Tuning spin-orbit coupling in 2D materials for spintronics: a topical review

Enhanced Nonlinear Response by Manipulating the Dirac Point at the (111) LaTiO_{3}/SrTiO_{3} Interface

Tunable spin-orbit interaction (SOI) is an important feature for future spin-based devices. In the presence of a magnetic field, SOI induces an asymmetry in the energy bands, which can produce nonlinear transport effects (V∼I^{2}). Here, we focus on such effects to study the role of SOI in the (111) LaTiO_{3}/SrTiO_{3} interface. This system is a convenient platform for understanding the role of SOI since it exhibits a single-band Hall response through the entire gate-voltage range studied. We report a pronounced rise in the nonlinear longitudinal resistance at a critical in-plane field H_{cr}. This rise disappears when a small out-of-plane field component is present. We explain these results by considering the location of the Dirac point formed at the crossing of the spin-split energy bands. An in-plane magnetic field pushes this point outside of the Fermi contour, and consequently changes the symmetry of the Fermi contours and intensifies the nonlinear transport. An out-of-plane magnetic field opens a gap at the Dirac point, thereby significantly diminishing the nonlinear effects. We propose that magnetoresistance effects previously reported in interfaces with SOI could be comprehended within our suggested scenario.

Magnetic doping in transition metal dichalcogenides

Transition metal dichalcogenides (TMDCs) are materials with unique electronic properties due to their two-dimensional nature. Recently, there is a large and growing interest in synthesizing ferromagnetic TMDCs for applications in electronic devices and spintronics. Apart from intrinsically magnetic examples, modification via either intrinsic defects or external dopants may induce ferromagnetism in non-magnetic TMDCs and, hence expand the application of these materials. Here, we review recent experimental work on intrinsically non-magnetic TMDCs that present ferromagnetism as a consequence of either intrinsic defects or doping via self-flux approach, ion implantation or e-beam evaporation. The experimental work discussed here is organized by modification/doping mechanism. We also review current work on density functional theory calculations that predict ferromagnetism in doped systems, which also serve as preliminary data for the choice of new doped TMDCs to be explored experimentally. Implementing a controlled process to induce magnetism in two-dimensional materials is key for technological development and this topical review discusses the fundamental procedures while presenting promising materials to be investigated in order to achieve this goal.

2D-materials-integrated optoelectromechanics: recent progress and future perspectives

The discovery of two-dimensional (2D) materials has gained worldwide attention owing to their extraordinary optical, electrical, and mechanical properties. Due to their atomic layer thicknesses, the emerging 2D materials have great advantages of enhanced interaction strength, broad operating bandwidth, and ultralow power consumption for optoelectromechanical coupling. The van der Waals (vdW) epitaxy or multidimensional integration of 2D material family provides a promising platform for on-chip advanced nano-optoelectromechanical systems (NOEMS). Here, we provide a comprehensive review on the nanomechanical properties of 2D materials and the recent advances of 2D-materials-integrated nano-electromechanical systems and nano-optomechanical systems. By utilizing active nanophotonics and optoelectronics as the interface, 2D active NOEMS and their coupling effects are particularly highlighted at the 2D atomic scale. Finally, we share our viewpoints on the future perspectives and key challenges of scalable 2D-materials-integrated active NOEMS for on-chip miniaturized, lightweight, and multifunctional integration applications.

Engineering van der Waals Materials for Advanced Metaphotonics

The outstanding chemical and physical properties of 2D materials, together with their atomically thin nature, make them ideal candidates for metaphotonic device integration and construction, which requires deep subwavelength light-matter interaction to achieve optical functionalities beyond conventional optical phenomena observed in naturally available materials. In addition to their intrinsic properties, the possibility to further manipulate the properties of 2D materials via chemical or physical engineering dramatically enhances their capability, evoking new science on light-matter interaction, leading to leaped performance of existing functional devices and giving birth to new metaphotonic devices that were unattainable previously. Comprehensive understanding of the intrinsic properties of 2D materials, approaches and capabilities for chemical and physical engineering methods, the resulting property modifications and novel functionalities, and applications of metaphotonic devices are provided in this review. Through reviewing the detailed progress in each aspect and the state-of-the-art achievement, insightful analyses of the outstanding challenges and future directions are elucidated in this cross-disciplinary comprehensive review with the aim to provide an overall development picture in the field of 2D material metaphotonics and promote rapid progress in this fast emerging and prosperous field.

