Free Electrons Holes and Novel Surface Polar Order in Tetragonal BaTiO3 Ground States

TL;DR

This study uncovers new surface polar orders in tetragonal BaTiO3 that generate electron and hole gases, using DFT and experiments, revealing their dependence on surface termination and intrinsic constraints on depolarization.

Contribution

It introduces a comprehensive DFT analysis of ion relaxation in BaTiO3 surfaces and experimentally confirms the origin of electron and hole gases from spontaneous polarization.

Findings

Discovery of surface-dependent electron and hole gases in BaTiO3.

Experimental validation of gases originating from spontaneous polarization.

Identification of intrinsic constraints on depolarization fields due to surface polar order.

Abstract

We find novel polar orders that yield electron (e-) and hole (h+) gas and depend on surface terminations, using density functional theory (DFT) that, unlike existing reports, relaxed all the ion positions of ATiO3 having spontaneous polarization Ps (A: alkali earth metal). By the experiments of atomic-oxygen cleaned surfaces of BaTiO3, we find both e- and h+ gas that are proven to originate from Ps and constrain electrostatic potential, which has been missing. These experiments that remarkably agree with the DFTs of defect free BaTiO3 reveal the properties of Ps-originated e-h+ and, for ferroelectric basics, an e-h+-posed intrinsic constraint on depolarization field arising from Ps in proper time ranges.