Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session X35: 2D Materials - Magnetism and Magnetotransport IIFocus
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Sponsoring Units: DMP GMAG Chair: Tiancong Zhu, Ohio State University Room: LACC 409B |
Friday, March 9, 2018 8:00AM - 8:36AM |
X35.00001: Discovery of intrinsic ferromagnetism in two-dimensional van der Waals crystals Invited Speaker: Xiang Zhang In this talk, I will present our discovery of the intrinsic ferromagnetism in 2D vdW crystals, including the prominent dimensionality effect and unprecedented magnetic field control of the Curie temperature in a nearly-ideal 2D Heisenberg ferromagnet. Given the revealed significant role of interlayer coupling, I will discuss our further work on various bilayer magnetic systems. After some remarks on the fundamental physics in 2D magnetism and existing challenges in this emerging field, I will discuss the possible directions towards advancing 2D magnets for practical spintronic devices. |
Friday, March 9, 2018 8:36AM - 8:48AM |
X35.00002: First-principles calculations of few-layer Chromium Trihalides CrCl3, CrBr3, and CrI3 Lucas Webster, Jia-An Yan One of the latest advances in the field of two-dimensional (2D) materials is the discovery of intrinsic ferromagnetism in monolayers of CrGeTe3 and CrI3– in these structures, thermal fluctuations could be counteracted by magnetic anisotropy, as required by the Mermin-Wagner Theorem. Combining ferromagnetism in 2D materials is desirable as a means to expand the possibilities for microelectronic devices. In this work, using first-principles calculations, we present a detailed study of the energetics, atomic structures, electronic structures, and lattice dynamics of Chromium Trihalides CrCl3, CrBr3, and CrI3 down to the two-dimensional limit, as all three compounds exhibit a strong intralayer ferromagnetic coupling. Calculations using various exchange-correlation functionals and pseudopotentials are tested and compared with experimental data. The effect of strain and stacking geometry in the magnetic properties of these materials will also be discussed. |
Friday, March 9, 2018 8:48AM - 9:00AM |
X35.00003: Epitaxial growth and characterization of few-layer 1T-NbSe2 Cliff Chen, Weimin Zhou, Justin Horowitz, Peng Wei Recently discovered ferromagnetic order in two-dimensional van der Waals crystals has attracted intense research. Spin ordering in these materials is carried by magnetic transition metal elements such as chromium. Here we demonstrate signatures of spin-ordered states in a strongly correlated 2D material: 1T-NbSe2. In contrast to the superconducting 2H-NbSe2 phase, strong electron correlation in 1T-NbSe2 gives rise to a Mott insulator phase. Stable 1T-NbSe2 can be grown under high temperatures in an ultra-high vacuum environment using molecular beam epitaxy (MBE). In this talk, we will present the epitaxial growth of high quality few-layer 1T-NbSe2 on the well-defined atomic terraces of a sapphire substrate. The sample crystal structure is confirmed by both x-ray diffraction (XRD) and in-situ reflection high energy electron diffraction (RHEED). The 1T-NbSe2 phase is further proved by X-ray photoelectron spectroscopy (XPS). Electrical transport studies of the grown samples revealed charge density wave (CDW) behavior. Spin density wave signatures are further captured via magnetotransport. Our work paves the way for investigating a wider range of spin ordered states such as antiferromagnetism in 2D materials. |
Friday, March 9, 2018 9:00AM - 9:12AM |
X35.00004: Large Valley Splitting in Monolayer Transition Metal Dichalcogenides by Proximity Coupling to Magnetic Substrate Lei Xu, Yuan Feng Lifting the valley degeneracy in monolayer transition metal dichalcogenides is indispensable for valley operations. In this work, we show that valley polarization can be achieved by proximity coupling monolayer transition metal dichalcogenides to magnetic insulator substrate, and the magnitude of valley splitting can be continually tuned by external pressure and strain. Due to the sizeable Berry curvature and time-reversal symmetry breaking in transition metal dichalcogenides when placed on magnetic substrate, a spin and valley polarized anomalous Hall current can be obtained in the presence of in-plane electrical field, which provides a mean to detect the valleys by electric measurements and forms the basis for valleytronic device. These findings are also applicable to other valley materials coupling to magnetic insulators. |
