Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session K38: 2D Magnetism IIFocus
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Sponsoring Units: GMAG DMP Chair: Giovanni Vignale, University of Missouri Room: BCEC 206B |
Wednesday, March 6, 2019 8:00AM - 8:12AM |
K38.00001: Spin Transfer Torques in Monolayer WSe2/PMA Heterostructures Steve Novakov, Nguyen Vu, Bhakti Jariwala, Azim Kozhakhmetov, Guanyu Zhou, Christopher Hinkle, Joshua Robinson, John Heron
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Wednesday, March 6, 2019 8:12AM - 8:24AM |
K38.00002: Imaging the Electronic Structure of Magnetic and Non-Magnetic Atomic Impurities in Monolayer Semiconductors Caleb Zerger, Alex Contryman, Hong Li, Xiaolin Zheng, Hari Manoharan It has been predicted that a two dimensional dilute magnetic semiconductor (2D DMS) can be formed by suitably doping monolayer transition metal dichalcogenides with magnetic materials. Here we present scanning tunneling microscopy (STM) and spectroscopy (STS) data on these doped monolayers. We identify and characterize multiple impurity types resulting from the doping procedure, observing both magnetic and non-magnetic impurities. Using a full spectral mapping in position and energy space, and through comparison to density functional theory calculations, we can associate these impurity states with different impurity types and incorporation sites. For example, we have found transition metal substitutional dopants and a dichalcogenide substitutional dopant. Specific dopant arrangements demonstrate strong spin splitting, a necessary ingredient towards achieving a 2D DMS state in monolayer semiconductors. |
Wednesday, March 6, 2019 8:24AM - 8:36AM |
K38.00003: Magneto-Optical Kerr Effect microscopy on the intercalated transition metal dichalcogenides FexTaS2 Arielle Little, Dylan Rees, Spencer Doyle, Caolan John, Eran Maniv, James G. Analytis, Joseph Orenstein The layered transition metal dichalcogenide TaS2 exhibits a broad range of phases including CDW states and superconductivity [1]; the polymorph 1T-TaS2 has even been predicted to host a spin liquid [2]. Also of interest is the doping of such materials with metallic ions. In TaS2 the intercalation of Iron between layers stabilizes ferromagnetic order. The magnetic properties vary drastically with the amount of intercalant, x [3]. For example, for x=0.33 chiral superlattice structures form [4]. Here we employ scanning Magneto-Optical Kerr Effect (MOKE) microscopy to image the onset of magnetic order and in some instances optical birefringence in compounds with intercalation from x=0.26 to 0.33. In the x=0.33 compound we report a sharp structural phase transition coincident with the magnetic transition and image the resulting domains on a 10-μm scale. |
Wednesday, March 6, 2019 8:36AM - 9:12AM |
K38.00004: Thermodynamic "valley noise" in monolayer semiconductors: access to intrinsic valley relaxation timescales Invited Speaker: Mateusz Goryca The new class of atomically-thin transition-metal dichalcogenide (TMD) semiconductors such as monolayer MoS2 and WSe2 has focused broad attention on the concept of "valleytronics", founded on the idea of encoding information in an electron’s valley degree of freedom. A key parameter is therefore the intrinsic timescale of an electron’s inter-valley relaxation, and recent optical pump-probe studies have shown long (microsecond) valley relaxation of resident carriers in monolayer TMDs [1,2]. However, a significant drawback of all such pump-probe experiments is that they are by design perturbative: the optical pumping that drives the carrier polarization away from equilibrium also inevitably introduces "dark" excitons, whose presence may mask the intrinsic valley relaxation of resident carriers. |
Wednesday, March 6, 2019 9:12AM - 9:24AM |