Magnetic NiFe thin films composing MoS2 nanostructures for spintronic application

We demonstrate a nanostructure layer made of Ni₈₀Fe₂₀ (permalloy:Py) thin film conjugated MoS₂ nano-flakes. Layers are made based on a single-step co-deposition of Py and MoS₂ from a single solution where ionic Ni and Fe and MoS₂ flakes co-exist. Synthesized thin films with MoS₂ flakes show increasing coercivity and enhancement in magneto-optical Kerr effect. Ferromagnetic resonance linewidth as well as the damping parameter increaseed significantly compared to that of the Py layer due to the presence of MoS₂. Raman spectroscopy and elemental mapping is used to show the quality of MoS₂ within the Py thin film. Our synthesis method promises new opportunities for electrochemical production of functional spintronic-based devices.

Strain Effects on Rashba Spin-Orbit Coupling of 2D Hole Gases in GeSn/Ge Heterostructures

A demonstration of 2D hole gases in GeSn/Ge heterostructures with a mobility as high as 20 000 cm² V^-1 s^-1 is given. Both the Shubnikov-de Haas oscillations and integer quantum Hall effect are observed, indicating high sample quality. The Rashba spin-orbit coupling (SOC) is investigated via magneto-transport. Further, a transition from weak localization to weak anti-localization is observed, which shows the tunability of the SOC strength by gating. The magneto-transport data are fitted to the Hikami-Larkin-Nagaoka formula. The phase-coherence and spin-relaxation times, as well as spin-splitting energy and Rashba coefficient of the k-cubic term, are extracted. The analysis reveals that the effects of strain and confinement potential at a high fraction of Sn suppress the Rashba SOC caused by the GeSn/Ge heterostructures.

Interplay of spin-orbit coupling and Coulomb interaction in ZnO-based electron system

Spin-orbit coupling (SOC) is pivotal for various fundamental spin-dependent phenomena in solids and their technological applications. In semiconductors, these phenomena have been so far studied in relatively weak electron-electron interaction regimes, where the single electron picture holds. However, SOC can profoundly compete against Coulomb interaction, which could lead to the emergence of unconventional electronic phases. Since SOC depends on the electric field in the crystal including contributions of itinerant electrons, electron-electron interactions can modify this coupling. Here we demonstrate the emergence of the SOC effect in a high-mobility two-dimensional electron system in a simple band structure MgZnO/ZnO semiconductor. This electron system also features strong electron-electron interaction effects. By changing the carrier density with Mg-content, we tune the SOC strength and achieve its interplay with electron-electron interaction. These systems pave a way to emergent spintronic phenomena in strong electron correlation regimes and to the formation of quasiparticles with the electron spin strongly coupled to the density.

Signatures of nontrivial Rashba metal states in a transition metal dichalcogenides Josephson junction

Nontrivial Rashba metal states in conventional semiconductor materials generated by both Rashba spin-orbit coupling and ferromagnetic exchange coupling coexisting were recently predicted and exploited. Single layered transition metal dichalcogenides (TMDC) featuring those states and their potential applications have been less focused. We find that, in the materials with Rashba spin-orbit coupling only, nontrivial Rashba metallic states can be manipulated by an external gate voltage. Based on extensive numerical simulations, the relationships between the supercurrent and nontrivial Rashba metallic states in the TMDC Josephson junction have been investigated. It is shown that, in the absence of the Rashba spin-orbit coupling, the critical supercurrent exhibits a stark difference between normal Rashba metal state and anomalous Rashba metal state in the finite junction as compared to the case of the short junction. While in the case of the finite Rashba spin-orbit coupling, the critical supercurrent demonstrates a reentrant behavior when Fermi level sweeps from anomalous Rashba metal state to Rashba ring metal state. Intriguingly, not only at the conversion of the nontrivial Rashba metallic states but also in the Rashba ring metal state the reentrant behavior exhibits again, which could be well explained by the mixing of spin-triplet Cooper pairs with spin-singlet Cooper pairs in Ising superconductor. Such a reentrant effect offers a new way to detect Ising superconductivity based on the TMDC systems. Meanwhile our study also clarified that the nontrivial Rashba metallic state plays an important role in controlling the supercurrent in the TMDC Josephson junction, which is useful for designing future superconducting devices.

