KTaO₃—The New Kid on the Spintronics Block

Abstract

Long after the heady days of high‐temperature superconductivity, the oxides came back into the limelight in 2004 with the discovery of the 2D electron gas (2DEG) in SrTiO3 (STO) and several heterostructures based on it. Not only do these materials exhibit interesting physics, but they have also opened up new vistas in oxide electronics and spintronics. However, much of the attention has recently shifted to KTaO3 (KTO), a material with all the “good” properties of STO (simple cubic structure, high mobility, etc.) but with the additional advantage of a much larger spin‐orbit coupling. In this state‐of‐the‐art review of the fascinating world of KTO, it is attempted to cover the remarkable progress made, particularly in the last five years. Certain unsolved issues are also indicated, while suggesting future research directions as well as potential applications. The range of physical phenomena associated with the 2DEG trapped at the interfaces of KTO‐based heterostructures include spin polarization, superconductivity, quantum oscillations in the magnetoresistance, spin‐polarized electron transport, persistent photocurrent, Rashba effect, topological Hall effect, and inverse Edelstein Effect. It is aimed to discuss, on a single platform, the various fabrication techniques, the exciting physical properties and future application possibilities of this family of materials.

Quantum Linear Magnetoresistance and Nontrivial Berry Phase in High‐Mobility Elemental Tellurium

“…[ 31 ] We also detected the planar Hall effect and anisotropic MR (Supporting Information) which may arise from different mechanisms. [ 52,53 ] Combining the observed quantum linear MR and the nonzero Berry phase, the systematic magnetotransport measurements on the high‐mobility crystals provide crucial evidence for the nontrivial topology of the p ‐type Te bulk. Before this, the nontrivial π Berry phase was only reported in high‐pressure Te and gate‐tuned n ‐type tellurene which only characterizes the conduction bands.…”

Direct visualization of Rashba-split bands and spin/orbital-charge interconversion at KTaO3 interfaces

“…Another family of oxide 2DEGs is based on KTaO 3 (KTO) instead of STO 14 . Bulk STO and KTO share several features: they are both quantum paraelectrics and become n-type conductors when doped with minute amounts of impurities 15 .…”

Coexisting two-dimensional electron and hole gases highly confined at the interfaces of undoped KTaO3-sandwiching heterostructures

“…Fortunately, these are favorable for enhancing the carrier mobility. It is expected that the 2D carrier concentrations can be controlled by applying a gating field, and thus these pairs of 2DEG and 2DHG could be useful for electronic and optoelectronic device applications such as novel transistors and sensors [23]. On the other hand, some spontaneous inter-interface excitons can be formed, with their life-time manipulated by applying a gating field or changing the thickness of the intermediate KTO layer, and interesting Bose-Eistein condensation could be observed in such exciton systems.…”

“…For perovskite KTaO 3 (KTO), some excellent properties have been observed, such as high mobilities and signatures of spin-orbit coupling in low-temperature magnetoresistance data [14], spin-charge conversion of 2DEG at KTO surface [15,16], spin-polarized 2DEGs at a mag- * Email: bgliu@iphy.ac.cn netic EuO/KTO interface [17], and 2D superconductivity discovered at the KTO-based oxide interfaces [18][19][20][21][22] (see [23] for an overall review). In view of realistic applications, a main challenge is the epitaxial growth of KTO because of very reactive potassium atoms.…”