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 P36: Novel Topological and Magnetic MaterialsFocus Live
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Sponsoring Units: GMAG DMP FIAP DCOMP Chair: Olle Heinonen, Argonne National Laboratory |
Wednesday, March 17, 2021 3:00PM - 3:12PM Live |
P36.00001: Canted Spin Texture and Quantum Spin Hall Effect in WTe2 José Garcia Aguilar, Marc Vila, Chuang-Han Hsu, Xavier Waintal, Vitor Manuel Pereira, Stephan Roche We report an unconventional quantum spin Hall phase in the monolayer Td-WTe2, which exhibits hitherto unknown features in other topological materials. The low-symmetry of the structure in-duces a canted spin texture in theyzplane, which dictates the spin polarization of topologically protected boundary states. Additionally, the spin Hall conductivity gets quantized (2e2/h) witha spin quantization axis parallel to the canting direction. These findings are based on large-scale quantum simulations of the spin Hall conductivity tensor and nonlocal resistances in multi-probe geometries using a realistic tight-binding model elaborated from first-principle methods. The observation of this canted quantum spin Hall effect, related to the formation of topological edge stateswith nontrivial spin polarization, demands for specific experimental design and suggests interestingalternatives for manipulating spin information in topological materials. |
Wednesday, March 17, 2021 3:12PM - 3:24PM Live |
P36.00002: Phononic control of magnetism in a topological insulator Hari Padmanabhan, Nathan Koocher, Peter Kim, Maxwell Poore, Vladimir A Stoica, Danilo Puggioni, Huaiyu Wang, Seng Huat Lee, Maxwell Wetherington, Richard D Schaller, Richard Averitt, James M Rondinelli, Venkatraman Gopalan Magnetism in topological materials can give rise to unconventional topological phases that exhibit intriguing behavior such as the quantized magnetoelectric effect and the quantum anomalous Hall effect. The key to harnessing these properties in device applications lies in the ability to control magnetism. In this work, we demonstrate for the first time, the optical control of magnetism in a topological insulator. This is achieved in the recently discovered magnetic topological insulator MnBi2Te4. |
Wednesday, March 17, 2021 3:24PM - 3:36PM Live |
P36.00003: Anomalous Hall effect in ferromagnetic semiconductors: the case of Ga1-xMnxAs1-yPy Xinyu Liu, Logan Riney, Jiashu Wang, Sining Dong, Lei Guo, Ren-Kui Zheng, Xiang Li, Seul-Ki Bac, Jacek Kossut, Malgorzata Dobrowolska, J K Furdyna, Yi-Ting Hsu, Badih A Assaf While the carrier-mediated ferromagnetism on GaMnAs has been thoroughly studied in the past, the origin of anomalous Hall effect (AHE) in magnetic III-V systems remains elusive, particularly for the quaternary GaMnAsP. We have thus measured the AHE in a series of GaMnAsP alloys grown by molecular beam epitaxy with varying amounts of phosphorus, both as-grown and annealed. We carried out extensive measurements of transverse and longitudinal resistivity as a function of both magnetic field and temperature. By applying a modified Brillouin-Weiss model for the magnetization, we decompose the AHE into an “extrinsic” contribution (due to scattering) and a dominating “intrinsic” contribution (due to the Berry curvature). Our data strongly indicates that the influence of the intrinsic mechanism is important in samples with low phosphorus content. Importantly, we find that the amount of the intrinsic contribution is tunable by controlling the phosphorus concentration in GaMnAsP alloys. Consequently, our results suggest that the valence bands of III-Mn-V compounds could host a strong Berry curvature, possibly driven by the strain effects that are known to occur in GaMnAsP grown on GaAs. |
Wednesday, March 17, 2021 3:36PM - 3:48PM Live |
