Phase separation and enhanced superconductivity in plastically deformed La_1.875Ba_0.125CuO₄ under uniaxial strain

We report our investigation of the superconducting transition and structural response in La_1.875Ba_0.125CuO₄ (LBCO) under uniaxial strain. Our in-situ X-ray diffraction and AC susceptibility studies reveal a dramatic phase separation when applying gentle uniaxial compressive stress along the orthorhombic a-axis. With increasing strain, the fraction of the low-temperature tetragonal (LTT) phase is suppressed at the expense of the low-temperature less-orthorhombic (LTLO) phase and a disordered phase, which is broadly consistent with the recent study[1]. Our transmission electron microscopy study shows that the latter phase is composed of orthorhombic nano-domains. The phase separation is irreversible, resulting in plastic deformation of the crystal. This phase separation is accompanied by the appearance of a second superconducting transition at Tc2 around 32 K, in addition to the Tc1 ~ 3 K in the unstrained sample. Both transition temperatures are enhanced continuously with increasing strain, i.e. from ~3 K to ~20 K for Tc1 and from ~32 K to ~36 K for Tc2. The superconducting volume fraction appears to follow that of the structural volume fraction changes. Under 0.32% uniaxial compressive strain, both the LTT superconducting and structural volume fractions drop to ~40%, while those of the disordered phase grow to ~25%.

[1] Z. Guguchia, et al., Proc. Natl. Acad. Sci. 121, 1 (2024).