Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session M36: 2D Spintronics, Valleytronics, and Orbital Hall EffectFocus Live
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Sponsoring Units: GMAG DMP FIAP DCOMP Chair: Kelly Yunqiu Luo, Cornell University |
Wednesday, March 17, 2021 11:30AM - 12:06PM Live |
M36.00001: Spin-polarized conductivity and spin-dependent Seebeck effect in magnetized graphene Invited Speaker: Talieh Ghiasi Graphene-based van der Waals heterostructures have shown to be an excellent choice for two-dimensional (2D) spintronic devices as the superior spin and charge transport properties of graphene are enriched via the proximity to other 2D materials. By the proximity effects, one can induce spin-orbit and magnetic exchange interactions in the graphene which provide strong coupling between charge and spin currents [1-4]. In particular, our recent spin transport measurements in graphene in the proximity of a 2D interlayer antiferromagnet, Chromium Sulfide Bromide (CrSBr) have shown strong spin polarization of conductivity in graphene (∼14%) that arises from a large induced exchange interaction. The strong spin-polarization of conductivity also results in the observation of the spin-dependent Seebeck effect in graphene. This is the first-time experimental realization of the active role of the magnetic graphene in the electrical and thermal generation of spin currents, addressing the most technologically relevant aspects of the magnetism in graphene. Also, the high sensitivity of the spin-transport in graphene to the magnetization of the outer-most layer of the CrSBr provides the tool to read out a single magnetic sublattice. The spin-polarization of conductivity and spin-dependent Seebeck coefficient in magnetic graphene, together with its exceptional long-distance spin transport suggest a magnetic graphene as an ultimate building block for ultra-compact magnetic memory and sensory devices and provide substantial advances in 2D spintronic and caloritronic systems [4]. |
Wednesday, March 17, 2021 12:06PM - 12:18PM Live |
M36.00002: Chiral spin excitations in graphene with proximity-induced spin-orbit coupling Abhishek Kumar, Saurabh Maiti, Dmitrii Maslov Interacting electron systems with spin-orbit coupling (SOC) support collective spin excitations (CSE) even in the absence of the magnetic field. Motivated by recent observations of CSE in the surface state of a 3D topological insulator, Bi2Se3, and in semiconductor heterostructures, we develop a theory of CSE in doped graphene with proximity-induced SOC both of Rashba or valley-Zeeman (VZ) types. We show that these two types of SOC lead to different spectra of CSE. For the case of Rashba SOC there exist, just like in a single-band 2D Fermi liquid (FL), three branches of CSE: a doubly degenerate branch (in-plane polarized), and a non-degenerate branch (out-of-plane polarized). However, there also exists another in-plane doubly-degenerate branch. Both branches show up as peaks in the spin susceptibility and optical conductivity. For graphene with VZ SOC, spin and pseudospin degrees of freedom are decoupled, and there exists only a single out-of-plane polarized branch of CSE. In contrast to the case of Rashba SOC, the VZ CSE shows up only the spin susceptibility but not in the optical conductivity. This difference in CSE spectra can be used to experimentally distinguish the two types of SOC in the experiment. |
Wednesday, March 17, 2021 12:18PM - 12:30PM Live |
M36.00003: Spin Filtering Manipulation in WS2-based Magnetic Tunnel Junctions Victor Zatko, Marta Galbiati, Regina Galceran, Florian Godel, Pierre Brus, Odile Bezencenent, Marie-Blandine Martin, Bernard Servet, Mauro Och, Cecilia Mattevi, Frédéric Petroff, Albert Fert, Bruno Dlubak, Pierre Seneor Spintronics has opened a new paradigm through the use of the spin variable as the vector of information and has been largely applied from hard drives read-heads to the STT-MRAMs. While very recent, the introduction of 2D materials in Magnetic Tunnel Junctions (MTJs) has already shown some promising properties[1][2]. The recent advent of the wide TMDC family of 2D semiconductors opened new opportunities for further tailoring of spintronics properties. We will detail a protocol to fabricate spin valves based on CVD grown WS2 with step by step characterizations in support (Raman, photoluminescence, AFM). The fabrication process is further validated by the measurements of magnetoresistance spin signals above state of the art for 2D semiconductors based MTJs. We then present experimental results on WS2 spin-filtering tunability with thickness that we discuss in light of its peculiar thickness band-structure evolution, with Density Functional Theory calculations in support. Our work opens the way to the integration of different members of the very large TMDCs family, in order to reveal their spin transport properties in MTJs[3]. |
Wednesday, March 17, 2021 12:30PM - 12:42PM Live |