K38.00005: Gap opening at the Rashba-split states of Janus transition metal dichalcogenides induced by two-dimensional ferromagnetic semiconductor CrI3 Tao Hu, Wei Ren The Janus transition metal dichalcogenides intrinsically have Rashba spin splitting due to the presence of the out-of-plane mirror-symmetry breaking and strong spin-orbit coupling (SOC). Using first-principles calculations, we show that a van der Waals (vdW) heterostructure consisting of WSeTe and CrI3 has inherently coexistence of Rashba and Zeeman spin splitting near the Brillouin zone center (Γ). The Rashba spin splitting states of WSeTe open an exchange gap of 29 meV when it is placed on the top of two-dimensional (2D) ferromagnetic semiconductor CrI3. The spin-polarized subbands of Rashba-split states are separated at the Γ point because of the existence of effective perpendicular magnetic field stemmed from CrI3. In the crossover region of Rashba and Zeeman spin splitting, the spin orientations of the two subbands are locked to the momenta. A single-circle Fermi contour could emerge if the Fermi level is tuned to locate exactly within the gap between the two subbands. Our results provide important insight into the manipulation of spin states for future spintronics applications of 2D vdW heterostructures. |
Wednesday, March 6, 2019 9:24AM - 9:36AM |
K38.00006: Influence of the spatial fluctuations of the Rashba field and magnetization on the electron and spin transport in 2D systems Anna Dyrdal, Sylwia Kudla, Vitalii Dugaev, Evgeny Sherman, Jozef Barnas, Jamal Berakdar In a general case, spin-orbit coupling contains regular (spatially uniform, periodic) and a random component. Local imperfections, such as a random distribution of donors or impurities, may lead to local modifications of the spin-orbit coupling. Similarly, magnetization in the system may reveal spatial fluctuations as well. Such fluctuations modify transport properties of the system and induce a variety of observable effects [Physica E 42, 2157 (2010); Nat. Phys.12, 920(2016)]. |
Wednesday, March 6, 2019 9:36AM - 9:48AM |
K38.00007: Electrically switching the valley polarization of 2D semiconductor Lizhong Li, Shengwei Jiang, Zefang Wang, Jie Shan, Kin Fai Mak Controlling the valley degree of freedom in transition metal dichalcogenide(TMD) semiconductors is essential to the application of valleytronic devices. The strong magnetic coupling between monolayer TMD semiconductor WSe2 and few layer CrI3, a recently discovered 2D magnet, provides a novel approach for lifting the valley degeneracy and switching the valley polarization. In this talk we present our result on gate controlled valley polarization in CrI3/WSe2 heterostructures. Due to the short-range magnetic proximity interaction, WSe2 shows clear valley polarization controlled by the spin orientation of CrI3. We demonstrate that the magnetic coupling between WSe2 and CrI3 could be effectively tuned by gate voltage, allowing reversible and efficient electrical switching of the valley polarization in WSe2. |
Wednesday, March 6, 2019 9:48AM - 10:24AM |
K38.00008: Electrical gate control of photon-spin and photon-charge conversions in van der Waals heterostructures Invited Speaker: Yunqiu (Kelly) Luo Semiconductor heterostructures of dissimilar materials are inherently limited by their interface quality, lattice mismatch, and intrinsic defects. These obstacles are overcome in van der Waals (vdW) heterostructures forming atomically sharp interfaces even between very different 2D materials. Our recent work demonstrates the efficient optically-created spin-polarized charge transfer across monolayer MoS2/graphene vdW interface, including complete control of spin polarization with photon helicity and photon energy up to room temperature. However, the underlying mechanisms and the qualitative trends in both photon-spin and photon-charge conversions remain elusive. To investigate origins of photon-charge conversion, we build a dual-gated MoS2/graphene field-effect device which allows multi-variable control of photon energy, bias voltage, and top and bottom gates. We observe an intriguing bias enhancement of photoconductivity, which behaves oppositely across graphene Dirac point. We further investigate the origins of photoconductivity using both above and below MoS2 band gap photon excitation. Gate dependence and photon intensity dependence indicate strongly towards the dominance of graphene photothermoelectric effect for below gap excitation, while an interplay of MoS2 photovoltaic effect and graphene photothermoelectric effect mediates above gap excitation. DFT and analytical models connect the role of bias voltage as an independent density of states modulation, which enhances the charge tunneling efficiency across the vdW barrier. In the end, we will briefly discuss the ultrafast photon-spin transfer across monolayer WSe2/graphene interface, as well as an outlook to the challenges and future of photon-spin and photon-charge conversions based on vdW hybrid systems. |
