Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session Y52: Spin Transport and Relaxation in 2D, Topological, and Semiconductor MaterialsFocus Recordings Available
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Sponsoring Units: GMAG DMP FIAP Chair: Simranjeet Singh, Carnegie Mellon Univ. Room: McCormick Place W-475A |
Friday, March 18, 2022 8:00AM - 8:36AM |
Y52.00001: New developments in two-dimensional spintronics using large-scale graphene Invited Speaker: M. Venkata Kamalakar Graphene is the two-dimensional (2D) platform for spin current communication and an integration medium for advanced 2D spintronic components. In devices where spin currents are generated by electrical spin injection into graphene, the contacts can lead to significant charge transfer doping of graphene, modifying its electrical properties locally and overall device performance. Also, substrate roughness puts a limit on the spin transport capability of large-scale graphene. This presentation will discuss our results on how device engineering using chemical vapor deposited graphene [1] can minimize contact-induced spin relaxation, enabling the realization of the longest spin communication of 45 µm and very high spin parameters in graphene at room temperature [2]. Furthermore, realizing highly resilient flexible ferromagnetic nanowires [3] allows us to fabricate flexible graphene spin devices and observe high diffusive spin transport in graphene [4], despite the rough topography of flexible substrates. At the same time, knowing the ability to maintain the integrity of metal-oxide graphene interfaces can increase their spintronic efficiencies in such devices [5]. These developments enhance our understanding of spin relaxation originating from contacts and substrates while opening up new grounds for flexible-integrated large-scale 2D spin current circuits and novel spin components. |
Friday, March 18, 2022 8:36AM - 8:48AM |
Y52.00002: Spin-orbit phenomena in proximitized graphene L. Antonio B Benitez Moreno When graphene is in proximity to a transition metal dichalcogenide (TMDC), it acquires an enhanced spin-orbit interaction (SOI) together with a complex spin texture with out-of-plane and winding in-plane components. In this talk, I will discuss the relevant consequences of this unique type of SOI for spin manipulation in graphene-TMDC heterostructures. Firstly, I will present our recent experiments which demonstrate how the spin dynamics in graphene is strongly modified by the proximity to a TMDC. The results show that the spin lifetime depends on the spin orientation, and it is larger for spins pointing out of the graphene plane compared with the spins pointing in-plane. Such anisotropic features indicate that the spin–valley coupling present in the TMDC is imprinted in the graphene and felt by the propagating spins [1,2]. I will further present an unprecedented electric-field tunability of the spin relaxation in graphene-WS2 heterostructures. The characteristic spin relaxation varies from highly anisotropic to nearly isotropic when the applied an electric field. Finally, I will demonstrate that the enhanced SOI leads to spin-charge interconversion in graphene. By using spin precession measurements, we can separate the contributions of the spin Hall effect (SHE) and the spin galvanic effect (SGE). Remarkably, their corresponding conversion efficiencies can be tailored by electrostatic gating in magnitude and sign [3]. |
Friday, March 18, 2022 8:48AM - 9:00AM |
Y52.00003: Proximity effects in graphene/chromia heterostructures keke he, Ather Mahmood, Will Echtenkamp, Peter A Dowben, Christian Binek, Jonathan P Bird Chromia is an insulating magneto-electric, exhibiting antiferromagnetic character in bulk up to an elevated Neel temperature of 307 K. The magneto-electric nature of the chromia allows the direction of the net magnetism at this surface to be reversed electrically, by application of an appropriate voltage in the simultaneous presence of magnetic field and with low power dissipation. As a substrate, chromia offers room-temperature control of proximity effects in graphene, in marked contrast to previously reported ferromagnetic insulators. In this study, we have investigated the signatures of spin transport in heterostructures formed from CVD-graphene and chromia (Cr2O3). The non-local resistance measured in spin Hall geometry shows a maximum as the gate voltage is swept through the Dirac point. This spin-Hall signal is shown to persist beyond the Neel temperature of 307 K, and to persist instead all the way up to 350 K. This suggests that the signal is driven by strong spin-orbit coupling between the 2D layer and the chromia. These results are a step towards all-electric access to spin polarized currents at room temperature in graphene/Cr2O3 heterostructures, making them a highly promising system for future antiferromagnetic spintronic applications. |
Friday, March 18, 2022 9:00AM - 9:12AM |
