Session L42: Spin Phenomena in Nonmagnetic 2D Materials

Extracting spin-orbit coupling strength using realistic device simulations

We do realistic device simulations of semiconductor nanowires and calculate the weak antilocalization correction. We match our numerical simulation to magnetoconductance measurements and accurately extract the spin-orbit coupling strength from experiment. Unlike analytic approaches, our approach considers the electrostatic potential, works for arbitrary wire cross sections, and arbitrary finite mean free paths, including the complicated cross over regime where mean free path is comparable to wire width. After verifying our technique against known analytical results, we extract the spin-orbit coupling strength of semiconductor nanowires produced by selective area growth.
  

Far out-of-equilibrium spin populations trigger giant spin injection into atomically thin MoS2

Injecting spins from ferromagnetic metals into semiconductors efficiently is a crucial step towards the seamless integration of charge- and spin-information processing in a single device. However, efficient spin injection into semiconductors has remained an elusive challenge due to the extremely low injection efficiencies originating from impedance mismatch, or technological challenges due to stability and cost issues. In Co/MoS₂, by making use of the strongly out-of-equilibrium character of the injected spins, we demonstrate a highly-efficient spin injection from a ferromagnet (Co) into a semiconductor (MoS₂), thus overcoming the crippling problem of impedance mismatch. Astonishingly, we measure a giant spin current that is orders of magnitude larger than typical injected spin current densities using currently available techniques. Our result demonstrates that technologically relevant spin currents can be driven by ultralow-intensity laser pulses, finally enabling ultrashort spin-current pulses to be a technologically viable information carrier for terahertz spintronics. Nature Physics, DOI:10.1038/s41567-018-0406-3 (2019).   

Introducing CVD WS2 in Magnetic Tunnel Junctions

The use of the spin variable as the vector of information has been largely applied from hard drives read-heads to MRAMs, with novel propositions beyond spin-logics for neuromorphic, stochastic and quantum calculations. While very recent, the introduction of 2D materials such as graphene and the 2D insulator h-BN in Magnetic Tunnel Junctions (MTJs) vertical spin valves has already shown some promising properties[1]. The advent of the 2D semiconductors families opened new opportunities for further tailoring of spintronics properties[2]. Here, we will present results on the scarcely studied WS₂ 2D semiconductor for spintronics. We will detail a protocol to fabricate spin valves based on CVD grown WS₂, with step by step characterizations in support (Raman spectroscopy, photoluminescence, AFM). We will then present our spin transport measurements obtained in a CVD WS₂ based MTJ. Our measured MR signals, above state of the art for 2D semiconductor based MTJs, validates our fabrication approach. These results open the way to the integration of different members of the very large 2D semiconductor families, TMDC and beyond, in order to reveal their spin transport properties in MTJs.
[1] Piquemal-Banci et al. J. Phys. D 50, 203002 (2017)
[2] Galbiati et al. Phys. Rev. Appl. 12, 044022 (2019)   

Defect induced magnetism in monolayer semiconducting PtSe2

Intrinsic disorders such as metal vacancies and antisites in 2D TMDCs are generally accompanied by local magnetic moments which could induce long range magnetic ordering if disorder concentration is sufficient. Towards this, we have recently shown that platinum (Pt) vacancy defects located at the surfaces of multilayer metallic PtSe₂ act as magnetic centers [1]. For both fundamental studies and technological applications, it is critical to explore magnetism in ultra-thin PtSe₂ as this material shows a complete switching from a metal to a semiconductor as its thickness is reduced [2]. Here, we will present our magneto-transport measurements together with first-principles calculations which clearly demonstrate that Pt vacancy defect yields an antiferro- (ferro-)magnetic ordering in mono (bi-)layer PtSe₂. We will also demonstrate that these air-stable crystals can be employed as a magnetic substrate to induce novel proximity phenomena into adjacent nonmagnetic layers. Our findings broaden the horizon of 2D magnets to include normally nonmagnetic stable materials by defect engineering.
[1] A. Avsar, A. Ciarrocchi, M. Pizzochero, D. Unuchek, O. V. Yazyev, and A. Kis, Nature Nanotechnol. 14, 674 (2019).
[2] A. Ciarrocchi, A. Avsar, D. Ovchinnikov, and A. Kis, Nature Comm. 9, 919 (2018).   

Weak anti-localization in undoped Ge/GeSn heterostructures

In this work, we demonstrate weak anti-localization (WAL) in undoped Ge/GeSn heterostructures. We used gated Hall-bar devices to induce two-dimensional hole gases (2DHGs) in strained GeSn quantum wells for magnetotransport experiment at 1.2 to 10 K. Transition from weak localization to WAL is observed as the density increases due to density dependent spin-orbit coupling. By fitting to the HLN formula, phase coherence and spin-orbit times of 2DHGs as well as the spin-orbit splitting energy in undoped Ge/GeSn heterostructures are extracted. Our data show that the scattering lifetime is shorter than the spin-orbit time at all densities, indicating that the system is in the spin-diffusive regime.   