Wednesday, March 6, 2019 10:24AM - 10:36AM |
K38.00009: Unconventional transport in low-density two-dimensional Rashba systems Joel Hutchinson, Joseph Maciejko Rashba spin-orbit coupling appears in 2D systems lacking inversion symmetry, and causes the spin-splitting of otherwise degenerate energy bands into an upper and lower helicity band. In this talk, we explore how impurity scattering affects transport in the ultra-low density regime where electrons are confined to the lower helicity band. A previous study has investigated the conductivity in this regime using a treatment in the first Born approximation. In this work, we use the full T-matrix to uncover new features of the conductivity. We first present results for the conductivity within a semiclassical Boltzmann framework and show that it exhibits an unconventional density dependence due to the unusual features of the group velocity in the single particle dispersion, as well as quantized plateaus as a function of the logarithm of the electron density. We support this with results from the Kubo formula and find that these plateaus persist in the full quantum theory. We suggest that this quantization may be seen in a pump-probe experiment. |
Wednesday, March 6, 2019 10:36AM - 10:48AM |
K38.00010: Magnetism of a Magnetic-ion-intercalated Transition Metal Dichalcogenide with a Chiral Structure Kai Du, Jae Wook Kim, Fei-Ting Huang, Seong Joon Lim, Kasun Gamage, Junjie Yang, Myung-Geun Han, Sang-Wook Cheong Chiral magnets are materials where various intriguing spin orders including skyrmions, chiral soliton lattice, and topological domain walls can be found in the presence of the structural chirality. On the other hand, the hunting for exotic magnetic systems that are possible to fabricate functional two-dimensional devices for applications is also in high demand. In this work, we demonstrate that a magnetic-ion-intercalated transition metal dichalcogenide (TMD) is a layered chiral magnet according to our magnetic measurements and Lorenz transmission electron microscopy studies. Using transmission electron microscopy and magnetic force microscopy, we investigated its chiral structural domains and magnetic domains, respectively. Their domain topologies and relations will also be discussed. |
Wednesday, March 6, 2019 10:48AM - 11:00AM |
K38.00011: Spin-Phonon Coupling, Spin Waves, and Other Magnetic Phenomena in Layered XPS3 Materials via Raman Spectroscopy Amber McCreary, Jun Cao, Xi Ling, Robert D McMichael, Angela Hight Walker Raman spectroscopy is a powerful, non-destructive optical method to probe the fundamental physics of two-dimensional (2D) layered materials through inelastic scattering. An amazing amount of information is quantified from the spectra such as layer thickness, disorder, edge and grain boundaries, strain, etc. More interestingly for 2D materials is that Raman efficiently probes the evolution of the electron-phonon and spin-phonon interactions as a function of temperature, laser energy, polarization, and magnetic field. Using our unique magneto-Raman capabilities, we study the magnetic properties of the metal phosphorus trisulfide family (XPS3, where X = Fe, Mn, and Ni) which are layered antiferromagnetic semiconductors. While the three materials have the same crystal structure, their varying spin structures result in distinct behavior as a function of temperature and magnetic field, which will be presented herein. In FePS3, we investigate the splitting and shifting of a non Γ-point phonon mode below the Neel temperature that is not present in MnPS3, as well as the emergence of a spin wave with anomalous symmetry behavior. In addition, we have studied the apparent two-magnon mode in NiPS3 under various conditions. |
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