Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session B35: Spin in Monolayer Materials |
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Sponsoring Units: DCMP Chair: Ahmet Avsar, École Polytechnique Federale de Lausanne Room: LACC 409B |
Monday, March 5, 2018 11:15AM - 11:27AM |
B35.00001: Tailoring the Valley Polarization in Monolayer MoS2 by Plasmonic Chiral Structures Chien-Ju Lee, Hung-Chih Li, Wen-Hao Chang Monolayer transition metal dichalcogenides (TMDs) are semiconductors exhibiting direct band gaps at two degenerate but inequivalent valleys of electronic bands, which can endow carriers with valley pseudo-spins for future valleytronic applications. Although the control of valley polarization can be created by the helicity of light, it remains difficult to create valley polarization by lifting the valley degeneracy using magnetic field due to the small valley Zeeman splitting for typical monolayer TMDs. Here, we report on manipulating valley polarization in monolayer MoS2 by plasmonic chiral structures. The chiral structures are based on Archimedean spiral antennas fabricated on single-crystalline Au flakes using focused ion beam. The resonance wavelength of these structures can be controlled by their geometry. As the chiral structure is on resonance with the excitonic emission of MoS2, an imbalance between the left- and right-circularly polarized components of the optical density of states is created, which in turn affects the valley decay dynamics, giving rise to a net valley polarization and a high degree of circular polarization even at room temperature. Our results demonstrate a route for tailoring the valley-dependent dynamics of carriers in monolayer TMDs. |
Monday, March 5, 2018 11:27AM - 11:39AM |
B35.00002: Graphene with Spin-Orbit Coupling: Transport and Wave Packet Dynamics in presence of Barriers Ranjani Seshadri, Diptiman Sen, Krishnendu Sengupta I will be discussing some interesting phenomena that arise when we consider Spin-Orbit (S-O) |
Monday, March 5, 2018 11:39AM - 11:51AM |
B35.00003: Spin magnetism induced by chemical tuning of topology in Graphene Kazuyuki Takai, Kentaro Tajima, Takuya Isaka, Tomoki Yamashina, Yoshiaki Matsuo Introducing vacancies into the graphene lattice causes the asymmetry in sub-lattices, resulting in spin-polarized localized states near the Dirac point. Chemical modification also tunes topology of graphene lattice due to vacancies in pi-electron network of graphene induced by covalent bonding to other elements. In this study, we have tuned the topology of honeycomb lattice by oxidizing graphene in two methods, and examined its structure and magnetism. Graphene oxide was synthesized by Hummers and Brodie methods (HGO and BGO). According to FT-IR, XPS and XRD, -OH and C-O-C groups are more likely to be introduced to HGO and BGO, respectively. HGO shows an order of magnitude larger localized spin density than that of BGO. The difference in functional groups between HGO and BGO explains the larger spin magnetism for HGO. In the case of introduction of C-O-C, oxygen atom bonds to adjacent carbon atoms, remaining symmetry in sub-lattices of graphene. In contrast, attaching -OH group occurs randomly on carbon atoms of graphene and breaks symmetry in two sub-lattices, resulting in the emergence of the localized states and the spin magnetism. |
Monday, March 5, 2018 11:51AM - 12:03PM |
B35.00004: Spin Response and Spin Waves in a Two Dimensional Transition Metal Dichalchogenide Dibya Mukherjee, Arijit Kundu, Herbert Fertig Transition metal dichalcogenide (TMD) monolayers are 2D materials with a direct bandgap. Due to the lack of inversion symmetry and strong spin-orbit coupling (SOC), they are promising candidates for applications, such as vallytronic and spintronic devices. In such systems repulsive interactions among the electrons are found to have an important impact on their band structures, as well as on their topological properties. Because of the important qualitative effects of SOC, spin response is a particularly useful probe of these systems. We report on a theoretical study of spin response functions of this system using the time-dependent Hartree-Fock approximation. Our study shows how collective modes (i.e, spin-waves) can emerge in a simple model TMD with short-range repulsive interactions. We also show that their behavior as a function of chemical potential contains simple signatures of the unique valence band structure in this type of system. |
Monday, March 5, 2018 12:03PM - 12:15PM |
B35.00005: Interplay of valley polarization and dynamic nuclear polarization in 2D transition metal dichalcogenides Girish Sharma, Sophia Economou, Edwin Barnes The interplay of Ising spin-orbit coupling and non-trivial band topology in transition metal dichalcogenides (TMDs) produces anomalous transport and optical properties that are very different from a regular 2D electron gas. The spin-momentum locking of optically excited carriers near a valley point can give rise to an anomalous spin-valley Hall current under the application of an in-plane electric field. TMDs also exhibit strong electronnuclear hyperfine interactions, but their effect on spin-valley-locked currents remains unknown. Here, we show that hyperfine interactions can create a feedback mechanism in which spin-valley currents generate significant dynamical nuclear polarization which in turn Zeeman shifts excitonic transitions out of resonance with an optical driving field, saturating the production of spin-valley polarization. We propose an experimental signature of dynamic nuclear polarization which can be detected via measurements of the anomalous Hall current. Our results help to elucidate the interplay of valley polarization and nuclear spin dynamics in TMDs. |