Y52.00004: Tunable spin splitting in graphene on a magnetic oxide insulator Junxiong Hu, Ariando Ariando In crystals, the spin degeneracy is lifted when time-reversal symmetry is broken, leading to many interesting spintronic, topological phenomena and applications. Owing to the short range of the magnetic-exchange interaction, the spin splitting can be induced in graphene by magnetic proximity effect. For spin-polarized graphene, the frequency of quantum oscillations is determined not only by the Fermi energy, but also by the spin splitting energy. In this work, we demonstrate a strong spin splitting of graphene on a magnetic insulator, Tm3Fe5O12 (TmIG), which can be tuned over a broad range. From the gate voltage tuning of quantum oscillations, we extract that the spin splitting energy can be as large as 138 meV at 2 K. Based on the direction and strength of cooling fields, the spin splitting energy can be further tuned between 107 meV and 200 meV, shifting the quantum oscillations and quantum Hall plateau. DFT calculations reveal that an electron doping to graphene is induced by TmIG and the proximity-induced spin splitting energy can be up to 150 meV, consistent with that obtained from transport measurements. Finally, the spin polarization of graphene π orbitals is probed directly using the x-ray magnetic circular dichroism (XMCD) at the C K absorption edges. |
Friday, March 18, 2022 9:12AM - 9:24AM |
Y52.00005: Substrate Effects on Spin Relaxation in 2D Dirac Materials from First-Principles Density-Matrix Dynamics Junqing Xu, Adela Habib, Ravishankar Sundararaman, Yuan Ping Effects of substrates are crucial for spin relaxation and transport in 2D Dirac Materials. Substrates provide support of samples and profoundly modify the physical properties of materials through proximity effect and interlayer couplings. However, in past theory substrate effects were often described by simple models. More importantly, the modifications of phonons and electron-phonon (e-ph) couplings by substrates were ignored or treated very approximately due to theoretical and computational difficulties. Such limitations have been overcome by our recently developed First-Principles Density-Matrix (FPDM) approach [1]. Here, through FPDM simulations with e-ph and spin-orbit couplings, we investigate the substrate effects on spin relaxation and transport properties of two 2D Dirac materials – graphene and germanene on different substrates. We find that for graphene, spin lifetime is strongly reduced by the substrate due to the induced "internal magnetic fields" which activate a new spin relaxation mechanism. For spin relaxation in germanene, spin relaxation becomes dominated by intervalley scattering at low temperatures if with the substrate. And the change of spin relaxation by a substrate is largely due to the substrate-induced modification of e-ph interactions. |
Friday, March 18, 2022 9:24AM - 9:36AM |
Y52.00006: Gate Control of Spin-Layer-Locking FETs and Application to Monolayer LuIO Antimo Marrazzo, Rong Zhang, Matthieu J Verstraete, Nicola Marzari, Thibault Sohier A recent 2D spinFET concept proposes to switch electrostatically between two separate sublayers with strong and opposite intrinsic Rashba effects, exploiting the spin-layer-locking mechanism in centrosymmetric materials with local dipole fields. In this talk, I will discuss a novel monolayer material within this family, lutetium oxide iodide (LuIO), that we identified with first-principles simulations. It displays one of the largest Rashba effects among 2D materials (up to kR = 0.08 Å–1), leading to a π/2 rotation of the spins over just 1 nm. The monolayer was predicted to be exfoliable from its experimentally known 3D bulk counterpart, with a binding energy lower than graphene. I will present our characterisation and our simulations of the interplay between the two gate-controlled parameters for such devices: doping and spin channel selection. Finally, I will discuss how the ability to split the spin channels in energy diminishes with doping, leading to specific gate-operation guidelines that can apply to all devices based on spin-layer locking. |
Friday, March 18, 2022 9:36AM - 9:48AM |
Y52.00007: Dynamical Spin-Orbit-Based Spin Transistor Inanc Adagideli, Fahriye N Gursoy, Cosimo Gorini, Phillipp Reck, Klaus Richter Spin-orbit interaction (SOI) has been a key tool to steer and manipulate spin-dependent transport properties in two-dimensional electron gases. Here we demonstrate how spin currents can be created and efficiently read out in nano- or mesoscale conductors with time-dependent and spatially inhomogenous Rashba SOI. Invoking an underlying non-Abelian SU(2) gauge structure we show how time-periodic spin-orbit fields give rise to spin-motive forces and enable the generation of pure spin currents of the order of several hundred nano-Amperes. In a complementary way, by combining gauge transformations with "hidden" Onsager relations, we exploit spatially inhomogenous Rashba SOI to convert spin currents (back) into charge currents. In combining both concepts, we devise a spin transistor that integrates efficient spin current generation, by employing dynamical SOI, with its experimentally feasible detection via conversion into charge signals. We derive general expressions for the respective spin- and charge conductances, covering large parameter regimes of SOI strength and driving frequencies, far beyond usual adiabatic approaches such as the frozen scattering matrix approximation. We check our analytical expressions and approximations with full numerical spin-dependent transport simulations and demonstrate that the predictions hold true in a wide range from low to high driving frequencies. |
Friday, March 18, 2022 9:48AM - 10:00AM |
Y52.00008: Effects of Nuclear Field Gradients on Spin Transport Nicholas J Harmon, Truman Schulz, Bryan Stevens, Dana Coleman Signatures of large nuclear fields, induced by the process of dynamic nuclear polarization, have been observed in spin transport and optical orientation experiments with n-GaAs [1,2,3]. In this work, the electron spin polarization is modeled using the spin drift-diffusion equation with various terms capturing the complex electron-nuclear spin dynamics in the system. For some boundary conditions, analytic solutions for the steady state polarization are ascertained for arbitrary applied field. Importantly, we extend the conventional spin drift-diffusion equation to include a term accounting for the nuclear field gradient which induces a spin-dependent force on the carriers. Experiments are proposed that would demonstrate the influence of nuclear field gradients on spin transport. |
