Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session V36: 2D Materials - Semimetals and Orbital OrderFocus
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Sponsoring Units: DMP Chair: David Tomanek, Michigan State Univ Room: LACC 410 |
Thursday, March 8, 2018 2:30PM - 3:06PM |
V36.00001: Control of spin-orbit torques through crystal symmetry Invited Speaker: Gregory Stiehl In experiments performed to date, spin-orbit torques have an important limitation -- the component of torque that can compensate magnetic damping is required by symmetry to lie within the device plane. This means that spin-orbit torques can drive the most current-efficient type of magnetic reversal (antidamping switching) only for magnetic devices with in-plane anisotropy, not the devices with perpendicular magnetic anisotropy that are needed for high-density applications. Here we show experimentally that this state of affairs is not fundamental, but rather one can change the allowed symmetries of spin-orbit torques in spin-source/ferromagnet bilayer devices by using a spin-source material with low crystalline symmetry. WTe2, a semi-metallic transitional metal dichalcogenide, is one such low symmetry material. Consistent with the symmetries of the WTe2 crystal structure, we generate an out-of-plane antidamping torque when current is applied along a low-symmetry axis of WTe2/Permalloy bilayers, but not when current is applied along a high-symmetry axis [1]. We show directly that the sign of this out-of-plane antidamping torque reverses across a monolayer step in the WTe2, and that the magnitude of this torque depends only weakly on the thickness of WTe2 from 16 nm down to the single monolayer limit [2]. Finally, we compare our observations of spin-orbit torques generated in WTe2 to that another low symmetry crystal, TaTe2 – a monoclinic metallic transition metal dichalcogenide. |
Thursday, March 8, 2018 3:06PM - 3:18PM |
V36.00002: Hidden spin polarization in TaTe$_2$, applications in spintronics Nikhil Sivadas, Gregory Stiehl, Dan Ralph, Craig Fennie We investigate the relationship between the hidden spin polarization and underlying crystal symmetries in TaTe$_2$ using first-principles calculations. Time reversal symmetry combined with bulk inversion symmetry prohibits the presence of a net k-dependent spin-polarization, but does not forbid the presence of a local spin-polarization [1]. Here, we report the presence of such a 'hidden' spin-polarization in TaTe$_2$, a centrosymmetric layered material. On application of an electric field, a net spin polarization is generated in the Te layers, which can result in a local spin-orbit torque (SOT) when interfaced with a ferromagnet. Interestingly, we find that the nature of the SOT changes from Rashba-like to Dresselhaus-like as TaTe$_2$ undergoes a transition from the high symmetry P-3m1 phase to the room temperature C12/m1 phase. Further, we find that the Dresselhaus-like torque can reverse its sign within the C12/m1 space group when the local symmetries are altered. Finally, we compare our first principle calculations to the experimental observation of temperature-dependent Dresselhaus-like SOT in C12/m1 TaTe2/Ni_{80}Fe_{20} heterostructures. |
Thursday, March 8, 2018 3:18PM - 3:30PM |
V36.00003: Using electrostatic doping to control structural phases in monolayer MoTe2 Ying Wang, Jun Xiao, Hanyu Zhu, Yao Li, Yousif Alsaid, King Yan Fong, Yao Zhou, Siqi Wang, Wu Shi, Yuan Wang, Alex Zettl, Evan Reed, Xiang Zhang Monolayers of transition-metal dichalcogenides (TMDs) exhibit numerous crystal phases with distinct structures, symmetries and physical properties. Exploring the physics of transitions between these different structural phases in two dimensions may provide a means of switching material properties, with implications for potential applications. Structural phase transitions in TMDs have so far been induced by thermal or chemical means; purely electrostatic control over crystal phases through electrostatic doping was recently proposed, but has not yet been realized. Here we report first experimental demonstration that electrostatic doping could drive phase transition between the hexagonal and monoclinic phases of monolayer molybdenum ditelluride (MoTe2). Such transition is accompanied with hysteresis measured in Raman and Second Harmonic Generation. Polarization-resolved spectroscopies reveal crystal orientation before and after phase transition. It potentially bridges unique properties in monolayer materials such as valley degree of freedom, Ising pairing and topological transport. Such an unprecedented electrostatic driven structural phase transition opens new door for applications like electronic memory and low power switching. |
Thursday, March 8, 2018 3:30PM - 3:42PM |
