The Tetragonal to Orthorhombic structural phase transition in multiband FeAs-based superconductors

TL;DR

This study investigates the temperature-dependent structural phase transition in FeAs-based superconductors, revealing how chemical pressure influences the transition temperature and its competition with superconductivity.

Contribution

It provides detailed insights into the tetragonal to orthorhombic transition and its relation to doping, chemical pressure, and superconducting properties in FeAs-based materials.

Findings

The phase transition occurs between 120 K and 150 K, centered at 137 K in NdOFeAs.

Chemical pressure shifts the transition temperature and enhances superconducting T_c.

Misfit strain reduction correlates with optimal doping and superconductivity.

Abstract

We report the temperature dependent x-ray powder diffraction of the FeAs-based superconductors in the range between 300 K and 95 K. In the case of NdOFeAs we have detected the structural phase transition from the tetragonal phase, with P4/nmm space group, to the orthorhombic phase,with Cmma space group, over a broad temperature range from 150 K to 120 K, centered at T0 137K. This transition is reduced, by about 30K, by the internal chemical pressure going from LaOFeAs to NdOFeAs. On the contrary the superconducting critical temperature increases from 27K to 51 K going from LaOFeAs to NdOFeAs doped samples. The FeAs layers in all undoped 1111 and 122 systems suffer a tensile misfit strain. The tensile misfit strain is reduced in 1111 and in 122 samples and at optimum doping the misfit strain is close to zero. This result shows that the normal striped orthorhombic Cmma phase competes with the superconducting tetragonal phase. In the orthorhombic clusters the charges can move only along the stripes in the b direction and are localized by the magnetic interaction.