Symmetry Breaking in a Polar Metal Probed with XUV Second Harmonic Generation

It was first predicted over 50 years ago that polar metals could form through a 2^nd order phase transition, but the first experimentally realized polar metal, LiOsO₃, was discovered only recently [1]. In LiOsO₃, a continuous phase transition occurs at T_c = 140 K, where it transitions from a nonpolar metallic to a polar metallic phase through the loss of inversion symmetry. Previous measurements have shown that the transition involves a coordinated 0.5 Å displacement of Li-ions along the polar axis. To gain insight into the nature of the Li-coordination environment in the polar phase, we turn to extreme ultraviolet second harmonic generation (XUV-SHG) at a free electron laser (XFEL). Here, we directly probe the dielectric environment around the Li-ion below T_c by tuning the incident XFEL energy to be half-resonant with energies around the Li K-edge [2]. We extract the effective X⁽²⁾ and use ab initio simulations to relate the nonlinear response to the Li coordination environment. Our results provide insight into the Li-bonding environment and pave the way for future ultrafast time-resolved studies of phase transitions involving structural distortions.

[1] Y. Shi et al., Nat. Mat. 12, 1024 (2013).
[2] E. Berger et al., arxiv.org/abs/2010.03134 (2020)