Monday, March 5, 2018 12:15PM - 12:27PM |
B35.00006: Strained graphene in the quantum Hall regime Daiara Faria, Carlos Leon, Leandro Lima, Andrea Latge, Nancy Sandler The coupling between electronic and mechanical properties in 2D materials has become an important tool to control their properties. In this route, graphene nanoribbons with longitudinal deformations have been proposed as electronic waveguides for valley polarized currents [1]. As strained-folds can be easily engineered on graphene samples [2], we propose a theoretical study of the strain effects on a graphene membrane in the quantum Hall regime. A continuum model description allows to obtain analytic expressions for corrections to the Landau levels while a numerical tight-binding band structure calculation confirms these results. Using recursive Green’s function method, we obtain new extra conducting channels due to the "new edges” formed at the fold. We discuss the magnetic field effect on the system that protects these extra channels against edge disorder. Our results are consistent with recent experimental measurements [3]. [1] R. Carrillo-Bastos et al., PRB 94, 125422 (2016). [2] Y. Jiang et al., Nano Lett. 17, 2839 (2017). [3] See abstract: Valley-selective Channels in Strained Graphene Wrinkles, by the group of Eva Andrey. |
Monday, March 5, 2018 12:27PM - 12:39PM |
B35.00007: Nuclear Magnetic Resonance Line Shapes of Electron Crystals in 13C Graphene Rene Cote, Jean-Michel Parent Inhomogeneous electronic states such as Wigner and Skyrme crystals are expected to form in graphene in a small range of filling factor near integer fillings [1]. In these states, the spatial variation of the spin density leads to an inhomogeneous broadening of the nuclear magnetic resonance (NMR) line shape of the 13C nuclei. In GaAs quantum wells, Wigner crystals in different Landau levels have NMR line shapes that are qualitatively different [2]. It is thus natural to ask if this also hold for Wigner crystals in graphene. In this talk, we first show that in graphene, one must consider, in addition to the spin hyperfine interaction (both contact and dipolar), a second contribution of equal importance that arises from the coupling between the orbital motion of the electrons and the nuclei i.e. one must consider both the Knight and the chemical shifts. Taking this fact into account, we compute the line shape of different crystals and discuss the conditions, such as the linewidth of the bare resonance, needed to differentiate between different types of Wigner and Skyrme crystals. |
Monday, March 5, 2018 12:39PM - 12:51PM |
B35.00008: Nematic, ferroelectric, quantum Hall states in atomically thin transition metal dichalcogenides Patrick Cheung, Fan Zhang Rotational and inversion symmetries relate different valleys in the Brillouin zones of atomically thin transition metal dichalcogenides. When such systems are subject to a perpendicular magnetic field, spontaneous symmetry breaking occurs when the lowest Landau levels are only partially filled. The nematic and/or ferroelectric characteristics of the broken symmetry states make the control and manipulation of the valley and layer degrees of freedom of the ground state possible by additional external fields that break the rotational and/or inversion symmetries. |
Monday, March 5, 2018 12:51PM - 1:03PM |
B35.00009: Two dimensional ferromagnetic hafnene monolayer Arqum Hashmi, Jisang Hong Few years ago, it was claimed that the two-dimensional ferromagnetic planar Hf monolayer could be synthesized on Ir(111). However, many open questions was still remained. Hereby, we unravel the structural stability and its influence on the magnetism using first principles calculations. Despite the ferromagnetic state in planar free-standing Hf layer, extensive systematic calculations with phonon spectra reveal that a planar free-standing Hf layer is unstable and it has a non-magnetic high-buckled structure in ground state. We also find a structural transition from buckled to flat honeycomb geometry on Ir(111) substrate. Nonetheless, the 2D hafnene has no magnetic state due to the strong hybridization with Ir(111) surface. The evolution from non-magnetic to ferromagnetic state incorporated with structural transition is found by adding BN as a spacer layer on Ir(111) substrate (BN/Ir(111)). Besides, we find that the 2D Hf on BN/Ir(111) has a giant perpendicular magnetic anisotropy of 3.41 meV. |
Monday, March 5, 2018 1:03PM - 1:15PM |
B35.00010: Angle- and spin-resolved photoemission spectroscopy study of monolayer NbSe2 Wei Yao, Mingzhe Yan, Kun Zhao, Ke Deng, Kenan Zhang, Andrew Weber, Koji Miyamoto, Taichi Okuda, Hugo Dil, Ming Shi, Shuaihua Ji, Shuyun Zhou The coexistence of superconductivity and charge density wave (CDW) at low temperature makes NbSe2 an ideal system to study the interplay between superconductivity and CDW. When its thickness approaches to one single layer, the impact of spin-orbit coupling on the electronic structure of NbSe2 becomes more and more significant, leading to spin-valley locking and so-called “Ising superconductivity”. Here we report the electronic and spin structural studies of a monolayer NbSe2 thin film using Angle-resolved photoemission spectroscopy (ARPES) and Spin-Resolved ARPES. |