Friday, March 18, 2022 10:00AM - 10:12AM Withdrawn |
Y52.00009: First-principles prediction of long spin lifetimes in materials with uniform internal magnetic fields Christian Multunas, Ravishankar Sundararaman, Mani Chandra, Jian Shi, Yuan Ping, Lifu Zhang Materials with long spatio-temporal spin-relaxation scales are critical for the realization of spintronic devices. The Dyakonov-Perel (DP) mechanism, wherein randomized spin-precession/mixing occurs due to scattering in a non-uniform internal magnetic field profile in the Brillouin zone, is the dominant relaxation mechanism in systems with broken inversion symmetry. We show that a wide class of hybrid perovskite materials exhibit near-perfect uniformity in the direction of the internal magnetic field, which is known to strongly suppress DP spin relaxation. Using a first-principles density-matrix framework for predicting spin relaxation in a wide class of materials, we computationally demonstrate increased spin life times in these materials. Finally, we investigate the impact of additional symmetry breaking, such as chirality, on the spin dynamics in such materials. |
Friday, March 18, 2022 10:12AM - 10:24AM |
Y52.00010: Ab initio calculation of T2 using Lindbladian dynamics Mani Chandra, Junqing Xu, Christian Multunas, Adela Habib, Yuan Ping, Ravishankar Sundararaman A fundamental requirement for the development of spintronics are materials in which spin ensembles are sustained for sufficiently long length and time scales. The T1 time scale, and the corresponding spin-diffusion length scale, characterizes the decay of an initial spin polarization. However, when prepared in a superposition of energy eigenstates, the decay of the superposition is characterized by time scale T2, which can be much smaller than T1. The characterization of T2 in materials is crucial for the development of spin-qubit interconnects in addition to traditional spintronic applications. Using a recently-developed first-principles Lindbladian density-matrix simulation framework, we directly simulate Hahn spin echo measurements to extract T2 in exactly the same way as in experiments. We present the intrinsic, limiting T2 time scales accounting for spin-phonon relaxation for several materials spanning a wide range of dimensionality, anisotropy and symmetry, and compare them to the corresponding T1 spin relaxation time scales. |
Friday, March 18, 2022 10:24AM - 10:36AM |
Y52.00011: Dresselhaus spin-orbit interaction in p-AlGaAs/GaAs/AlGaAs square quantum well Alexey Suslov, Irina L Drichko, Ivan Y Smirnov, Kirk Baldwin, Loren N Pfeiffer, Kenneth W West The effect of spin-orbit interaction was studied in a high-quality p-AlGaAs/GaAs/AlGaAs square quantum well using the surface acoustic wave (SAW) technique in the frequency range 30 MHz – 300 MHz, at temperatures 20 mK – 300 mK, in the magnetic field of up to 18 T and at various acoustic intensities. The structure grown on a GaAs (100) substrate was symmetrically doped with carbon on both sides of the well and had hole concentration p = 1.2 1011 cm−2 and mobility μ = 1.8 106 cm2/V s at 300 mK. The attenuation and relative velocity of SAW were measured and then the complex ac conductance was calculated. At low magnetic fields B < 2 T observed Shubnikov–de Haas-type oscillations of the ac conductance undergo beatings induced by a spin-orbit interaction. Applying the fast Fourier transform analysis we separated the conductance contributions from the two heavy hole subbands split by the spin-orbit interaction. For each of the subbands the values of the effective masses and quantum relaxation times have been computed, and then the energy of the spin-orbit interaction ΔSO = (0.18 +/- 0.05) meV was obtained. We concluded that the spin-orbit splitting is governed by the Dresselhaus mechanism due to the small magnitude of the spin-orbit interaction and to the square quantum well profile. |
Friday, March 18, 2022 10:36AM - 10:48AM |
Y52.00012: First Principles Search for Fully-Compensated Ferrimagnetic Spin Filter Materials in the Equiatomic Quaternary Heusler Family Matthew E Matzelle, Gavin Winter, Christopher A Lane, Arun Bansil Fully-compensated ferrimagnets (FCFs) are ideal candidates for spin filter applications due to their intrinsic asymmetric band gap between the opposite spin channels. In FCFs, magnetic moments of various atoms cancel to produce a net zero magnetic moment. The strong spin polarization is generated purely by intrinsic exchange splittings and covalent bonding between the atomic species. The FCFs are robust against external coercive fields, making this class of materials suitable for use in nanoscale devices without the need to correct for the deleterious effects of stray coercive fields. Using state-of-the-art first-principles calculations we systematically examine the equiatomic quaternary Heusler family of CrMNAl (M= Ti, Zr, Hf and N = V, Nb, Ta). We identify several new FCFs with large exchange splitting of ~200 meV and small band gaps of ~100 meV beyond CrVTiAl, which has already been shown to be a promising spin-filter candidate material. Robustness against combinatorial mixing and hydrostatic stress is presented. |
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