Metastable ferroelectricity in optically strained $SrTiO_3$

TL;DR

This paper demonstrates that mid-infrared optical pulses can induce a metastable ferroelectric phase in SrTiO3 at room temperature, revealing a novel photo-induced phase transition and coupling mechanism.

Contribution

It introduces a method to optically induce and stabilize ferroelectric order in SrTiO3 at high temperatures, showing a new photo-flexoelectric coupling effect.

Findings

Optical pulses induce a long-lived polar phase in SrTiO3.

The induced phase exhibits a large second-order nonlinearity.

Evidence suggests a photo-induced ferroelectric transition.

Abstract

Fluctuating orders in solids are generally considered high-temperature precursors of broken symmetry phases. However, in some cases these fluctuations persist to zero temperature and prevent the emergence of long-range order, as for example observed in quantum spin and dipolar liquids. $SrTiO_3$ is a quantum paraelectric in which dipolar fluctuations grow when the material is cooled, although a long-range ferroelectric order never sets in. We show that the nonlinear excitation of lattice vibrations with mid-infrared optical pulses can induce polar order in $SrTiO_3$ up to temperatures in excess of 290 K. This metastable phase, which persists for hours after the optical pump is interrupted, is evidenced by the appearance of a large second-order optical nonlinearity that is absent in equilibrium. Hardening of a low-frequency mode indicates that the polar order may be associated with a photo-induced ferroelectric phase transition. The spatial distribution of the optically induced polar domains suggests that a new type of photo-flexoelectric coupling triggers this effect.