Pressure-induced enhancement of two-dimensionality in LaO1-xFxBi(Se/S)2 superconductors

Application of high pressure (HP) induces evolution from metallic to semimetallic state in the layered superconducting LaO1-xFxBiSe2 phase, as concluded from ab initio calculations based on experimental P-dependent structural data. These changes in conduction properties are associated with a reported decrease in the superconducting transition temperature, T-c. However, further increase in pressure induces structural transformation in LaO1-xFxBiSe2, which results in a strong enhancement of its two-dimensionality with a related improvement in superconducting performance. In addition, the HP structural anisotropy induces a loss of long-range order correlations along the stacking direction of the LaO/F and BiSe2 layers yielding formation of stacking faults. A similar but even more pronounced structural transition was observed in the related LaO1-xFxBiS2 phase which is also accompanied by an increase in T-c. This demonstrates that enhancement of two-dimensionality in layered superconducting systems acts in favor of their superconducting performance.