Electric-Field Control of Spin Current Generation and Detection in Ferromagnet-Free SrTiO3-Based Nanodevices

Enhancement of the Rashba Effect in a Conducting SrTiO3 Surface by MoO3 Capping

Systems having inherent structural asymmetry retain the Rashba-type spin-orbit interaction, which ties the spin and momentum of electrons in the band structure, leading to coupled spin and charge transport. One of the electrical manifestations of the Rashba spin-orbit interaction is nonreciprocal charge transport, which could be utilized for rectifying devices. Further tuning of the Rashba spin-orbit interaction allows additional functionalities in spin-orbitronic applications. In this work, we present our study of nonreciprocal charge transport in a conducting SrTiO3 (001) surface and its significant enhancement by a capping layer. The conductive strontium titanate SrTiO3 (STO) (001) surface was created through oxygen vacancies by Ar+ irradiation, and the nonreciprocal signal was probed by angle- and magnetic field-dependent second harmonic voltage measurement with an AC current. We observed robust directional transport in the Ar+-irradiated sample at low temperatures. The magnitude of the nonreciprocal signal is highly dependent on the irradiation time as it affects the depth of the conducting layer and the impact of the topmost conducting layer. Moreover, the nonreciprocal resistance was significantly enhanced by simply adding a MoO3 capping layer on the conductive STO surface. These results show a simple methodology for tuning and investigating the Rashba effect in a conductive STO surface, which could be adopted for various two-dimensional (2D) conducting layers for spin-orbitronic applications.

Giant tunable Rashba spin splitting in two-dimensional polar perovskites TlSnX3 (X = Cl, Br, I)

The electronic structures and Rashba effect of two-dimensional polar tetragonal perovskites TlSnX₃ (X = Cl, Br, I) are investigated by first-principles density functional theory, and intrinsic Rashba effects are found around the Γ point. In particular, TlSnI₃ has the largest Rashba constant of 1.072 eV Å^-1. Additionally, TlSnBr₃ and TlSnI₃ respond strongly to the applied electric field, and the electric field responsivity of TlSnI₃ can reach 0.790 e Ų. Therefore, due to the large Rashba constants and strong electric field responses, these 2D polar perovskites are promising semiconductor materials with short channel lengths. The nano-scale short spin coherence length can keep the spin coherence of spin FETs, which is superior to the traditional 3D micron spin FETs, and will show a broad application prospect in the Rashba semiconductor field.

Tunable Spin and Orbital Edelstein Effect at (111) LaAlO3/SrTiO3 Interface

Converting charge current into spin current is one of the main mechanisms exploited in spintronics. One prominent example is the Edelstein effect, namely, the generation of a magnetization in response to an external electric field, which can be realized in systems with lack of inversion symmetry. If a system has electrons with an orbital angular momentum character, an orbital magnetization can be generated by the applied electric field, giving rise to the so-called orbital Edelstein effect. Oxide heterostructures are the ideal platform for these effects due to the strong spin–orbit coupling and the lack of inversion symmetries. Beyond a gate-tunable spin Edelstein effect, we predict an orbital Edelstein effect an order of magnitude larger then the spin one at the (111) LaAlO3/SrTiO3 interface for very low and high fillings. We model the material as a bilayer of t2g orbitals using a tight-binding approach, whereas transport properties are obtained in the Boltzmann approach. We give an effective model at low filling, which explains the non-trivial behaviour of the Edelstein response, showing that the hybridization between the electronic bands crucially impacts the Edelstein susceptibility.

A Two-Dimensional Superconducting Electron Gas in Freestanding LaAlO3/SrTiO3 Micromembranes

Freestanding oxide membranes constitute an intriguing material platform for new functionalities and allow integration of oxide electronics with technologically important platforms such as silicon. Sambri et al. recently reported a method to fabricate freestanding LaAlO₃/SrTiO₃ (LAO/STO) membranes by spalling of strained heterostructures. Here, we first develop a scheme for the high-yield fabrication of membrane devices on silicon. Second, we show that the membranes exhibit metallic conductivity and a superconducting phase below ∼200 mK. Using anisotropic magnetotransport we extract the superconducting phase coherence length ξ ≈ 36-80 nm and establish an upper bound on the thickness of the superconducting electron gas d ≈ 17-33 nm, thus confirming its two-dimensional character. Finally, we show that the critical current can be modulated using a silicon-based backgate. The ability to form superconducting nanostructures of LAO/STO membranes, with electronic properties similar to those of the bulk counterpart, opens opportunities for integrating oxide nanoelectronics with silicon-based architectures.

Spin-Charge Interconversion in KTaO3 2D Electron Gases

Oxide interfaces exhibit a broad range of physical effects stemming from broken inversion symmetry. In particular, they can display non-reciprocal phenomena when time reversal symmetry is also broken, e.g., by the application of a magnetic field. Examples include the direct and inverse Edelstein effects (DEE, IEE) that allow the interconversion between spin currents and charge currents. The DEE and IEE have been investigated in interfaces based on the perovskite SrTiO₃ (STO), albeit in separate studies focusing on one or the other. The demonstration of these effects remains mostly elusive in other oxide interface systems despite their blossoming in the last decade. Here, the observation of both the DEE and IEE in a new interfacial two-dimensional electron gas (2DEG) based on the perovskite oxide KTaO₃ is reported. 2DEGs are generated by the simple deposition of Al metal onto KTaO₃ single crystals, characterized by angle-resolved photoemission spectroscopy and magnetotransport, and shown to display the DEE through unidirectional magnetoresistance and the IEE by spin-pumping experiments. Their spin-charge interconversion efficiency is then compared with that of STO-based interfaces, related to the 2DEG electronic structure, and perspectives are given for the implementation of KTaO₃ 2DEGs into spin-orbitronic devices is compared.

