  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.
