Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session L21: Spins in 2D MaterialsFocus
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Sponsoring Units: GMAG DMP FIAP DCOMP Chair: Paul Crowell, Univ of Minn - Minneapolis Room: LACC 309 |
Wednesday, March 7, 2018 11:15AM - 11:27AM |
L21.00001: Spin Inversion in Graphene Spin Valves by Gate-Tunable Magnetic Proximity Effect at One-Dimensional Contacts Jinsong Xu, Simranjeet Singh, Jyoti Katoch, Guanzhong Wu, Tiancong Zhu, Igor Zutic, Roland Kawakami Graphene has remarkable opportunities for spintronics due to its high mobility and long spin diffusion length, especially when encapsulated in hexagonal boron nitride (h-BN). Here, for the first time, we demonstrate gate-tunable spin transport in such encapsulated graphene-based spin valves with one-dimensional (1D) ferromagnetic contacts. An electrostatic backgate tunes the Fermi level of graphene to probe different energy levels of the spin-polarized density of states (DOS) of the 1D ferromagnetic contact, which interact through a magnetic proximity effect (MPE) that induces ferromagnetism in graphene. In contrast to conventional spin valves, where switching between high- and low-resistance configuration requires magnetization reversal by an applied magnetic field or a high-density spin-polarized current, we provide an alternative path with the gate-controlled spin inversion in graphene. The resulting tunable MPE employing a simple ferromagnetic metal holds promise for spintronic devices and to realize exotic topological states. |
Wednesday, March 7, 2018 11:27AM - 11:39AM |
L21.00002: Measurements of pure spin current using highly conductive graphene/Co and graphene/Al/Co interfaces Gordon Stecklein, Jiaxi Hu, Qun Su, Steven Koester, Paul Crowell Experiments using graphene nonlocal spin valves have historically employed high resistance graphene/ferromagnet interfaces to confine spins to the graphene channel. In contrast, here we focus on how the pure spin current absorbed by a ferromagnetic contact increases as the conductance of the ferromagnet/graphene interface is improved. We show that the spin current density js, when reported for clarity as equivalent charge density je=js(4πe/h), increases from 103 to 109 A/m2 as the interface conductance per unit area is increased from 10-5 to 10-2 S/μm2. These conductive interfaces are developed by encapsulating exfoliated graphene flakes in AlOx by atomic layer deposition, which allows resist residue to be removed prior to etching away the protective AlOx layer and depositing the contacts. Furthermore, we show that inserting a thick conductive Al interlayer increases the spin injection/detection efficiency and spin lifetime without reducing the interface conductance. Finally, we comment on the challenge of switching by spin-transfer torque in graphene and suggest potential paths forward. |
Wednesday, March 7, 2018 11:39AM - 11:51AM |
L21.00003: Robust spin generation in graphene layers using interfacial spin-orbit coupling Simranjeet Singh, Tiancong Zhu, Huma Yusuf, Roland Kawakami, P Chris Hammel Generation of spin currents in graphene is fundamentally important for understanding spin related phenomena as well as for spintronic applications. Electrical spin injection, wherein a charge current is applied from a ferromagnetic electrode into graphene, is widely used method to generate spin currents in graphene. However, this method requires state of the art material growth techniques to integrate the tunnel barrier at the ferromagnet/graphene interface. As an alternative, the spin orbit coupling (SOC), in a non-magnetic heavy metal or at the interface of two different materials, can induce a spin polarization owing to spin dependent scattering of charge carriers. Here we present experiments showing large spin accumulation in graphene channels induced by SOC at the platinum/graphene interface. Clean Pt/graphene interfaces are prepared by mechanically placing graphene flakes onto Pt nanowires. We will discuss the dependence of the generated spin accumulation in graphene on charge carrier density in graphene, temperature, and magnitude of charge current in the Pt. This demonstration of large spin accumulation in graphene obtained without using a ferromagnet material represents a significant advance. |
Wednesday, March 7, 2018 11:51AM - 12:03PM |
L21.00004: Study of proximity induced SOC and spin injection in graphene - multilayer WSe2 Van der Waals structures Madhushankar Bettadahalli Nandishaiah, Siddhartha Omar, Bart Van Wees Graphene with long electron spin diffusion length (up to 30 µm) has emerged as a promising two dimensional (2D) material for studying spin electronics.1 However, it lacks spin orbit coupling (SOC) in its pristine form which could facilitate electrical manipulation of both charge and spin transport. However, 2D transition metal di-chalcogenides (TMD) like WSe2 have strong SOC which can be placed on top of graphene to investigate proximity-induced SOC in graphene.2 We study such interaction via probing spin transport in graphene encapsulated by multilayer WSe2 (5-10 nm thick) as a function of electrical tuning of charge carrier concentration in graphene and WSe2. Further, spin injection into graphene using WSe2 is demonstrated to understand the nature of interface in order to realize all 2D spin electronics devices. |
Wednesday, March 7, 2018 12:03PM - 12:15PM |
