Ba¹³⁷ and Ba¹³⁵ NMR measurements of the high-T_c superconductor Ba_1-xK_xBiO₃

Potassium-doped barium bismuthate, Ba_1-xK_xBiO₃, is a high-T_c superconductor that exhibits local inversion-breaking distortions despite being globally centrosymmetric [1]. Ba_1-xK_xBiO₃ is exceptionally difficult to measure with NMR due to its long relaxation time, spin-spin diffusion, and broad spectrum, so many spin-echo pulses must be acquired to achieve a reasonable signal. In spite of these experimental difficulties, here we present Ba¹³⁷ and Ba¹³⁵ NMR measurements for two single-crystal samples, one metallic and one insulating. Both samples exhibit very broad spectra of several MHz as well as frequency shifts that can be replicated by assuming quadrupolar relaxation as the primary relaxation mechanism. The spectra exhibit no angle dependence, which is indicative of a highly anisotropic Ba environment that lacks the expected axial symmetry along [111]. This observation supports the picture that Ba_1-xK_xBiO₃ is a locally non-centrosymmetric system.

[1] S. Griffitt, M. Spaic, J. Joe, Z. Anderson, D. Zhai, M. J. Krogstad, R. Osborn, D. Pelc, and M. Greven, arXiv:2209.01361.