Magnetic order and anisotropy at 3d ferromagnet/2D material heterojunctions

Layered (opto-)electronic devices integrated with two dimensional (2D) materials is an exciting research field that packs with new possibilities and challenges. For example, a high spin injection efficiency from ferromagnetic (FM) electrode to 2D-materials is thought possible, but it needs to work with the complex orbital hybridization and proximity effect at heterojunctions. Here we report the magnetic properties of two 3d ferromagnet/2D material heterojunctions; Co/MoS₂ and Fe/graphene. Both macroscopic magnetic orders and microscopic magnetic configurations of these two heterojunctions are examined. In addition, the interplay between FM materials and 2D materials are studied with the X-ray micro-spectroscopy. We find a heterojunction can exhibit the magnetic anisotropy, but it seems crystalline structure of FM layer is not responsible for it. I will present the experimental data followed by a brief discussion.   

Spin-to-charge interconversion in van der Waals heterostructures

Graphene has been known as an excellent material for long-distance spin transport due to its weak spin-orbit coupling (SOC). However, the same reason makes graphene an adverse candidate for different spintronics applications in which strong SOC is required, such as the Datta and Das proposal or spin-charge interconversions. It has recently been predicted theoretically that SOC can be induced in graphene so that exotic spin-orbit phenomena such as spin Hall effect (SHE) or Rashba-Edelstein effect can be obtained. In our work, by using van der Waals heterostructure-based lateral spin valves, we experimentally demonstrate spin-to-charge conversion (SCC) due to SHE in graphene via spin-orbit proximity with MoS₂, a transition metal dichalcogenide (TMD)¹. The combination of long-distance spin transport and large spin-to-charge conversion in a van der Waals heterostructure gives rise to a hitherto unreported efficiency for the spin-to-charge voltage output. Using a similar approach, we observed large multidirectional SCC in Weyl semimetal MoTe₂. Here, due to the low symmetry of MoTe₂ crystal, we detect, along with the conventional SCC, an unconventional SCC where the spin polarization and the charge current are parallel. Our finding enables the simultaneous conversion of spin currents with any in-plane spin polarization in one single experimental configuration. All in all, these exceptional effects obtained by the unique properties of 2D materials open exciting opportunities in a variety of future spintronic applications.

C. K. Safeer, et al. Nano Letters, 19, 1074 (2019).
C. K. Safeer, et al. arXiv: 1910.04642 (2019).   

Two-dimensional spin-valley locking spin valve

Valleytronics is an emerging field of research which employs energy valleys in the band structure of two-dimensional (2D) electronic materials to encode information. A special interest has been triggered by the associated spin-valley coupling which reveals rich fundamental physics and enables new functionalities. Here, we propose exploiting the spin-valley locking in 2D materials with a large spin-orbit coupling and electric field reversible valley spin polarization, such as germanene, stanene, a 1T′ transition metal dichalcogenide (TMDC) monolayer, and a 2H-TMDC bilayer, to realize a valley spin valve (VSV). The valley spin polarization in these materials can be switched by an external electric field, which enables functionalities of a valley spin polarizer or a valley spin analyzer. When placed in series, they constitute the proposed VSV—a device whose conductance state is ON or OFF depending on the relative valley spin polarization of the polarizer and the analyzer. Using quantum-transport calculations based on an adequate tight-binding model, we predict a giant VSV ratio of nearly 100% for both germanene- and stanene-based VSV devices. Our results demonstrate the implication of the spin-valley coupling in 2D materials for the novel device concept promising for valleytronics.   

Group-IV tellurides as a route towards all-in-one spin transistors

Simple displacive ferroelectrics GeTe and SnTe have been studied for decades but their intriguing physics based on spin-orbit coupling remained unexplored until few years ago. In particular, they are prototype examples of ferroelectric Rashba semiconductors, a class of materials in which the unique coupling between spin and polar degrees of freedom enables the electrical manipulation of spins. Such a property, combined with a sizable spin Hall effect recently predicted in these materials opens a perspective to integrate different functionalities and construct ferromagnets-free spin devices. Here, we reveal even more intriguing phenomena emerging at the limit of the monolayer. More specifically, we explore the persistent spin texture intrinsically present in the ferroelectric phase which protects the spin from decoherence and supports extraordinarily long spin lifetime. Our first-principles calculations followed by symmetry arguments revealed that such a spin wave mode can be externally detuned by perpendicular electric field, leading to spin randomization and decrease in spin lifetime. We propose this mechanism as an operation principle of an all-in-one spin transistor, in which spin injection/detection can be accomplished via spin Hall effects.   

