Band Structure and Spin–Orbital Texture of the (111)‐KTaO₃ 2D Electron Gas

Abstract

metal-insulator transitions, among others. It is this plethora of bulk properties that enrich the phase diagrams of transition metal oxides compared to those of conventional semiconductors, and which renders 2DEGs in correlated materials so promising for the discovery of new functionalities and emergent physical phenomena. This is exemplified by the observation of superconductivity, [3] magnetism, [4] strong electron lattice interactions, [5] and quasi 1D conductivity [6] in the 2DEG stabilized in (001)-oriented SrTiO 3 (STO) which is, by far, the most studied system among ABO 3 transition metal oxides. At present it is possible to create confined electron systems in other several oxides including: KTaO 3 , BaTiO 3 , and TiO 2 by different synthesis strategies. [7][8][9][10] While the various methods used to engineer such 2DEGs may differ, the physics of the resulting system is always dominated by the host crystal. This is an advantageous situation since different methods allow 2DEGs to be obtained in diverse forms, thereby expanding the range of experimental probes to which these systems are accessible. A significant step forward in the understanding of these systems came with the experimental observation of the electronic structure of 2DEGs by angle resolved photoemission spectroscopy (ARPES). [11][12][13][14][15][16] 2D electron gases (2DEGs) in oxides show great potential for the discovery of new physical phenomena and at the same time hold promise for electronic applications. In this work, angle-resolved photoemission is used to determine the electronic structure of a 2DEG stabilized in the (111)-oriented surface of the strong spin-orbit coupling material KTaO 3 . The measurements reveal multiple sub-bands that emerge as a consequence of quantum confinement and form a sixfold symmetric Fermi surface. This electronic structure is well reproduced by self-consistent tight-binding supercell calculations. Based on these calculations, the spin and orbital texture of the 2DEG is determined. It is found that the 2DEG Fermi surface is derived from bulk J = 3/2 states and exhibits an unconventional anisotropic Rashba-like lifting of the spindegeneracy. Spin-momentum locking holds only for high-symmetry directions and a strong out-of-plane spin component renders the spin texture threefold symmetric. It is found that the average spin-splitting on the Fermi surface is an order of magnitude larger than in SrTiO 3 , which should translate into an enhancement in the spin-orbitronic response of (111)-KTaO 3 2DEGbased devices. SpintronicsThe realization of 2D electron gases (2DEGs) with remarkable physical properties at complex oxide interfaces has become one of the major driving forces in the field of oxide electronics. [1,2] The correlated materials used in these heterostructures display fascinating bulk properties, ranging from high temperature superconductivity to ferroelectricity and MottThe ORCID identification number(s) for the author(s) of this article can be found under https://doi.

Spontaneous rotational symmetry breaking in KTaO$_3$ interface superconductors

“…S4 [18]). Considering that the existence of striking asymmetric twofold oscillations in magnetoresistance manifests deep inside the superconducting region and vanishes in the normal state, we can straightforwardly rule out the possibilities of ex- trinsic contributions, such as the magnetic field induced Lorentz force effect [34] and the electronic band structure inherent to the KTaO 3 with respect to the underlying threefold lattice symmetry [35] [also see Fig. 1(c)], and thus demonstrate that this anisotropy is an intrinsic property of the superconducting phase in YAlO 3 /KTaO 3 .…”

“…In the YAlO 3 /KTaO 3 the lack of inversion symmetry, however, tends to mix spin-singlet and spin-triplet driven by strong spin-orbit coupling [39]. Indeed, the conducting electrons with strong spin-orbit coupling originating from the heavy Ta 5d orbitals has been revealed at KTaO 3 interfaces [35,40]. Due to the absence of a mirror plane parallel to the interface of YAlO 3 /KTaO 3 , the point group of YAlO 3 /KTaO 3 is C 3v , which does not contain the symmetry element of an inversion.…”

A strong polarizing field in thin-film paraelectrics

“…surf . Consistent with the need to screen the associated polar discontinuities, the surfaces readily form two-dimensional electron gases that have been well characterized using angle-resolved photoemission [26,27,28]. (Note that an interface of KTaO 3 is electrostatically similar to a surface, provided that its neighbor is not a coherent I-V perovskite oxide, and so we use the term surface also for interfaces with systems that are not the vacuum).…”

Spin-Phonon Resonances in Nearly Polar Metals with Spin-Orbit Coupling

“…53 BaTiO 3 can be then expected to have a comparable λ. Perovskites consisting of heavier elements, such as LiOsO 3 or KTaO 3 (KTO), are expected to have stronger SOC, and hence stronger λ. Indeed, for KTO the SOC-related splitting in the band structure of around 400 meV 54,55 , an order of magnitude larger than in STO. For LiOsO 3 it has been reported that, unlike other 5d transition-metal oxides, the effect of SOC on the band structure is small 56 .…”