M36.00004: Spontaneous Valley Polarization of Interacting Carriers in a Monolayer Semiconductor Scott Crooker, Jing Li, Mateusz Goryca, Xiaodong Xu, Nathan P Wilson, Andreas V. Stier We report magneto-absorption spectroscopy of electrostatically-gated WSe2 monolayers in high magnetic fields up to 60 tesla. When doped with a 2D Fermi sea of mobile holes, well-resolved sequences of optical transitions are observed in both left- and right-circular polarizations, which unambiguously and separately indicate the number of filled Landau levels (LLs) in both K and K' valleys. This reveals the interaction-enhanced valley Zeeman energy, which is found to be highly tunable with hole density p. We exploit this tunability to align the LLs in K and K', and find that the 2D hole gas becomes unstable against small changes in LL filling and can spontaneously valley-polarize [1]. These results cannot be understood within a single-particle picture, highlighting the importance of exchange interactions in determining the ground state of 2D carriers in monolayer semiconductors. [1] J. Li et al., Phys. Rev. Lett. 125, 147602 (2020). |
Wednesday, March 17, 2021 12:42PM - 12:54PM Live |
M36.00005: Spatially-resolved Measurements of Spin Valley Polarization in Monolayer WSe2 Spencer Batalden, Fauzia Mujid, Jiwoong Park, Vanessa A Sih Time-resolved Kerr microscopy is used to generate and measure spin valley polarizations in MOCVD-grown monolayer tungsten diselenide (WSe2). The Kerr signal reveals bi-exponential decay with fit time constants of ~100 ps and ~3 ns. Measurements are performed on several triangular flakes from the same growth cycle and reveal larger spin valley polarization near the edges of the flakes. This spatial dependence is consistent across different WSe2 flakes, but no correlation is found with spatially resolved reflectivity or photoluminescence microscopy data. Time-resolved pump-probe overlap measurements further reveal that the Kerr signal’s spatial dependence is not due to spin diffusion on the same timescale. |
Wednesday, March 17, 2021 12:54PM - 1:06PM Live |
M36.00006: Imaging the valley and orbital Hall effect in monolayer MoS2 Paul M Haney, Vivek Amin, Fei Xue The topological properties of a material's electronic structure are encoded in its Berry curvature, a quantity which is intimately related to the transverse electrical conductivity. In transition metal dichalcogenides with broken inversion symmetry, the nonzero Berry curvature results in a valley Hall effect. We identify a previously unrecognized consequence of Berry curvature in these materials: an electric-field-induced change in the electrons' charge density orientation. We use first-principles calculations to show that measurements of the electric-field-induced change in the charge density or local density of states in MoS2 can be used to measure its energy-dependent valley and orbital Hall conductivity. |
Wednesday, March 17, 2021 1:06PM - 1:18PM Live |
M36.00007: Orbital Hall effect in bilayer transition metal dichalcogenides Tarik Cysne, Luis Canonico, Marcio Costa, Nicholas V Nardelli, Roberto B. Muniz, Tatiana Rappoport The orbital Hall effect (OHE) is analogous to the spin Hall effect (SHE) and consists in the appearance of a transverse orbital angular momentum current as a response to a longitudinally applied electric field. It has been predicted that transition metal dichalcogenides (TMDs) monolayers exhibit rather large OHE in the absence of SHE. However, valley Hall effect (VHE) also contributes to a transverse flow of orbital angular momentum current in monolayer TMDs. Therefore, it becomes experimentally difficult to discriminate between the orbital and the valley Hall effects in them. |
Wednesday, March 17, 2021 1:18PM - 1:30PM Live |
M36.00008: Orbital and Spin Hall effect: Effect of symmetry breaking and Density-Functional results for MoS2 PRATIK SAHU, Sayantika Bhowal, Sashi Satpathy The Orbital Hall effect (OHE) is the transverse flow of orbital moment in a solid in response to an applied electric field, analogous to the flow of spin moment in the spin Hall effect (SHE). The effect is fundamentally different in solids with broken inversion symmetry, where an intrinsic orbital moment is present even without the electric field. Using a tight-binding model Hamiltonian for a simple cubic lattice with two atoms in the unit cell, we study the OHE as a function of the deviation from the inversion symmetry by computing the orbital and spin Berry curvatures. Our work shows that both OHE and SHE are much stronger in broken symmetry materials. As a concrete example, we present our results from density-functional calculations for the 2D transition-metal-dichalcogenide (TMDC) materials such as MoS2, a broken symmetry system. Our work suggests that the TMDC's and other broken symmetry solids, because of their more robust OHE, would be good materials for the experimental observation of the OHE, which is so far not conclusively established by experiments. |
Wednesday, March 17, 2021 1:30PM - 1:42PM Live |
M36.00009: Tunable Berry curvature and topological Nernst effect in biased bilayer WSe2 Vassilios Vargiamidis, Panagiotis Vasilopoulos, Neophytos Neophytou We investigate the Berry curvature and anomalous thermoelectric transport in bilayer WSe2 with broken inversion symmetry, e.g., due to a gate electric field, regardless of time-reversal symme-try. In the presence of spin-orbit coupling and a valley-contrasting Berry curvature, anomalous spin and valley Nernst responses are generated. We find that the Nernst signals exhibit peaks and dips as the chemical potential is varied that have the signs of the Berry curvatures of the bands and are proportional to their magnitudes. The anomalous valley Nernst coefficient is enhanced with increasing electric field strength. We also analyze the orbital magnetization and the orbital magnetic moment. The magnetization and its two contributions, one due to the magnetic moment and one due to the Berry curvature, are calculated and interpreted in terms of opposite circulating currents of the bands in the two layers. The results are pertinent to other transition metal dichal-cogenides and future caloritronic applications. |
