A polarity-induced defect mechanism for conductivity and magnetism at polar–nonpolar oxide interfaces

Abstract

The discovery of conductivity and magnetism at the polar-nonpolar interfaces of insulating nonmagnetic oxides such as LaAlO 3 and SrTiO 3 has raised prospects for attaining interfacial functionalities absent in the component materials. Yet, the microscopic origin of such emergent phenomena remains unclear, posing obstacles to design of improved functionalities. Here we present first principles calculations of electronic and defect properties of LaAlO 3 / SrTiO 3 interfaces and reveal a unifying mechanism for the origins of both conductivity and magnetism. We demonstrate that the polar discontinuity across the interface triggers thermodynamically the spontaneous formation of certain defects that in turn cancel the polar field induced by the polar discontinuity. The ionization of the spontaneously formed surface oxygen vacancy defects leads to interface conductivity, whereas the unionized Ti-on-Al antisite defects lead to interface magnetism. The proposed mechanism suggests practical design principles for inducing and controlling both conductivity and magnetism at general polar-nonpolar interfaces.

Constructing oxide interfaces and heterostructures by atomic layer-by-layer laser molecular beam epitaxy

“…Since its discovery 2 several competing mechanisms have been proposed and intensely debated to explain the origin of the interfacial 2D electron gas, including electronic reconstruction, 19 oxygen vacancies in the SrTiO 3 substrate, [20][21][22] and intermixing between the LaAlO 3 film and the SrTiO 3 substrate. 23,24 According to the electronic reconstruction mechanism, because the atomic layers are charge neutral in SrTiO 3 but charged in LaAlO 3 , a diverging electric potential is built up in a LaAlO 3 film grown on a TiO 2 -terminated SrTiO 3 substrate.…”

Interface superconductivity

“…For instance a Zener breakdown could occur, causing charge transfer from the LAO surface to the interface, thereby screening the field [26,27,28]. Alternatively, oxygen vacancies located at the LAO surface have been advocated as the providers of electrons required to heal the discontinuity [29,30]. Oxygen vacancies on the STO side of the interface and / or cation intermixing across the interface [31,32] have also been invoked as a possible source for the conduction observed -note that those do not imply a charge transfer from the LAO surface to the LAO/STO interface and thus do not solve the polar catastrophe problem [33].…”

“…In the latter category, electrons donated by oxygen vacancies near the interface partly contribute to the conduction band and partly to forming localized states orbiting neighboring Ti sites. Correlations are required to produce a magnetic instability [76,30,77] (see also Ref. [70]).…”

Room-Temperature Giant Charge-to-Spin Conversion at the SrTiO3–LaAlO3 Oxide Interface

“…[31][32][33] It is known that the defects, such as the oxygen vacancies (VO), can be present in the LAO layer during film growth and form localized states (LS) in the LAO band gap. [34][35][36][37] Hence, the IET process via these LS [31][32][33][38][39][40] via LS is the main mechanism of the spin transmission through the LAO layer, the SOT efficiency is expected to exhibit a very strong temperature dependence as indicated by the Glazman-Matveev (GM) theory, 31,33,38,41 , which describes the IET with a power law dependence on temperature.…”