Pure Spin Currents Driven by Colossal Spin-Orbit Coupling on Two-Dimensional Surface Conducting SrTiO3

Spin accumulation is generated by passing a charge current through a ferromagnetic layer and sensed by other ferromagnetic layers downstream. Pure spin currents can also be generated in which spin currents flow and are detected as a nonlocal resistance in which the charge current is diverted away from the voltage measurement point. Here, we report nonlocal spin-transport on two-dimensional surface-conducting SrTiO₃ (STO) without a ferromagnetic spin-injector via the spin Hall effect (and inverse spin Hall effect). By applying magnetic fields to the Hall bars at different angles to the nonlocal spin-diffusion, we demonstrate an anisotropic spin-signal that is consistent with a Hanle precession of a pure spin current. We extract key transport parameters for surface-conducting STO, including: a spin Hall angle of γ ≈ (0.25 ± 0.05), a spin lifetime of τ ∼ 49 ps, and a spin diffusion length of λ_s ≈ (1.23 ± 0.7) μm at 2 K.

Heterojunction interface-induced enhancement of position-sensitive photodetection in the nano-film of Ti/SrTiO3 based on the p-type silicon

Complex oxide perovskites exhibit a range of novel, to the best of our knowledge, physical phenomena and have gained popularity as a material system in the past decades. Strontium titanate (SrTiO₃) is an iconic material among oxide perovskite due to its unusual electronic transport behavior and has been investigated in many electronic devices. In this Letter, a type of SrTiO₃ nano-film-induced enhancement of lateral photovoltaic effect (LPE) is observed in the heterojunction of Ti/SrTiO₃/p-type Si. Optimizing the thickness of SrTiO₃, the LPE sensitivity can reach 123.2 mV/mm, which is much higher than the sensitivity in the control samples of Ti/Si (55.3 mV/mm) and SrTiO₃/Si (∼0mV/mm). These findings offer an effective way to improve the sensitivity and will be helpful in the development of oxide-based photodetection devices.

Spin transport in polarization induced two-dimensional electron gas channel in c-GaN nano-wedges

A two-dimensional electron gas (2DEG), which has recently been shown to develop in the central vertical plane of a wedge-shaped c-oriented GaN nanowall due to spontaneous polarization effect, offers a unique scenario, where the symmetry between the conduction and valence band is preserved over the entire confining potential. This results in the suppression of Rashba coupling even when the shape of the wedge is not symmetric. Here, for such a 2DEG channel, relaxation time for different spin projections is calculated as a function of donor concentration and gate bias. Our study reveals a strong dependence of the relaxation rate on the spin-orientation and density of carriers in the channel. Most interestingly, relaxation of spin oriented along the direction of confinement has been found to be completely switched off. Upon applying a suitable bias at the gate, the process can be switched on again. Exploiting this fascinating effect, an electrically driven spin-transistor has been proposed.

Tunable Rashba Spin Splitting in Two-Dimensional Polar Perovskites

Two-dimensional (2D) Rashba semiconductors with structure inversion asymmetry and a spin-orbit coupling (SOC) effect show promising applications in nanospintronics, such as spin field effect transistors (FETs). Here, we systematically investigate the electronic structures and Rashba effect of 2D polar perovskites ABX₃ (A = Cs⁺ or Rb⁺; B = Pb²⁺ or Sn²⁺; X = Cl, Br, or I) by first-principles density functional theory calculations. We demonstrate that, except for the cubic case, 2D polar perovskites from tetragonal and orthorhombic three-dimensional (3D) bulks exhibit a strong intrinsic Rashba effect around the Γ point, due to their structure inversion asymmetry and the strong SOC effect of heavy atoms. In particular, 2D orthorhombic RbSnI₃ shows the largest Rashba constant of 1.176 eV Å among these polar perovskites, which is comparable to that of 3D bulk perovskites previously reported in experiments and theory. Furthermore, several 2D polar perovskites also show a strong electric field response. In particular, 2D tetragonal RbPbI₃ and tetragonal CsPbI₃ have strong electric field responses of >0.5 e Ų. Therefore, 2D polar perovskites as promising Rashba semiconductors possess large Rashba constants and strong electric field responses, resulting in a short spin channel length of tens of nanometers to preserve the spin coherence in spin FETs, superior to conventional 3D micrometer spin FETs.

Two-Dimensional Giant Tunable Rashba Semiconductors with Two-Atom-Thick Buckled Honeycomb Structure

Spin field-effect transistors (SFETs) based on the Rashba effect could manipulate the spin of electrons electrically, while seeking desirable Rashba semiconductors with large Rashba constant and strong electric-field response, to preserve spin coherence remains a key challenge. Herein, we propose a series of 2D Rashba semiconductors with two-atom-thick buckled honeycomb structure (BHS) according to high-throughput first-principles density functional theory calculations. BHS semiconductors show large Rashba constants that are favorable to be integrated into nanodevices superior to conventional bulk materials, and they can be fabricated by mechanical exfoliation or chemical vapor deposition. In particular, 2D AlBi monolayer has the largest Rashba constant (2.77 eVÅ) of all 2D Rashba materials. Furthermore, 2D BiSb monolayer is a promising candidate for SFETs due to its large Rashba constant (1.94 eVÅ) and strong electric field response (0.92 eŲ). Our designed 2D-BiSb-SFET shows shorter spin channel length (42 nm with strain) than conventional SFETs (2-5 μm).