Quantum fluctuation of ferroelectric order in polar metals

Since its discovery in less than a decade ago, "polar metallic phase" has ignited significant research interest, as it further functionalizes the switchable electric polarization with additional electric transport capability, granting them great potential in next-generation electronic devices. The polar metallic phase is a metallic phase of matter containing long-range ferroelectric order in atomic position, typically found in carrier-doped FE materials. Distinct from the typical FE insulating phase, this phase hosts FE order as a spontaneous inversion symmetry breaking in the absence of global FE polarization. Unexpectedly, FE order is found to be dramatically suppressed by metallic carriers and destroyed at only moderate ∼ 12% carrier density. Here, we propose a general mechanism based on carrier-induced quantum fluctuations to explain this puzzling phenomenon. Owing to their kinetic energy, quantum carriers can be strongly dressed by a polarizable medium and form polaronic quasi-particles, inside which the local FE correlation is disrupted. Consequently the long-range FE order must diminish before the size and density of the polarons is large enough to disconnect the remaining FE-allowed region. We demonstrate such polaron formation and its kinetic-driven growth via a simple model using exact diagonalization, perturbation and quantum Monte Carlo approaches. This proposed quantum mechanism also provides an intuitive picture for many unexplained experimental findings, which can facilitate new designs of multifunctional FE-electronic nanodevices through additional quantum effects.