Linear and nonlinear optical probe of the ferroelectric-like phase transition in a polar metal, LiOsO3

TL;DR

This study investigates the optical properties of LiOsO3, a polar metal, revealing a continuous phase transition at 140 K and detailed insights into its nonlinear optical susceptibility and domain wall structure.

Contribution

It provides the first detailed optical characterization of LiOsO3's phase transition and domain structure, highlighting similarities and differences with ferroelectric insulators.

Findings

Strong optical birefringence observed

Nonlinear susceptibility similar to LiNbO3 except along polar axis

Continuous phase transition at 140 K with specific domain wall structures

Abstract

LiOsO3 is one of the first materials identified in a recent literature as a 'polar metal', a class of materials that are simultaneously noncentrosymmetric and metallic. In this work, the linear and nonlinear optical susceptibility of LiOsO3 is studied by means of ellipsometry and optical second harmonic generation (SHG). Strong optical birefringence is observed using spectroscopic ellipsometry. The nonlinear optical susceptibility extracted from SHG polarimetry reveals that the tensor components are of the same magnitude as in isostructural insulator LiNbO3, except the component along the polar axis d33, which is suppressed by an order of magnitude. Temperature-dependent SHG measurements in combination with Raman spectroscopy indicate a continuous order-disorder type polar phase transition at 140 K. Linear and nonlinear optical microscopy techniques reveal 109 deg/71 deg ferroelastic domain walls, like in other trigonal ferroelectrics. No 180 deg polar domain walls are observed to emerge across the phase transition.