Evolution of two bulk-superconducting phases in Sr0.5RE0.5FBiS2 (RE: La, Ce, Pr, Nd, Sm) by external hydrostatic pressure effect

TL;DR

This study investigates how applying external hydrostatic pressure induces and enhances bulk superconductivity in Sr0.5RE0.5FBiS2 compounds with various rare-earth elements, revealing pressure-dependent phase transitions and structural changes.

Contribution

It demonstrates pressure-induced evolution of bulk superconducting phases and structural transitions in Sr0.5RE0.5FBiS2, providing insights into superconductivity mechanisms in BiCh2-based materials.

Findings

Bulk superconductivity is induced at pressures below 1 GPa.

High-pressure phase exhibits significantly higher Tc (~10 K).

Structural transition from tetragonal to monoclinic occurs at ~1.1 GPa.

Abstract

Polycrystalline samples of Sr1-xRExFBiS2 (RE: La, Ce, Pr, Nd, and Sm) were synthesized via the solid-state reaction and characterized using synchrotron X-ray diffraction. Although all the Sr0.5RE0.5FBiS2 samples exhibited superconductivity at transition temperatures (Tc) within the range of 2.1-2.7 K under ambient pressure, the estimated superconducting volume fraction was small. This indicated the non-bulk nature of superconductivity in these samples under ambient pressure. A dramatic evolution of the bulk superconducting phases was achieved on applying an external hydrostatic pressure. Near pressures below 1 GPa, bulk superconductivity was induced with a Tc of 2.5-2.8 K, which is termed as the low-P phase. Moreover, the high-P phase (Tc = 10.0-10.8 K) featuring bulk characteristics was observed at higher pressures. Pressure-Tc phase diagrams indicated that the critical pressure for the emergence of the high-P phase tends to increase with decreasing ionic radius of the doped RE ions. According to the high-pressure X-ray diffraction measurements of Sr0.5La0.5FBiS2, a structural phase transition from tetragonal to monoclinic also occurred at approximately 1.1 GPa. Thus, this phase transition indicates a pressure-induced superconducting-superconducting transition similar to the transition in LaO0.5F0.5BiS2. Bulk superconducting phases in Sr0.5RE0.5FBiS2 induced by the external hydrostatic pressure effect are expected to be useful for evaluating the mechanisms of superconductivity in BiCh2-based superconductors.