Structural Phase Diagram of LaO1−xFxBiSSe: Suppression of the Structural Phase Transition by Partial F Substitutions

Kazuhisa Hoshi,Chikako Moriyoshi,Yoshihiro Kuroiwa,Yosuke Goto,Yoshikazu Mizuguchi,Takeshi Yajima,Shunsuke Sakuragi,Akira Miura

doi:10.3390/condmat5040081

Kazuhisa Hoshi, Chikako Moriyoshi + Show 6 more

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https://doi.org/10.3390/condmat5040081

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Abstract

Recently, the anomalous two-fold-symmetric in-plane anisotropy of superconducting states has been observed in a layered superconductor system, LaO1−xFxBiSSe (x = 0.1 and 0.5), with a tetragonal (four-fold symmetric) in-plane structure. To understand the origin of the phenomena observed in LaO1−xFxBiSSe, clarification of the low-temperature structural phase diagram is needed. In this study, we have investigated the low-temperature crystal structure of LaO1−xFxBiSSe (x = 0, 0.01, 0.02, 0.03, and 0.5). From synchrotron X-ray diffraction experiments, a structural transition from tetragonal to monoclinic was observed for x = 0 and 0.01 at 340 and 240 K, respectively. For x = 0.03, a structural transition and broadening of the diffraction peak were not observed down to 100 K. These facts suggest that the structural transition could be suppressed by 3% F substitution in LaO1−xFxBiSSe. Furthermore, the crystal structure for x = 0.5 at 4 K was examined by low-temperature laboratory X-ray diffraction, which confirmed that the tetragonal structure is maintained at 4 K for x = 0.5. Our structural investigation suggests that the two-fold-symmetric in-plane anisotropy of superconducting states observed in LaO1−xFxBiSSe was not originated from structural symmetry lowering in its average structure. To evaluate the possibility of the local structural modification like nanoscale puddles in the average tetragonal structure, further experiments are desired.

Highlights

BiCh2-based (Ch = S, Se) superconductors were discovered in 2012 [1,2], which was followed by the development of related layered compounds
Structural symmetry may be a switch of pairing symmetry of superconductivity in BiCh2-based compounds, and further investigation on the relationship between the crystal structure and mechanisms of superconductivity should deeply be studied for the BiCh2-based superconductor family
For LaO1−xFxBiSSe, there has still been a possibility of the emergence of the two-fold-symmetric in-plane anisotropy of MR due to structural symmetry lowering because the parent phase (F-free) LaOBiSSe undergoes a structural transition from tetragonal to monoclinic at 300–400 K [3,16]

Summary

Introduction

Through room-temperature structural analysis using X-ray diffraction (XRD), the crystal structure was determined to be a tetragonal type (P4/nmm) having four-fold symmetry in the ab-plane [16]. The two-fold-symmetric anisotropy of physical properties, including MR, magnetization, specific heat, and superconducting gap, in superconducting states has been observed, in spite of three-fold (AxBi2Se3) or four-fold (IBSCs) symmetry in its crystal structure.

Results

Conclusion