paper provides a detailed thermodynamic analysis of the phase diagram of LaFe$_{1-x}$Co$_x$AsO, offering new insights into the suppression of magnetic and nematic phases and the nature of superconductivity in this compound.

Findings

Gradual suppression of antiferromagnetic order with Co doping.

Complete suppression of nematic order at 7.5% Co.

Superconductivity appears around 6% Co doping, with no microscopic coexistence with magnetism.

Abstract

In this work we revisit the phase diagram of Co-doped LaFeAsO using single crystals and thermodynamic methods. From magnetic susceptibility studies we track the doping evolution of the antiferromagnetic phase, revealing a continuous suppression of $T_\mathrm{N}$ up to 5$\%$ Co doping. In order to study the evolution of the so-called nematic phase, the temperature dependence of the lengths changes along the $a$ and $b$ orthorhombic directions, $\Delta L/L_0$, was determined by high-resolution capacitance dilatometry. The results clearly show a gradual reduction of the orthorhombic distortion $\delta$ and of $T_\mathrm{S}$ with increasing Co content up to 4.5$\%$, while it is completely suppressed for 7.5$\%$ Co. Bulk superconductivity was found in a small doping region around 6$\%$ Co content, while both $T_\mathrm{c}$ and the superconducting volume fraction rapidly drop in the neighbouring doping regime. Ultimately, no microscopic coexistence between the superconducting and magnetic phases can be assessed within our resolution limit, in sharp contrast with other iron-pnictide families, e.g., electron- and hole-doped BaFe$_2$As$_2$.