Nonlinear Optical Susceptibility and Ferroic Domains in Polar Metals*

Polar metals are materials that are simultaneously metallic and exhibit ferroelectric-like polar order, despite the presence of free charges that typically screen long-range electrostatic forces. The existence of a ferroelectric-like polar state in a conductor leads to the question – how do properties typical of insulating ferroelectrics manifest themselves in a metal? In this work, we investigate two such properties – nonlinear optical susceptibility, and domain formation.

Two polar metals, with polar displacements originating from fundamentally different mechanisms, LiOsO₃ and Ca₃Ru₂O₇, have been studied. Second harmonic generation (SHG) polarimetry measurements show that both materials have a large SHG susceptibility, with anisotropy an order of magnitude greater than comparable classical ferroelectrics. An in-house developed cryogenic scanning confocal SHG microscope is used to probe the mesoscale structure of these materials. The experiments reveal an abundance of ferroic domain structures in both materials. Notably, Ca₃Ru₂O₇ exhibits a rich ferroelectric-like domain structure, a feature that has never before been seen in a metal. The existence of such properties in a metallic state make for an enticing prospect for future optoelectronic applications.