Unidirectional charge orders induced by oxygen vacancies on SrTiO$_3$(001)

TL;DR

This study uses atomic-scale microscopy to reveal how oxygen vacancies induce unidirectional charge orders and surface anisotropy changes in SrTiO$_3$(001), shedding light on electronic-lattice interactions in oxide heterostructures.

Contribution

It provides the first detailed atomic-scale characterization of oxygen vacancy-induced unidirectional charge orders on SrTiO$_3$(001) surfaces.

Findings

Unidirectional stripe orders appear on c(4 × 2) surface.

Long-range unidirectional charge order forms on (2 × 2) surface.

Surface anisotropy decreases with increasing oxygen vacancies.

Abstract

The discovery of high-mobility two-dimensional electron gas and low carrier density superconductivity in multiple SrTiO$_3$-based heterostructures has stimulated intense interest in the surface properties of SrTiO$_3$. The recent discovery of high-T$_c$ superconductivity in the monolayer FeSe/SrTiO$_3$ aroused the upsurge and underscored the atomic precision probe of the surface structure. By performing atomically resolved cryogenic scanning tunneling microscopy/spectroscopy characterization on dual-TiO$_{2}$-${\delta}$-terminated SrTiO$_3$(001) surfaces with ($\sqrt{13}$ $\times$ $\sqrt{13}$), c(4 $\times$ 2), mixed (2 $\times$ 1), and (2 $\times$ 2) reconstructions, we disclosed universally broken rotational symmetry and contrasting bias- and temperature-dependent electronic states for apical and equatorial oxygen sites. With the sequentially evolved surface reconstructions and simultaneously increasing equatorial oxygen vacancies, the surface anisotropy reduces, and the work function lowers. Intriguingly, unidirectional stripe orders appear on the c(4 $\times$ 2) surface, whereas local (4 $\times$ 4) order emerges and eventually forms long-range unidirectional c(4 $\times$ 4) charge order on the (2 $\times$ 2) surface. This work reveals robust unidirectional charge orders induced by oxygen vacancies due to strong and delicate electronic-lattice interaction under broken rotational symmetry, providing insights into understanding the complex behaviors in perovskite oxide-based heterostructures.