Monday, March 5, 2018 1:15PM - 1:27PM |
B35.00011: Ab initio study of the magnetic and transport properties of VSe2 Adolfo Fumega, Warren Pickett, Victor Pardo VSe2 is a transition metal dichalcogenoide formed by layers that bond via van der Waals forces |
Monday, March 5, 2018 1:27PM - 1:39PM |
B35.00012: Conductance oscillations in Chern insulator junctions: valley-isospin dependence and Aharonov-Bohm effects Nojoon Myoung, Hee Chul Park The transport properties of Chern insulator junctions generated by bipolar junctions in quantum Hall graphene are theoretically studied in the coherent regime. Coherent transport across the junction exhibits two mesoscopic features: valley-isospin dependence of the quantum Hall conductance, and the Aharonov-Bohm (AB) effects with the interface channels. We demonstrate that the valley-isospin dependence can be measured in a graphene sample with perfect edge terminations, resulting in conductance oscillation for the smallest Chern number case. On the other hand, while conductance plateaus are found to be unclear for larger Chern numbers, the conductance exhibits an oscillatory behavior of which period is relatively longer than the valley-isospin dependent oscillation. This conductance oscillation is ascribed to the AB effect, which is implicitly created by the split metallic channels near the junction interface. We point out that a possible origin of the unclear plateaus previously speculated to be incompleteness in realistic devices is the low-visibility conductance oscillation due to unequal beam splitting. |
Monday, March 5, 2018 1:39PM - 1:51PM |
B35.00013: Carrier and Strain Tunable Intrinsic Magnetism in Two-Dimensional MAX3 Transition Metal Chalcogenides Bheema Lingam Chittari, Dongkyu Lee, Allan MacDonald, Euyheon Hwang, Jeil Jung We present a density functional theory study of the carrier-density and strain dependence of magnetic order in two-dimensional (2D) MAX3 (M= V, Cr, Mn, Fe, Co, Ni; A= Si, Ge, Sn, and X= S, Se, Te) transition metal trichalcogenides. Our ab initio calculations show that this class of compounds includes wide and narrow gap semiconductors and metals and half-metals, and that most of these compounds are magnetic. Although antiferromagnetic order is most common, ferromagnetism is predicted in MSiSe3 for M= Mn, Ni, in MSiTe3 for M= V, Ni, in MnGeSe3, in MGeTe3 for M=Cr, Mn, Ni, in FeSnS3, and in MSnTe3 for M= V, Mn, Fe. Among these compounds CrGeTe3 and VSnTe3 are ferromagnetic semiconductors. Our calculations suggest that the competition between antiferromagnetic and ferromagnetic order can be substantially altered by strain engineering, and in the semiconductor case also by gating. The associated critical temperatures can be enhanced by means of carrier doping and strains. |
Monday, March 5, 2018 1:51PM - 2:03PM |
B35.00014: Orbital-dependent quasiparticle scattering interference in 3R-NbS2 Tadashi Machida, Ryuji Suzuki, Yuhki Kohsaka, Katsuya Iwaya, Ryotaro Arita, Masayuki Ochi, Tetsuo Hanaguri, Yoshihiro Iwasa We measured the quasiparticle interference (QPI) in a valleytronics-candidate material 3R-NbS2 using spectroscopic-imaging scanning tunneling microscopy. We find that the QPI signal of the inter-valley scattering is noticeably weaker than that of the intra-valley scattering. Numerical simulations reveal that the selection rules associated with the spin and orbital degrees of freedom (DOF) suppress the QPI signal of the inter-valley scattering and that the orbital selection rule plays a major role in the QPI. These findings indicate that not only the spin but also orbital DOF should be taken into account for a better understanding of inter-valley scattering process. Our findings offer a new perspective for better understanding of the valley related phenomena and for developing a novel technique of the valley manipulation. |
Monday, March 5, 2018 2:03PM - 2:15PM |
B35.00015: Valley-selective Channels in Strained Graphene Wrinkles Junxi Duan, Xinyuan Lai, Jinhai Mao, Zhenyuan Zhang, Daiara Faria, Nancy Sandler, Eva Andrei Pristine graphene has two degenerate valleys with linear dispersion. Under the influence of strain, the shift of the carbon atoms generates a vector potential which can give rise to a so-called pseudo-magnetic (PM) field. Unlike a real magnetic field, the PM field has opposite signs in the two valleys and provides a handle to control the valley degree of freedom. Theory has shown that in a strained wrinkle with Gaussian shape, the propagation of carriers along the wrinkle is valley-selective [1]. The carriers from one valley can only move in one direction while the ones from the other valley move in the opposite direction. By transferring single-layer graphene on top of an array of pillars fabricated on the surface of hBN, we generate strained wrinkles in a controllable way [2]. Measuring the magneto and Hall resistance as a function of field and carrier density at low temperatures, we observe a series of quantum conductance plateaus. Surprisingly, we find that all the plateaus are shifted by two units of quantum conductance. Using the Landauer-Buttiker formalism we show that this shift is a direct consequence of a valley selective channel along the wrinkle. |
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