V36.00004: Dimensionality-driven orthorhombic MoTe2 at room temperature Rui He, Shazhou Zhong, Hyun Ho Kim, Gaihua Ye, Zhipeng Ye, Logan Winford, Adam Tsen We use variable-temperature Raman spectroscopy to study thin flakes of the type-II Weyl semimetal candidate MoTe2 protected from oxidation. In contrast to bulk crystals, which undergo a phase transition from monoclinic to the inversion symmetry breaking, orthorhombic phase below ~250 K, we find that in moderately thin samples below ~12 nm, a single orthorhombic phase exists up to and beyond room temperature by monitoring characteristic Raman lines from the orthorhombic phase. We propose a c-axis confinement mechanism to interpret this thickness-driven phase stabilization. |
Thursday, March 8, 2018 3:42PM - 3:54PM |
V36.00005: Dimensionality-tuned semimetal-to-metal transition in 1Td-MoTe2 Shazhou Zhong, Archana Tiwari, George Nichols, Fangchu Chen, Xuan Luo, Y.P. Sun, Adam Tsen We study magnetotransport in 1Td-MoTe2 ultrathin flakes at low temperature. The samples are fabricated by mechanical exfoliation and capped with hBN to protect from oxidation. In contrast to bulk crystals, which show large magnetoresistance due to near-perfect electron-hole compensation, we find greater charge imbalance with reduced thickness. The results may explain stabilization of the orthorhombic phase in ultrathin samples as found by He et al. |
Thursday, March 8, 2018 3:54PM - 4:06PM |
V36.00006: Visualizing the unoccupied bulk band structure of topological semimetals with resonant inelastic x-ray scattering. Anirudh Chandrasekaran, Stefanos Kourtis Recently, the resolving power of resonant inelastic x-ray scattering (RIXS) has been dramatically enhanced, thereby enabling ever more detailed characterization of novel materials. The detection of features of topological origin is of particular interest, especially in settings where other spectroscopies do not perform adequately. Here we will demonstrate how RIXS can be used to infer the unoccupied bulk band structure of topological semimetals, thus enabling us to visualize nodal points above the Fermi level. We model the RIXS cross section using low-energy k.p theories derived from realistic material calculations, such as Weyl semimetals MoTe2 and TaIrTe4, which have Weyl nodes in unoccupied bands. Our work promotes high-resolution RIXS as a viable method for the imaging of bulk three-dimensional band structures. |
Thursday, March 8, 2018 4:06PM - 4:18PM |
V36.00007: Mottness collapse in 1T-TaS2-xSex transition metal dichalcogenide: an interplay between localized and itinerant orbitals Shuang Qiao, Xintong Li, Naizhou Wang, Wei Ruan, Peng Cai, Zhenqi Hao, Hong Yao, Xianhui Chen, Jian Wu, Yayu Wang, Zheng Liu The layered transition metal dichalcogenide 1T-TaS2 has been recently found to undergo a Mott-insulator-to-superconductor transition induced by high pressure, charge doping, or isovalent substitution. By combining scanning tunneling microscopy (STM) measurements and first-principles calculations, we investigate the atomic scale electronic structure of 1T-TaS2 Mott insulator and its evolution to the metallic state upon isovalent substitution of S with Se. We identify two distinct types of orbital textures - one localized and the other extended - and demonstrate that the interplay between them is the key factor that determines the electronic structure. Especially, we show that the continuous evolution of the charge gap visualized by STM is due to the immersion of the localized-orbital-induced Hubbard bands into the extended-orbital-spanned Fermi sea, featuring a unique evolution from a Mott gap to a charge-transfer gap. This new mechanism of Mottness collapse revealed here suggests an interesting route for creating novel electronic states and designing future electronic devices. |
Thursday, March 8, 2018 4:18PM - 4:30PM |
V36.00008: Theory of Mott physics in 1T-NbSe2: An LDA+DMFT study Ebad Kamil, Jan Berges, Gunnar Schönhoff, Malte Schueler, Tim Wehling Transition metal dichalcogenides (TMDCs) often occur in two different crystal phases with octahedral (1T) and trigonal prismatic (2H) symmetry. For NbSe2, the 2H phase has been known to be the most stable and easier to synthesize polymorph. However, recently 1T-NbSe2 in the monolayer limit was synthesized epitaxially on a bilayer graphene and was suggested to be a Mott insulator [Yuki Nakata et al. NPG Asia Materials (2016) 8, e321] with √13 × √13 periodic density modulation. We perform ab-initio calculations to understand the emergence of an insulating behavior and charge density wave (CDW) in the monolayer 1T-NbSe2, which otherwise is predicted to be a metal in DFT and GW calculations. We provide an estimate for the local and non-local screened Coulomb interaction within the ab-initio formalism and present the findings of LDA+DMFT simulations that suggest the possibility of opening of a Mott insulating gap in the CDW phase. |