L21.00005: Opto-Valleytronic Spin Injection in Monolayer MoS2/Few-Layer Graphene Hybrid Spin Valves Yunqiu (Kelly) Luo, Jinsong Xu, Tiancong Zhu, Guanzhong Wu, Elizabeth McCormick, Wenbo Zhan, Mahesh Neupane, Roland Kawakami Two dimensional (2D) materials provide a unique platform for spintronics and valleytronics due to the ability to combine vastly different functionalities into one vertically-stacked heterostructure. Graphene has been demonstrated to be an exceptional material for spin transport at room temperature, however it lacks a coupling of the spin and optical degrees of freedom. In contrast, spin/valley polarization can be efficiently generated in monolayer transition metal dichalcogenides (TMD) via absorption of circularly-polarized photons. In this talk, we will present monolayer MoS2/multilayer graphene hybrid spin valves and demonstrate the opto-valleytronic spin injection across TMD/graphene interface. We observe that the magnitude and direction of spin polarization is controlled by both helicity and photon energy. In addition, Hanle spin precession measurements confirm optical spin injection, spin transport, and electrical detection up to room temperature. Our results demonstrate a 2D spintronic/valleytronic system that achieves optical spin injection and lateral spin transport at room temperature in a single device, which paves the way for multifunctional 2D spintronic devices for memory and logic applications. |
Wednesday, March 7, 2018 12:15PM - 12:27PM |
L21.00006: Spin and Valley Polarization Dynamics Probed by Time-Resolved Kerr Rotation in WSe2 Spencer Batalden, Xinlin Song, Saien Xie, Kibum Kang, Jiwoong Park, Vanessa Sih Using time-resolved Kerr rotation measurements, we observe long-lived spin signals up to ~80 ns in the MOCVD-grown monolayer transition metal dichalcogenide (TMD) WSe2 [1]. This signal is robust to transverse magnetic fields, providing evidence that the spin signals arise from the resident hole population. A recent study by Dey et al. has shown this robustness occurs solely in the p-type regime, due to spin-valley locking in the valence band [2]. Photoluminescence measurements also show a transition from free exciton to localized exciton emission as the temperature decreases. Wavelength-dependent Kerr rotation measurements then reveal that this polarization is only generated by excitation near the free exciton energy. Unlike a recent study on exfoliated MoSe2 which shows long spin lifetimes at room temperature, our polarization decays quickly as the temperature is increased [3]. Combined, these measurements support the future use of TMDs for creating spintronic and valleytronic devices. |
Wednesday, March 7, 2018 12:27PM - 1:03PM |
L21.00007: Magnetic Proximity Effects in Two-Dimensional Materials Invited Speaker: Igor Zutic Proximity effects can transform a given material through its adjacent regions to become superconducting, magnetic, or topologically nontrivial. In bulk materials, their size often dwarfs the lengths of proximity effects allowing their neglect. However, in 2D materials such as graphene, transition-metal dichalcogenides (TMDs) and 2D electron gas (2DEG), the situation is drastically different. Even short-range magnetic proximity effects exceed their thickness and strongly modify spin transport and optical properties[1,2]. Experimental confirmation[3] of our prediction for bias-controlled spin polarization reversal in Co/h-BN/graphene[1] suggests that magnetic proximity effects may overcome the need for an applied magnetic field and a magnetization reversal to implement spin logic[4]. In TMDs, where robust excitons dominate their optical response, magnetic proximity effects cannot be described by the single-particle description. We predict a conversion between optically inactive and active excitons by rotating the magnetization of the substrate[2]. Combined magnetic and superconducting proximity effects could enable elusive Majorana bounds states (MBS) for fault-tolerant quantum computing. Exchanging (braiding) MBS yields a noncommutative phase, a sign of non-Abelian statistics and nonlocal degrees of freedom protected from local perturbations. MBS could be manipulated and braided in proximity-induced superconductivity in a 2DEG with magnetic textures from the fringing fields of magnetic tunnel junctions[5,6]. |
Wednesday, March 7, 2018 1:03PM - 1:15PM |
L21.00008: Optical Mapping of Spin/Valley Diffusion and Drift in Transition Metal Dichalcogenides Michael Newburger, Elizabeth McCormick, Jinsong Xu, Roland Kawakami Transition Metal Dichalcogenides (TMDs) have garnered attention due to their potential for future spintronic applications. With the recent observations of long spin lifetimes and optical manipulation and detection of spins, the interest in this family of materials has only increased. However, an understanding of the spin/valley transport in these materials is crucial for most spintronic applications. In TMDs, spin/valley transport will be influenced by their strong spin-orbit coupling and Berry curvature, in addition to any inhomogeneities. |
Wednesday, March 7, 2018 1:15PM - 1:27PM |
