Competing electronic states emerging on polar surfaces

TL;DR

This study reveals that excess charge on polar surfaces of ionic compounds can form complex electronic states, such as charge density waves and localized polarons, which are more energetically favorable than the traditional 2DEG model, impacting surface properties and potential electronic applications.

Contribution

The paper demonstrates the coexistence of charge density waves and localized polarons on polar surfaces, challenging the conventional 2DEG model and highlighting the role of electron-lattice interactions.

Findings

Charge density waves coexist with localized polarons and bipolarons.

Surface electronic reconstructions are energetically favored over 2DEG.

Distinct spectroscopic signatures and suppression of ferroelectric distortions observed.

Abstract

Excess charge on polar surfaces of ionic compounds is commonly described by the two-dimensional electron gas (2DEG) model, a homogeneous distribution of charge, spatially-confined in a few atomic layers. Here, by combining scanning probe microscopy with density functional theory calculations, we show that excess charge on the polar TaO$_2$ termination of KTaO$_3$(001) forms more complex electronic states with different degrees of spatial and electronic localization: charge density waves (CDW) coexist with strongly-localized electron polarons and bipolarons. These surface electronic reconstructions, originating from the combined action of electron-lattice interaction and electronic correlation, are energetically more favorable than the 2DEG solution. They exhibit distinct spectroscopy signals and impact on the surface properties, as manifested by a local suppression of ferroelectric distortions. Controlling the degree of charge ordering and the transition from ferroelectric to paraelectric states could be of great benefit for the generation and transport of carriers in electronic applications.