Atomically-resolved theory of the magnitude of the bulk Rashba effect

Rashba effect based-mechanisms to control and generate spin-polarized states have been one of the cornerstones of spintronics. A strong Rashba effect -- as measured by the Rashba coefficient α_R -- is required for applications. However, the physical mechanism defining the R scale, i.e., the magnitude of α_R, is unknown. We find an intrinsic separation between strong Rashba effect associated with anti-crossing bands, and weak Rashba effect associated with no anti-crossing bands. As an application of the proposed theory, we use this design principle to guide the selection of strong Rashba compounds. First, we illustrate that since topological insulators (TIs) intrinsically have anti-crossing bands, strong Rashba compounds could then be identified almost effortlessly by isolating TIs that are non-centrosymmetric, thus establishing a case for cross-functionality of topological Rashba materials. Second, we predict 34 strong Rashba materials by performing DFT calculations for ~800 potential Rashba compounds and filtering those with anti-crossing bands. This reveals rationally designed cases including known GeTe and BiTeI, and compounds that were previously made but unnoticed as Rashba compounds BiTeCl (P6₃mc) and BaCdK2Sb2 (Pmc2₁).   

All-electrical creation and control of giant spin-galvanic effect in 1T′-MoTe2/graphene heterostructures at room temperature

The ability to engineer new states of matter and to control their electronic and spintronic properties by electric fields is at the heart of the modern information technology and driving force behind recent advances in van der Waals (vdW) heterostructures of two-dimensional materials. Here, we exploit a proximity-induced Rashba-Edelstein (REE) effect in vdW heterostructures of Weyl semimetal candidate MoTe₂ and CVD graphene, where an unprecedented gate-controlled switching of spin-galvanic effect emerges due to an efficient spin-to-charge conversion at room temperature [1]. The magnitude of the measured spin-galvanic signal is found to be an order of magnitude larger than the other systems, giving rise to a giant REE. The magnitude and the sign of the spin-galvanic signal are shown to be strongly modulated by gate electric field near the charge neutrality point, which can be understood considering the spin textures of the Rashba spin-orbit coupling-induced spin-splitting in conduction and valence bands of the heterostructure. These findings open opportunities for utilization of gate-controlled switching of spin-galvanic effects in spintronic memory and logic technologies.

References:
[1] Hoque, M.A. et al. doi:arXiv preprint arXiv:1812.02113 (2018).   

Highly tunable nonlinear Hall effects induced by spin-orbit couplings in strained polar transition-metal dichalcogenides

Recently, signatures of nonlinear Hall effects induced by Berry curvature dipoles have been found in atomically thin 1T'/T_d-WTe₂. In this work, we show that in strained polar transition-metal dichalcogenides(TMDs) with 2H-structures, Berry curvature dipoles created by spin degrees of freedom lead to strong nonlinear Hall effects. Under easily accessible uniaxial strain of order ~ 0.2%, strong nonlinear Hall signals, characterized by Berry curvature dipole in the order of ~ 1Å, arise in electron-doped polar TMDs such as MoSSe, which is easily detectable experimentally. Moreover, the magnitude and sign of the nonlinear Hall current can be easily tuned by electric gating and strain. These properties can be used to distinguish nonlinear Hall effects from classical mechanisms such as ratchet effects. Importantly, our system provides a potential scheme for building electrically switchable energy harvesting rectifiers.   

Research on the valley-related properties based on multilayer transition metal dichalcogenides

Monolayer transition metal dichalcogenides (TMDCs), owing to the unique valley-related phenomena, have aroused great attention. However, in multilayer TMDCs, for the presence of the spatial-inversion symmetry, the valley-related phenomena are absent, which limits the developments of valleytronics. In this report, by utilizing the ionic liquid (IL) covering on top of the multilayer TMDC samples, the spatial-inversion symmetry is broken by the out-of-plane electric field induced by IL. The valley-related properties are therefore induced. Firstly, we investigated the circular photogalvanic effect (CPGE) in multilayer MoS₂. Upon the electric field induced by IL, the valley-coupled CPGE signals emerge. Via the wavelength-dependent measurements, it is confirmed only with resonant excitation of K (K’) valley can the CPGE signals be detected. Furtherly, we studied the circular photon assisted valley Hall effect in multilayer WSe₂. With the application of IL, a Hall-oriented photocurrent signal in proportional to the drain-source voltage is detected, illustrating the valley Hall effect is successfully observed. These results imply the valley degree of freedom could be reborn in multilayer TMDCs.