P36.00004: Orbital hybridization in the spin-glass state of Mn-doped ZnTe Alexandria R Alcantara, Steven Barrett, Dina Matev, Ireneusz Miotkowski, Anant K Ramdas, Thomas Pekarek, Jason Haraldsen To gain insight into the spin-glass state of diluted magnetic semiconductors, we have examined the magnetic and electronic properties of Zn1-xMnxTe using density functional theory and magnetization measurements, where our measurements on the x = 0.43 and 0.55 samples demonstrate a clear spin-glass transition. Using a generalized gradient approximation, we investigate the electronic and magnetic properties for x = 0, 0.25, and 0.50 doping levels using the magnetic moment of Mn2+ as a guide for the dependence of the Hubbard onsite potential on the electronic structure. Simulations on both ferromagnetic (FM) and antiferromagnetic (AFM) configurations yield a distinct AFM ground state preference, where an onsite potential of up to 8 eV on the Mn 3d-orbitals is needed to harden the magnetic moment toward S = 5/2. From our analysis of the electronic structure evolution with doping and onsite potential, we confirm the semiconducting state of the material as well as show that Te-Mn pd-orbital hybridization is activated with the Mn doping above 25%, which is around the doping when the spin-glass transition begins to rise, which could be a precursor to the spin-glass state. |
Wednesday, March 17, 2021 3:48PM - 4:00PM Live |
P36.00005: Phonon vibration modes of layered MnBi2nTe3n+1 (n=1,2,3,4) topological heterostructures Yujin Cho, Jin Ho Kang, Liangbo Liang, Alexander Puretzky, Xiangru Kong, Subhajit Ghosh, Fariborz Kargar, Chaowei Hu, Ni Ni, Alexander A Balandin, David B Geohegan, Chee Wei Wong MnBi2Te4 is the first experimentally demonstrated intrinsic antiferromagnetic topological insulator, whose properties are tunable with additional Bi2Te3 layers between adjacent MnBi2Te4 layers. In this talk, we present the phonon modes of bulk MnBi2nTe3n+1 (n=1,2,3,4) at various temperatures and polarization configurations, using optical Raman spectroscopy. Our density functional theory calculation shows that the peaks at 66 cm-1 and 112 cm-1 are mainly generated by oscillating Mn-Te chains. In MnTe2Te4, below the transition temperature, we observed an abnormal change of those two peaks due to spin-phonon coupling. In MnBi4Te7, the additional Bi2Te3 induces Davydov splitting of the A1g mode at 136 cm-1 at room temperature. From the linear chain model, we estimated the out-of-plane interlayer force constant to be 4.48×1019 N/m3, 75% weaker than that of Bi2Te3. As the number of Bi2Te3 layer increases, the Raman spectrum is dominated by the vibrational modes in Bi2Te3. Our work provides useful guidance to tailoring their physical properties in future research. |
Wednesday, March 17, 2021 4:00PM - 4:12PM Live |
P36.00006: Prediction of momentum dependent spin splitting in low-Z collinear and noncollinear antiferromagnets even without spin-orbit coupling Linding Yuan, Zhi Wang, Jun-Wei Luo, Emmanuel I Rashba, alex zunger The conventional way of creating spin splitting without external field entails spin orbit coupling (SOC). The latter requires heavy elements that lead to weak anion-cation bonds and undesirable defects. Pekar and Rashba [Zh. Eksp. Teor. Fiz. 47, 1927 (1964)] envisioned an alternative mechanism in antiferromaagnets (AFM) that would generate momentum dependent spin splitting. We identify the magnetic symmetry conditions that produce AFM prototypes having spin splitting even without external magnetic field and even when the SOC is set to zero.[Yuan, Wang, Luo, Rashba and Zunger. Physical Review B 102.1 (2020): 014422.] The resulting spin splitting in AFM is not restricted to noncentrosymmetric crystals, has a magnitude comparable to best known giant SOC spin splitting, and does not rely on SOC. Band structures of specific antiferromagnetic compounds for both collinear and noncollinear are worked out within DFT showing the effects. [Yuan, Wang, Luo, and Zunger. arXiv:2008.08532 (2020).] We envision that use of the current design principles to identify an optimal antiferromagnet with spin-split energy bands would be beneficial for spintronic applications without the burden of requiring compounds containing heavy elements. |
Wednesday, March 17, 2021 4:12PM - 4:24PM Live |
