Thermal expansion of CaFe2As2: effect of cobalt doping and post-growth thermal treatment

TL;DR

This study investigates how cobalt doping and thermal treatment affect the anisotropic thermal expansion and phase transitions in CaFe2As2, revealing large pressure sensitivities and dominant c-axis responses.

Contribution

It provides detailed measurements of thermal expansion in CaFe2As2 with various dopings and treatments, highlighting anisotropic effects and large pressure derivatives compared to similar compounds.

Findings

Thermal expansion is anisotropic with opposite signs in-plane and c-axis.

Features associated with phase transitions have opposite signs, indicating different stress responses.

c-axis response dominates the thermal expansion behavior.

Abstract

We report thermal expansion measurements on Ca(Fe_(1-x)Co_x)_2As_2 single crystals with different thermal treatment, with samples chosen to represent four different ground states observed in this family. For all samples thermal expansion is anisotropic with different signs of the in-plane and c-axis thermal expansion coefficients in the high temperature, tetragonal phase. The features in thermal expansion associated with the phase transitions are of opposite signs as well, pointing to a different response of transition temperatures to the in-plane and the c-axis stress. These features, and consequently the inferred pressure derivatives, are very large, clearly and substantially exceeding those in the Ba(Fe_(1-x)Co_x)_2As_2 family. For all transitions the c-axis response is dominant.