Evidence for an extended critical fluctuation region above the polar ordering transition in LiOsO$_3$

TL;DR

This study reveals an extended critical fluctuation region above the transition temperature in LiOsO$_3$, characterized by subtle lattice distortions linked to dipole correlations, shedding light on the microscopic interactions driving its polar metallic phase.

Contribution

It uncovers a broad temperature range of critical fluctuations in LiOsO$_3$ using electromechanical coupling and symmetry analysis, highlighting interactions beyond current theories.

Findings

Critical fluctuations extend from $T_c$ to ~230 K.

Subtle symmetry-preserving lattice distortions are observed.

Extended fluctuation region explains anomalies in heat and Raman data.

Abstract

Metallic LiOsO$_3$ undergoes a continuous ferroelectric-like structural phase transition below $T_c$ = 140 K to realize a polar metal. To understand the microscopic interactions that drive this transition, we study its critical behavior above $T_c$ via electromechanical coupling - distortions of the lattice induced by short-range dipole-dipole correlations arising from Li off-center displacements. By mapping the full angular distribution of second harmonic electric-quadrupole radiation from LiOsO$_3$ and performing a simplified hyper-polarizable bond model analysis, we uncover subtle symmetry-preserving lattice distortions over a broad temperature range extending from $T_c$ up to around 230 K, characterized by non-uniform changes in the short and long Li-O bond lengths. Such an extended region of critical fluctuations may explain anomalous features reported in specific heat and Raman scattering data, and suggests the presence of competing interactions that are not accounted for in existing theoretical treatments. More broadly, our results showcase how electromechanical effects serve as a probe of critical behavior near inversion symmetry breaking transitions in metals.