Crystallographic phase transition and high-T_csuperconductivity in LaFeAsO:F

Abstract

Undoped LaFeAsO, parent compound of the newly found high-T c 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 (T c = 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.Parent compounds to the high-T c superconductors show a rapid decrease in their electrical resistivity (ρ), which is clearly seen on the resistivity-temperature (T) curves with a kink at ~160 K (T anom ). This anomaly has been attributed to the combined effect of a crystallographic phase transition at ~160 K, and an antiferromagnetic ordering of the Fe spins at a slightly lower temperature of ~140 K [27][28][29][30][31]. Both transitions can be simultaneously suppressed by the electron or hole doping, suggesting a close association of these phase transitions with the superconductivity observed in the doped compounds.The Fe-based and the Cu-based superconductors have a common feature in that superconductivity is attained by providing itinerant electron or hole carriers to the two-dimensional transport layers containing 3d transition metal elements. However, they differ distinctly from each other in that nine 3d electrons (one hole) are involved for Cu 2+ , which forms ionic bond with oxide ions, whereas six 3d electrons participate in a more complex interplay of Fe−Fe and Fe−As bonding.In this study, we examine the crystal structures, magnetic properties and superconductivity of undoped and 14 at.% F-doped LaFeAsO (T c = 20 K) by Rietveld refinement of synchrotron X-ray diffraction, DC magnetic measurements, and ab initio calculations. We demonstrate that the undoped LaFeAsO undergoes 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. Doping by F − ions in the LaO layers suppresses both transitions, which is ascribed to interrelated effects of geometric and electronic structural changes. Our results

Two-dome structure in electron-doped iron arsenide superconductors

“…That maximum was raised to 55 K in Ln-1111-type LnFeAsO 1 − x F x (Ln denotes lanthanide) 5 . The compound of LaFeAsO 1 − x F x is paramagnetic metal with tetragonal symmetry at room temperature and undergoes a tetragonal-orthorhombic transition around 150 K accompanied by a para-antiferromagnetic (AFM) transition 6,7 . Superconductivity emerges when the transitions are suppressed by carrier doping via element substitution or pressure application.…”

Structural and electronic response upon hole doping of rare-earth iron oxyarsenides Nd1−xSrxFeAsO (0 < x≤ 0.2)

“…In addition, the evolution of the electronic and structural properties with doping level is drastically different and non-symmetric to the electron-doped unit cell (P4/nmm) established before for the REFeAsO systems. 9,10 Rietveld analysis of the room temperature high statistics diffraction profiles confirmed the successful substitution of Sr 2+ at the Nd 3+ sites with the occupancies refining to 0.052(6), 0.091 (6) (Table S1, Fig. S1).…”

Phase stability and large in-plane resistivity anisotropy in the 112-type iron-based superconductor Ca1−xLaxFeAs2

“…Even though there are orthorhombic phases listed in this figure, these phases are different from (i.e. they belong to different space groups) the orthorhombic phases realized below the structural transitions in, for example, LaFeAsO, where the transition is from the tetragonal P 4/nmm to the orthorhombic Cmma (# 67) structure [48]. Similarly, in the 122 compounds that have a tetragonal structure with the ThCr 2 Si 2 type (space group F mmm) at high temperature, an electronic nematic phase breaks the four-fold rotational symmetry and hence induces the structural transition to an orthorhombic space group [49][50][51].…”