L21.00009: Spin-to-Charge Conversion in Transition-Metal-Dichalcogenide/Ferromagnet Heterostructure via Terahertz Emission Spectroscopy Liang Cheng, Xinbo Wang, Yang Wu, Mengji Chen, Weifeng Yang, Jian-Xin Zhu, Justin Song, Marco Battiato, Shijie Wang, Hyunsoo Yang, Ee Min Chia Molybdenum disulphide (MoS2), a well-known two-dimensional transition metal dichalcogenide, has been predicted to be an ideal material for spintronics because of its excellent spin-charge conversion efficiency, which originates from its strong spin-orbit coupling. To make efficient MoS2-based spintronics devices, it is necessary to elucidate the spin-charge conversion mechanism. Using THz emission spectroscopy on MoS2/Co heterostructure, we disentangle the contributions of the inverse spin Hall effect (ISHE) and inverse Rashba Edelstein effect (IREE) to the spin-to-charge conversion. The spin Hall angle is obtained to be ~0.15, which confirms the high spin-to-charge conversion efficiency in MoS2. In addition, the temperature dependence of spin-to-charge conversion is studied, revealing the robust nature of both ISHE and IREE in MoS2. |
Wednesday, March 7, 2018 1:27PM - 1:39PM |
L21.00010: Magnetic and spin-orbit proximity effects on the transport properties of hybrid heterostructures Alex Matos Abiague, Igor Zutic Proximity effects are commonly realized by bringing together two or more materials. The inherent lack of inversion symmetry at the interfaces yields the formation of interfacial spin-orbit fields (SOFs) which are crucial for novel phenomena, absent or fragile in the bulk. We theoretically investigate the interplay between SOFs and proximity-induced magnetism in hybrid SM/F heterostructures (SM and F stand for semiconductor and ferromagnet, respectively) as well as its effect on spectral and transport properties. In particular, we predict the emergence of novel magnetoresistive phenomena in planar SM/F multi-terminal devices, where the coexistence of magnetic and spin-orbit proximity effects leads to anisotropies in the longitudinal and transverse Hall-like responses with respect to both magnetization orientation and crystallographic axes. Potential device applications of the predicted phenomena are also discussed. |
Wednesday, March 7, 2018 1:39PM - 1:51PM |
L21.00011: Optically and Electrically Controllable Adatom Spin–orbital Dynamics in Transition Metal Dichalcogenides Bin Shao, Malte Schueler, Gunnar Schönhoff, Thomas Frauenheim, Gerd Czycholl, Tim Wehling We analyze the interplay of spin-valley coupling, orbital physics, and magnetic anisotropy taking place at single magnetic atoms adsorbed on semiconducting transition metal dichalcogenides, MX2 (M = Mo, W; X = S, Se). Orbital selection rules turn out to govern the kinetic exchange coupling between the adatom and charge carriers in the MX2 and lead to highly orbitally dependent spin-flip scattering rates, as we illustrate for the example of transition metal adatoms with d9 configuration. Our ab initio calculations suggest that d9 configurations are realizable by single Co, Rh, or Ir adatoms on MoS2, which additionally exhibit a sizable magnetic anisotropy. We find that the interaction of the adatom with carriers in the MX2 allows to tune its behavior from a quantum regime with full Kondo screening to a regime of ”Ising spintronics” where its spin–orbital moment acts as classical bit, which can be erased and written electronically and optically. |
Wednesday, March 7, 2018 1:51PM - 2:03PM |
L21.00012: Valley Polarization in Two-Dimensional Transition Metal Dichalcogenides Heterostructures with Ferromagnetic Semiconductor Tao Hu, Guodong Zhao, Yabei Wu, Wei Ren Lifted valleys' degeneracy is a crucial precondition for manipulating valley degrees of freedom and storing information in future spintronics. It was revealed that the Zeeman valley splitting (~0.2 meV/T) could be obtained by applying an external magnetic field, however, this is considered as an inefficient approach. Magnetic proximity effect has been demonstrated to be an effective way to realize exchange interactions, especially in the form of two-dimensional (2D) van der Waals (vdW) heterostructures. We have explored electronic properties of 2D vdW heterostructures using first-principles calculations. It is found that valley splittings of 2 meV and 1.6 meV are achieved in WSe2/CrI3 and MoSe2/CrI3 heterostructures due to the coexistence of inversion symmetry and time-reversal symmetry breaking. These values correspond to an effective exchange field of ~10 and 8 T, respectively. We also observe that the magnitude of the valley splitting is sensitive to the stacking pattern of the heterostructures. Moreover, the valley splittings and polarization at K and K′ point are switchable through vertical flipping of the CrI3 magnetization. |
Wednesday, March 7, 2018 2:03PM - 2:15PM |
L21.00013: spin/valley Hall effect from the mirror symmetry breaking in monolayer MoS2 Kyung-Han Kim, Hyun-Woo Lee For a 1H phase monolayer MoS2, we calculate the Berry curvature, which generates the intrinsic spin/valley Hall effect in the material. By using the kp perturbation theory, we investigate the effect of the mirror symmetry breaking on the monolayer MoS2 where the degree of the symmetry breaking can be controlled by a gate voltage. We show that the Berry curvature depends significantly on the gate voltage and this dependence arises from the spin-momentum coupling in the symmetry broken environment. We calculate the spin/valley Hall conductivity which can explain recent experimental results. We also extend this analysis to a bilayer MoS2. |
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