Thursday, March 8, 2018 4:30PM - 4:42PM |
V36.00009: Disorder-driven metal-insulator transition in Pt doped 1T-TiSe2 Kyungmin Lee, Nandini Trivedi, Junjing Zhao, Utpal Chatterjee, Thomas Neulinger, Zhenyu Wang, Vidya Madhavan, Jesse Choe, Chien-Lung Huang, Emilia Morosan We have conducted a multipronged investigation, which involves transport, Angle Resolved Photoemission Spectroscopy (ARPES) and Scanning Tunneling Spectroscopy (STS) measurements and theoretical modeling, of pristine and Pt doped 1T-TiSe2, a canonical Charge Density Wave (CDW) material. Transport data finds that at temperatures much lower than CDW transition temperature (TCDW), 1T-TiSe2 shows metallic behavior, while Pt doped samples display insulating behavior. STS reveals the formation of CDW domains with a large Mott gap separated by metallic or semi-metallic domain walls with a small pseudogap. We will present results obtained from a theoretical modeling of transport via the network of domain walls. Our combined study indicates an unconventional disorder-driven metal-insulator transition in Pt doped 1T-TiSe2 samples. |
Thursday, March 8, 2018 4:42PM - 4:54PM |
V36.00010: Correlation-driven Metal-Insulator Transition in Graphite Under High Magnetic Field Zhiming Pan, Ryuichi Shindou Graphite under high magnetic field exhibits a metal-insulator transition as well as insulator-metal re-entrant transition in the quasi-quantum limit at low temperature. The Hall conductivity experiment suggests that the electronic system is nearly in the charge-neutrality point. To explain the re-entrant transition, we employ a model with two-electron pockets and two-hole pockets, to construct a bosonized Hamiltonian that comprises of displacement fields along the field direction. We enumerate all possible umklapp progresses in which both the electron and hole pockets are involved and which are allowed in the model under the charge neutrality condition. Using the variational method and renormalization group argument, we show that there exists a critical interaction strength above which the umklapp terms become relevant, and the system enters a Mott insulator phase with spin nematic order. By showing how the critical interaction strength depends on Luttinger parameters of each pockets and temperature, we give a simple phenomenology of the insulator-metal re-entrant transition in graphite under high field. |
Thursday, March 8, 2018 4:54PM - 5:06PM |
V36.00011: Correlated Insulator Behaviour at Half-Filling in Magic Angle Graphene Superlattices Yuan Cao, Valla Fatemi, Ahmet Demir, Shiang Fang, Spencer Tomarken, Jason Luo, Javier Sanchez-Yamagishi, Kenji Watanabe, Takashi Taniguchi, Efthimios Kaxiras, Raymond Ashoori, Pablo Jarillo-Herrero Twist angle between different layers of a van der Waals heterostructure plays a crucial role in the ultimate electronic properties. So far, the study of the effect of twist angles in vdW heterostructures has been mostly concentrated in graphene/hexagonal boron nitride (h-BN) twisted structures, which exhibit relatively weak interlayer interaction due to the presence of a large bandgap in h-BN. Here we show that when two graphene sheets are twisted by an angle close to the theoretically predicted 'magic angle', the resulting flat band structure near charge neutrality gives rise to a strongly-correlated electronic system. These flat bands exhibit half-filling insulating phases at zero magnetic field, which we show to be a Mott-like insulator arising from electrons localized in the moiré superlattice. These unique properties of magic-angle twisted bilayer graphene (TwBLG) open up a new playground for exotic many-body quantum phases in a 2D platform made of pure carbon and without magnetic field. |
Thursday, March 8, 2018 5:06PM - 5:18PM |
V36.00012: Interactions and Renormalization of Semi-Dirac Fermions Valeri Kotov, Bruno Uchoa, Oleg Sushkov We develop full renormalization group (RG) analysis of 2D semi-Dirac |
Thursday, March 8, 2018 5:18PM - 5:30PM |
V36.00013: Phases of Interacting Dirac Matter Habib Rostami, Saikat Banerjee, Gabriel Aeppli, Alexander Balatsky We investigate the effects of interactions on the Dirac and Weyl nodal states in various dimensions. The logarithmic velocity renormalization of the Dirac excitation velocity in graphene is but one example of the effects of interactions on the nodal states. We expand this result and find the nodal velocity renormalization effects dependent on the the nature of the interaction and dimensionality. We find the unusual Lifshitz transitions that result in the novel phases with nodal line and nodal sphere that emerge from the Dirac node in case of strong interactions. Our results,while making contact with the established facts about effects of Coulomb interactions lead to a rich phase diagram for interacting Dirac matter. |
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