P36.00007: Experimental Signatures of Ground State and Photoexcited Edge Magnetism in Phosphorene Nanoribbons Arjun Ashoka, Christopher A Howard, Akshay Rao, Raj Pandya Zig-zag edge terminated phosphorene nanoribbons (z-PNRs) are mono-elemental intrinsically anisotropic two-dimensional (2D) semiconductors with a plethora of predicted exotic properties e.g. room-temperature edge ferromagnetism. Recently z-PNRs have been produced using a top-down chemical synthesis. Using femtosecond impulsive stimulated Raman spectroscopy on these z-PNRs, we demonstrate coupling between an excited state and a symmetry forbidden edge phonon mode allowing us to isolate signatures of excitations at the edge of the ribbons. Using polarisation sensitive pump-probe spectroscopy with sub-1T magnetic fields we demonstrate signatures of room temperature ground state ferromagnetism in these materials via a magnetic field-induced anisotropy. Calculations reveal spin-spin correlation lengths of at least 500 lattice sites in z-PNRs. We further find that the excited state dynamics cannot be fully resolved by a ground state orientation effect, suggesting the excited state edge-excitations additionally couple to the magnetic field. These results present a significant step towards room temperature optically controlled magnetism in semiconductors. |
Wednesday, March 17, 2021 4:24PM - 4:36PM Live |
P36.00008: Spin Texture Probed with Magnetoresistance in Ferromagnetic EuO1-x Thin Films Narendra Shrestha, Jinke Tang At present significant research effort is being placed in the study of topological particle-like magnetic skyrmions for electronic and spintronic applications. We have prepared thin films of EuO1-x on Si (001) substrate by pulsed laser deposition using europium (Eu) metal as a target in order to examine its spin-dependent transport. We present the angular dependence of magnetoresistance in EuO1-x thin films. We found that at low temperature below 30 K, ΔRout = R(Bx or y) – R(Bz) is maximum at 1 T, between 30K to 70K it is maximum at 2 T and at 90K it is maximum at 3T. And, it decreases with increasing applied field. Whereas, at high temperature above 90 K, ΔRout increases as the field is increased. And field in-plane ΔRin = R(Bx) – R(By) is almost zero. This could be because of the topological phase and its density variation present in EuO1-x below the transition temperature and suggest a two dimensional spin-texture. The magnetic anisotropy is also enhanced for the transport measurements, and is much larger than that obtained from magnetic measurements, which may result from strong spin-orbit coupling in EuO1-x and the presence of the spin texture. |
Wednesday, March 17, 2021 4:36PM - 4:48PM Live |
P36.00009: Dielectric and magnetic properties AYESHA USMAN, Zohra Kayani Ag-doped ZnO thin flms are prepared by the cost-efective sol–gel dip-coating method at room temperature. The Ag dopant percentage varies between (2–10) wt%. The magnetic and dielectric properties have been studied. The dielectric and magnetic properties of ZnO are signifcantly tailored by the increase in the Ag doping percentage. High dielectric constant and tangent loss have been observed at low frequencies which decreases with the increase in frequency. The AC conductivity is lower in the low-frequency region but has larger values in the high-frequency region. The ferromagnetic behavior of flms has been recorded at room temperature. Magnetic polarons play a pivotal role in the development of room temperature ferromagnetism in Ag-doped ZnO thin flms. So, ferromagnetism in thin flms is governed by bound magnetic polarons. As the doping concentration increased, the saturation magnetization decreased and coercivity increased due to the combined effect of the decrease in crystallite size, generation of large defects, and formation of bound magnetic polarons. These Ag doped ZnO thin flms are suitable for spintronics. |
Wednesday, March 17, 2021 4:48PM - 5:00PM Live |
P36.00010: Computational investigation of the spin-1 state of Ti-doped CdSe John Dimuna, Tucker Boyett, Ireneusz Miotkowski, Anant K Ramdas, Thomas Pekarek, Jason Haraldsen Using computational and experimental techniques, we examine the 2+ oxidation state of Ti when doped into CdSe. Through stoichiometry and confirmed through magnetization measurements, the weakly-doped material of Cd1-xTixSe (x = 0.0043) shows the presence of a robust spin-1 magnetic state of Ti, which is indicative of the 2+ oxidation state. Given the obscure nature of the Ti2+ state, we investigate the electronic and magnetic states using density functional theory. Using a GGA+U basis, we determine the electronic structure, magnetic moment density, and optical properties for a supercell of CdSe with an ultra-low concentration of Ti. We find that an onsite potential of 4-6 eV must be included in order to reproduce the magnetic moment of spin-1. The evolution of the electronic properties as a function of the Hubbard U shows that the Ti-d orbital drops below the Fermi around U = 4 eV along with the onset of a semiconducting state, and the impurity state mixes with the lower valence bands to produce the 2+ state for the Ti atom. |