High Quality Growth of Cobalt Doped GaN Nanowires with Enhanced Ferromagnetic and Optical Response

Group III-V semiconductors with direct band gaps have become crucial for optoelectronic and microelectronic applications. Exploring these materials for spintronic applications is an important direction for many research groups. In this study, pure and cobalt doped GaN nanowires were grown on the Si substrate by the chemical vapor deposition (CVD) method. Sophisticated characterization techniques such as X-ray diffraction (XRD), Scanning Electron Microscope (SEM), Energy Dispersive X-Ray Spectroscopy (EDS), Transmission Electron Microscopy (TEM), High-Resolution Transmission Electron Microscopy (HRTEM) and photoluminescence (PL) were used to characterize the structure, morphology, composition and optical properties of the nanowires. The doped nanowires have diameters ranging from 60-200 nm and lengths were found to be in microns. By optimizing the synthesis process, pure, smooth, single crystalline and highly dense nanowires have been grown on the Si substrate which possess better magnetic and optical properties. No any secondary phases were observed even with 8% cobalt doping. The magnetic properties of cobalt doped GaN showed a ferromagnetic response at room temperature. The value of saturation magnetization is found to be increased with increasing doping concentration and magnetic saturation was found to be 792.4 µemu for 8% cobalt doping. It was also depicted that the Co atoms are substituted at Ga sites in the GaN lattice. Furthermore N vacancies are also observed in the Co-doped GaN nanowires which was confirmed by the PL graph exhibiting nitrogen vacancy defects and strain related peaks at 455 nm (blue emission). PL and magnetic properties show their potential applications in spintronics.

Superhigh flexibility and out-of-plane piezoelectricity together with strong anharmonic phonon scattering induced extremely low lattice thermal conductivity in hexagonal buckled CdX (XS, Se) monolayers

Although CdX (X = S, Se) has been mostly studied in the field of photocatalysis, photovoltaics, their intrinsic properties, such as, mechanical, piezoelectric, electron and phonon transport properties have been completely overlooked in buckled CdX monolayers. Ultra-low lattice thermal conductivity [1.08 W m-1K-1(0.75 W m-1K-1)] and high p-type Seebeck coefficient [1300μV K-1(850μV K-1)] in CdS (CdSe) monolayers have been found in this work based on first-principles DFT coupled to semi-classical Boltzmann transport equations, combining both the electronic and phononic transport. The dimensionless thermoelectric figure of merit is calculated to be 0.78 (0.5) in CdS (CdSe) monolayers at room temperature, which is comparable to that of two-dimensional (2D) tellurene (0.8), arsenene and antimonene (0.8), indicating its great potential for applications in 2D thermoelectrics. Such a low lattice thermal conductivity arise from the participation of both acoustic [91.98% (89.22%)] and optical modes [8.02% (10.78%)] together with low Debye temperature [254 K (187 K)], low group velocity [4 km s-1(3 km s-1)] in CdS (CdSe) monolayers, high anharmonicity and short phonon lifetime. Substantial cohesive energy (∼4-5 eV), dynamical and mechanical stability of the monolayers substantiate the feasibility in synthesizing the single layers in experiments. The inversion symmetry broken along thezdirection causes out-of-plane piezoelectricity. |d33| ∼ 21.6 pm V-1, calculated in CdS monolayer is found to be the highest amongst structures having atomic-layer thickness. Superlow Young's modulus ∼41 N m-1(31 N m-1) in CdS (CdSe) monolayers, which is comparable to that of planar CdS (29 N m-1) and TcTe2(34 N m-1), is an indicator of its superhigh flexibility. Direct semiconducting band gap, high carrier mobility (∼500 cm2V-1s-1) and superhigh flexibility in CdX monolayers signify its gigantic potential for applications in ultrathin, stretchable and flexible nanoelectronics. The all-round properties can be synergistically combined together in futuristic applications in nano-piezotronics as well.

Quasi-2D Transport and Weak Antilocalization Effect in Few-layered VSe2

With strong spin-orbit coupling (SOC), ultrathin two-dimensional (2D) transitional metal chalcogenides (TMDs) are predicted to exhibit weak antilocalization (WAL) effect at low temperatures. The observation of WAL effect in VSe₂ is challenging due to the relative weak SOC and three-dimensional (3D) transport nature in thick VSe₂. Here, we report on the observation of quasi-2D transport and WAL effect in sublimed-salt-assisted low-temperature chemical vapor deposition (CVD) grown few-layered high-quality VSe₂ nanosheets. The WAL magnitudes in magnetoconductance can be perfectly fitted by the 2D Hikami-Larkin-Nagaoka (HLN) equation in the presence of strong SOC, by which the spin-orbit scattering length l_SO and phase coherence length l_ϕ have been extracted. The phase coherence length l_ϕ shows a power law dependence with temperature, l_ϕ∼ T^-1/2, revealing an electron-electron interaction-dominated dephasing mechanism. Such sublimed-salt-assisted growth of high-quality few-layered VSe₂ and the observation of WAL pave the way for future spintronic and valleytronic applications.