Friday, March 9, 2018 9:12AM - 9:24AM |
X35.00005: Bilayer CrI3 Ground State Revealed by Layer-sensitive Magnetic Sensor Ding Zhong, Kyle Seyler, Xiayu Linpeng, Nathan Wilson, Adrian Lonescu, Ilham Wilson, Michael McGuire, Di Xiao, Wang Yao, Kai-Mei Fu, Xiaodong Xu
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Friday, March 9, 2018 9:24AM - 9:36AM |
X35.00006: Electrical Detection of a Metamagnetic Transition in a van der Waals Magnetic Insulator Dahlia Klein, David MacNeill, Valla Fatemi, José Lado, David Soriano, Efrén Navarro-Moratalla, Joaquín Fernández-Rossier, Pablo Jarillo-Herrero The operation of modern magnetic memories relies on the interplay of magnetism and conductivity in ferromagnetic metals. In theory, higher performance is possible for tunneling through insulating magnets, but the requirement of ultrathin films has limited this concept. Here, we measure tunneling through atomically thin crystals of the magnetic insulator CrI3 prepared by mechanical exfoliation. Owing to the weak antiferromagnetic interlayer coupling in CrI3, we can orient the magnetization in adjacent layers to be either parallel or antiparallel using an external magnetic field. When we switch the interlayer magnetization alignment, we observe a 100%, 300%, and 550% conductance change for bilayer, trilayer, and tetralayer CrI3 barriers, respectively. We further observe evidence of collective modes in tunneling bias spectroscopy, establishing vertical tunneling as a spectroscopic probe for atomically thin magnetic materials. |
Friday, March 9, 2018 9:36AM - 9:48AM |
X35.00007: Structural phase transition and magnetic behavior in two-dimensional rhenium-doped molybdenum diselenide Amey Anant Apte, Vidya Kochat, Jordan Hachtel, Hiroyuki Kumazoe, Aravind Krishnamoorthy, Sandhya Susarla, Juan Carlos Idrobo, Fuyuki Shimojo, Priya Vashishta, Rajiv Kalia, Aiichiro Nakano, Chandra Sekhar Tiwary, Pulickel Ajayan Typical Group VIB transition metal dichalcogenides are layered van der Waals compounds with a semiconducting behavior due to 2H symmetry. The 1T/1T' symmetry imparts semimetallic behavior but isn't as energetically stable; these phases are typically obtained via lithium intercalation. Here, we demonstrate the 2H-1T' stable phase transition via controllable incorporation of rhenium in 2D MoSe2 using chemical vapor deposition. The phase transition is revealed via HAADF-STEM imaging and complimented by DFT simulations. In addition, we also show the emergence of magnetic ordering in such 2D alloys which pave the way for tuning these phase fractions for applications in electronics and/or spintronics. |
Friday, March 9, 2018 9:48AM - 10:00AM |
X35.00008: Structural and magnetic transition induced by bi-axial strain in 2D Fe3GeTe2 monolayer Jisoo Nam, Hosik Lee, Minseong Lee, Junhee Lee Two-dimensional materials have been in the spotlight these days because of their technological advantages such as highly tunable flexibility. In particular, van der Waals (VDW) bonded magnetic materials are of great importance as building blocks in spin-driven devices. Fe3GeTe2 is one of highly interesting ferromagnetic VDW materials because it maintains ferromagnetic properties down to a single layer with Curie temperature close to the room temperature. Using first-principles DFT calculations, we comprehensively studied on the coupling of magnetism to structural and electronic properties to find a way to enhance the ferromagnetic stabilities by strain. First, we scrutinize lattice instabilities by investigating the phonon dispersion of strained Fe3GeTe2 mono-layer and found a structural transition at around 7% strain. A linearly enhanced magnetization is observed at below the critical stain. An abrupt magnetization jump concomitant with the structural transition is expected beyond the critical strain. We will discuss the possible atomic-scale origin of the concomitant magnetic and structural transitions drive by the strain. |
Friday, March 9, 2018 10:00AM - 10:12AM |
