Mechanism of phase transitions and the electronic density of states in (La,Sm)FeAsO$_{1-x}$F$_x$ from ab initio calculations

TL;DR

This study uses ab initio calculations to explore phase transitions and electronic properties in LnFeAsO$_{1-x}$F$_x$ compounds, revealing mechanisms behind structural, magnetic, and superconducting phenomena.

Contribution

It provides a detailed mechanism for phase transitions and links electronic density of states to superconducting transition temperatures in these materials.

Findings

Complex potential energy surface explains structural and magnetic transitions.

Transition temperatures correlate with the electronic density of states at the Fermi level.

Transitions are likely universal in FeAs-layered compounds.

Abstract

The structure and electronic density of states in layered LnFeAsO$_{1-x}$F$_x$ (Ln=La,Sm; $x$=0.0, 0.125, 0.25) are investigated using density functional theory. For the $x$=0.0 system we predict a complex potential energy surface, formed by close-lying single-well and double-well potentials, which gives rise to the tetragonal-to-orthorhombic structural transition, appearance of the magnetic order, and an anomaly in the specific heat capacity observed experimentally at temperatures below $\sim$140--160 K. We propose a mechanism for these transitions and suggest that these phenomena are generic to all compounds containing FeAs layers. For $x>$0.0 we demonstrate that transition temperatures to the superconducting state and their dependence on $x$ correlate well with the calculated magnitude of the electronic density of states at the Fermi energy.