Crystallographic Phase Transition and High-Tc Superconductivity in LaFeAsO:F

TL;DR

This study links a structural phase transition and magnetic ordering in LaFeAsO to its superconductivity, showing F doping suppresses these transitions and induces superconductivity at 20 K.

Contribution

It provides detailed experimental and theoretical evidence connecting phase transitions and magnetic properties to high-Tc superconductivity in LaFeAsO:F.

Findings

Phase transition from tetragonal to orthorhombic at ~160 K.

Antiferromagnetic transition at ~140 K.

F doping suppresses transitions and induces superconductivity.

Abstract

Undoped LaFeAsO, parent compound of the newly found high-Tc superconductor, exhibits a sharp decrease in the temperature-dependent resistivity at ~160 K. The anomaly can be suppressed by F doping and the superconductivity appears correspondingly, suggesting a close associate of the anomaly with the superconductivity. We examined the crystal structures, magnetic properties and superconductivity of undoped (normal conductor) and 14 at.% F-doped LaFeAsO (Tc = 20 K) by synchrotron X-ray diffraction, DC magnetic measurements, and ab initio calculations to demonstrate that the anomaly is associated with a phase transition from tetragonal (P4/nmm) to orthorhombic (Cmma) phases at ~160 K as well as an antiferromagnetic transition at ~140 K. These transitions can be explained by spin configuration-dependent potential energy surfaces derived from the ab initio calculations. The suppression of the transitions is ascribed to interrelated effects of geometric and electronic structural changes due to doping by F- ions.