Wednesday, March 17, 2021 1:42PM - 1:54PM Live |
M36.00010: Ab initio spin dynamics and spin-phonon relaxation in graphene multilayers Adela Habib, Junqing Xu, Yuan Ping, Ravishankar Sundararaman Spin dynamics in graphene and its multilayers is of high interest because of the tremendous potential graphene holds for spintronics and spin-qubits. Experimental observations demonstrate that relaxation lifetime and lifetime anisotropy show qualitatively different trends from single to bi-layer graphene. Several model-based theoretical studies suggest that these distinct trends have roots in interlayer coupling and substrate induced relaxation channels. However, it is not easy for these studies to handle these relaxation mechanisms realistically. To this front, we present our investigations of spin dynamics and in particular spin-phonon relaxation fully from first-principles in single and bi-layer graphene in the presence of an external electric field and/or hBN substrate. Using Lindbladian dynamics of density matrices[1], we conduct a systematic study of electron(hole) spins evolution for microseconds in these systems all the while fully accounting for their scattering against phonons. We predict that the interplay between interlayer coupling-induced spin-orbit field and the substrate phonons contribute to the observed spin dynamics in experiments. |
Wednesday, March 17, 2021 1:54PM - 2:06PM Live |
M36.00011: Enhancing Tunneling Current in CrX3 (X = Cl, I) Junctions using Transition Metal Dichalcogenides Jonathan Heath, Marcelo A Kuroda Over the past few years, the fabrication of graphene/CrX3 (X = Cl, Br, I) magnetic tunnel junction (MTJ) devices has been achieved. While graphene/CrI3 MTJs have demonstrated large tunneling magnetoresistances (TMR), the conductance in these devices is disproportionately low compared to conventional devices due to the vanishing graphene states near the Fermi level. In order to circumvent this limitation, transition metal dichalcogenides NbSe2 and TaSe2 stand as alternative readily-available electrodes. Here we use first principles calculations within the density functional theory and Landauer’s formalism of ballistic transport to characterize 2H-MSe2 (M = Nb, Ta)/CrX3 (X = Cl, I) based MTJs. We rationalize the obtained TMR values in terms of the properties of the junctions (e.g. electrode density of states, complex band structure, band alignments, junction thickness, etc.). While the TMR results are comparable to those found in graphene/CrX3 heterostructures, we observe approximately two orders of magnitude larger conductance values, which facilitates experimental observations. The descriptions that account for the systems’ atomistic properties aid the rational design of future MTJs based on 2D materials. |
Wednesday, March 17, 2021 2:06PM - 2:18PM Live |
M36.00012: Orbital Hall insulating phase in transition metal dichalcogenide monolayers Luis Canonico, Tarik Cysne, Alejandro Molina-Sanchez, Roberto Muniz, Tatiana Rappoport The orbital-Hall effect (OHE), in resemblance to the spin-Hall effect (SHE), refers to the creation of a transverse flow of orbital angular momentum (OAM) that is induced by a longitudinally applied electric field. Despite that the OHE has been explored mostly in 3D metallic systems, recent theoretical results predicted the existence of OHE in 2D insulators, suggesting that it could be observable in a diverse pool of 2D materials. We showed that the 2H transition metal dichalcogenide (TMD) monolayers are orbital-Hall insulators [1]. They exhibit orbital-Hall conductivity plateaux within their main semiconducting gaps. The OHE in TMD occurs even in the absence of spin-orbit coupling, and it can be linked to exotic momentum-space Dresselhaus-like OT that arise from a combination of the orbital attributes and lattice symmetry. Our results open the possibility of using TMDs for orbital-current injection and orbital torque transfer that surpass their spin-counterparts in spin-orbitronic devices. |
Wednesday, March 17, 2021 2:18PM - 2:30PM On Demand |
M36.00013: Valley-controlled transport in graphene/WSe2 heterostructures under an off-resonant polarized light Muhammad Zubair, Panagiotis Vasilopoulos, Muhammad Tahir We investigate the electronic dispersion and transport properties of graphene/WSe2 heterostructures in the presence of a proximity-induced spin-orbit coupling λv, sublattice potential Δ, and an off-resonant circularly polarized light of frequency Ω and effective energy term ΔΩ. Using a low-energy effective Hamiltonian we find that the interplay between different perturbation terms leads to inverted spin-orbit coupled bands. At high Ω we study the band structure and dc transport using the Floquet theory and linear response formalism, respectively. We find that the inverted band structure transfers into the direct band one when the off-resonant light is present. The valley Hall conductivity switches sign when the polarization of the off-resonant light changes. The valley polarization vanishes for ΔΩ = 0 but it is finite for ΔΩ ≠ 0 and reflects the lifting of the valley degeneracy of the energy levels, for ΔΩ = 0, when the off-resonant light is present. The corresponding spin polarization, present for ΔΩ = 0, increases for ΔΩ ≠ 0. |
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