Wednesday, March 17, 2021 5:00PM - 5:12PM Live |
P36.00011: Resonant tunneling anisotropic magnetoresistance induced by magnetic proximity Chenghao Shen, Timothy Leeney, Alex Matos Abiague, Benedikt Scharf, Jong E Han, Igor Zutic We reveal that the interplay between Rashba spin-orbit coupling and proximity-induced magnetization in a two-dimensional electron gas leads to peculiar transport properties and large anisotropy of magnetoresistance. While the related tunneling anisotropic magnetoresistance (TAMR) has been extensively studied before, we predict an effect with a different origin arising from the evolution of a resonant condition with the in-plane rotation of magnetization and having a much larger magnitude. The resonances in the tunneling emerge from a spin-parity-time symmetry of the scattering states, which is generalized from the parity-time symmetry for the resonances in a simple potential barrier system. However, such a symmetry is generally absent from the system itself and only appears for certain parameter values. Without resonant behavior in the topological surface states of a proximitized three-dimensional topological insulator (TI), TAMR measurements can readily distinguish them from often misinterpreted trivial Rashba-like states inherent to many Tis [1]. |
Wednesday, March 17, 2021 5:12PM - 5:24PM Live |
P36.00012: Insights into the local atomic and magnetic structure of MnTe using Magnetic Pair Distribution Technique Raju Baral, Benjamin Frandsen MnTe is an antiferromagnetic semiconductor with potential applications as a high-performance thermoelectric and as a platform for spintronics. The hexagonal crystal structure supports antiferromagnetic ordering of the Mn2+ spins below 307 K. Interestingly, robust short-range magnetic correlations called paramagnons survive well above the transition temperature and are believed to be a key ingredient for the high value of the thermoelectric figure of merit zT in doped MnTe. We report comprehensive atomic and magnetic pair distribution function (PDF) analysis of neutron total scattering data collected from pure and Na-doped MnTe in the temperature range of 2 K – 500 K. The results provide a detailed view of the local atomic and magnetic structure of MnTe in all relevant temperature regimes and help establish a more complete understanding of the excellent properties observed in this system. In particular, the magnetic PDF analysis yields valuable information about the short-range spin correlations above TN and helps explain some unexpected magnetic behavior that has previously been observed in the low-temperature regime. |
Wednesday, March 17, 2021 5:24PM - 5:36PM Live |
P36.00013: Topologically nontrivial magnetism in Cr2Te3 ultrathin films Hang Chi, Yunbo Ou, Tim B. Eldred, Wenpei Gao, Valeria Lauter, Michael Dreyer, Robert E Butera, George J. de Coster, Charles Rong, Donald E Heiman, Jagadeesh S Moodera Transition metal chalcogenides are versatile for realizing topological phenomena. Quasi 2D Cr2Te3 MBE films enable control over charge/spin degrees of freedom, leveraged by strain tunability at the substrate/film interface. Cr2Te3 films display perpendicular magnetic anisotropy with a Curie temperature of ~ 160 K for thickness of 20 unit cells, whose structure and magnetic properties are examined by transmission electron microscopy, polarized neutron reflectometry and scanning tunneling microscopy. This is accompanied by rich magnetotransport properties: apart from the ordinary Hall effect with linear field dependence, the anomalous Hall effect manifests a unique temperature-dependent sign reversal. Novel topological Hall effect (THE) appears, related to the emergence of magnetic skyrmions, nanometer-sized quasiparticles with topological spin textures. The magnitude of the THE is tunable via interface strain that can be varied by film thickness and choice of substrate (Al2O3 and SrTiO3). The effective interface-driven tunability of non-coplanar spin textures in Cr2Te3 ultrathin films offers new opportunities for spintronics. |
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