Quantum fluctuation of ferroelectric order in polar metals

TL;DR

This paper investigates how quantum fluctuations induced by carriers suppress ferroelectric order in polar metals, revealing a quantum mechanism that explains experimental observations and guides future device design.

Contribution

It proposes a carrier-induced quantum fluctuation mechanism causing ferroelectric suppression in polar metals, supported by model calculations and quantum simulations.

Findings

Quantum fluctuations weaken ferroelectric order with increased carrier density.

Polaron formation explains the suppression of ferroelectricity.

The mechanism aligns with experimental observations in polar metals.

Abstract

Since its discovery a decade ago, "polar metallic phase" has ignited significant research interest, as it further functionalizes the switchable electric polarization of materials with additional transport capability, granting them great potential in next-generation electronic devices. The polar metallic phase is an unusual metallic phase of matter containing long-range ferroelectric (FE) order in the electronic and atomic structure. Distinct from the typical FE insulating phase, this phase spontaneously breaks the inversion symmetry but without global polarization. Unexpectedly, the FE order is found to be dramatically suppressed by carriers and destroyed at moderate ~10% carrier density. Here, we propose a general mechanism based on carrier-induced quantum fluctuations to explain this puzzling phenomenon. Basically, the quantum kinetic effect would drive the formation of polaronic quasi-particles made of the carriers and their surrounding dipoles. The disruption in dipolar directions can therefore weaken or even destroy the FE order. We demonstrate such polaron formation and the associated FE suppression via a simple model using exact diagonalization, perturbation, and quantum Monte Carlo approaches. This quantum mechanism also provides an intuitive picture for many puzzling experimental findings, thereby facilitating new designs of multifunctional FE electronic devices augmented with quantum effects.