X35.00009: Thickness dependence of anomalous hall effect in 2-dimensional Fe3GeTe2 Changgu Lee, Jinhwan Lee, Lan Wang, Cheng Tan In this talk, thickness-dependent magnetic properties of Fe3GeTe2, which is a 2-dimensional ferromagnetic conductor, will be presented. The properties were characterized by Hall effect measurement from 2K to room temperature. From the hall measurement, it exhibited the anomalous hall effect due to its intrinsic ferromagnetism. Interestingly, the magnetic properties such as coercivity changed significantly with decreasing thickness changing from weak ferromagnet to strong ferromagnet. With the changing angle of magnetic field, the coercivity changed significantly. The Curie temperature decreased with decreasing thickness below about 100nm, above which it remained unchanged over the temperature. |
Friday, March 9, 2018 10:12AM - 10:24AM |
X35.00010: Magnetic correlations in the quasi-2D itinerant ferromagnet Fe3-xGeTe2 Stuart Calder, Andrew May, Yaohua Liu, Tom Berlijn Compounds with two-dimensional (2D) layers weakly connected by van der Waals bonds offer access to novel physics via quantum confinement and are potential routes to new device applications. Neutron scattering can provide a powerful probe of these bulk quasi-2D materials, giving quantitiative insights into magnetic exchange interactions, low dimensionality and magnetostructural coupling. This understanding of the bulk material behavior affords insight into the important underlying interactions in these compounds and provides routes to understanding the materials down to a single 2D-monolayer. Here we present neutron scattering results on the quasi-2D itinerant ferromagnetic Fe3-xGeTe2. The neutron scattering results reveal insights into the magnetic correlations, 2D phenomena, magnetic anisotropy and short range ordering. |
Friday, March 9, 2018 10:24AM - 10:36AM |
X35.00011: Giant pseudo-magnetic fields and valley polarization by nanoscale strain engineering of graphene Jiaqing Wang, Chen-Chih Hsu, Marcus Teague, Meng-Huan Jao, Nai-Chang Yeh We report the use of nearly strain-free PECVD-grown graphene1 to induce controllable strain and pseudo-magnetic fields (BS) by placing graphene on synthesized tetrahedron nanocrystals (55nm laterally and 45nm in height).2 The nanocrystals were spin-coated on a Si substrates and then covered by a monolayer of h-BN followed by a monolayer of graphene. Scanning tunneling spectroscopic studies revealed giant BS values up to ~25000T in isolated tetrahedrons. In contrast, the maximum BS value became reduced to ~600T along the ridge of two correlated tetrahedrons. The effect of pseudo-magnetic field-induced local time-reversal symmetry breaking on valley polarization was confirmed by the alternating presence and absence of zeroth Landau level at two inequivalent sublattice sites.3 These empirical results were compared with Molecular Dynamics (MD) simulations for the magnitude and spatial distribution of strain-induced pseudo-magnetic fields, and the consistency ensured that properly designed arrays of nanostructures could induce the desirable BS values and spatial distributions to yield realistic valleytronic devices. |
Friday, March 9, 2018 10:36AM - 10:48AM |
X35.00012: Magnetothermoelectric Transport in Black Phosphorus Emilio Codecido, Jiawei Yang, Son Tran, Kenji Watanabe, Takashi Taniguchi, Ruoyu Chen, Chun Ning Lau Phosphorene, a 2D monolayer of black phosphorus, has been theoretically predicted to have outstanding thermoelectric potential due to its anisotropic transport behavior. Electrical and thermal conductances optimize at orthogonal directions, maximizing the thermoelectric figure of merit ZT along the armchair direction. Here, we further explore the Nernst effect in few-layer phosphorene. Transverse thermopower and its anisotropy transport behavior in black phosphorus as a function of temperature, carrier density, and magnetic field will be discussed. |
Friday, March 9, 2018 10:48AM - 11:00AM |
X35.00013: Brown-Zak oscillations and second-order magnetic Bloch states in graphene/hBN superlattices Roshan Krishna Kumar, Xi Chen, Gregory Auton, Artem Mishchenko, Denis Bandurin, Serge Morozov, Yang Cao, Ekaterina Khestanova, Moshe Ben Shalom, Andre Kretinin, Laurence Eaves, Konstantin Novoselov, Irina Grigorieva, Leonid Ponomarenko, Vladimir Falko, Andre Geim We report on a novel type of magneto-quantum oscillation which was recently observed and studied in graphene/hexagonal-boron nitride superlattices [1]. Surprisingly, the oscillations only become clearly noticeable above liquid helium temperatures (100 K), after Shubnikov-de Haas oscillations and the associated spectral gaps become smeared. These so-called BZ oscillations are extremely robust with respect to temperature and were found to exist even at 373 K (100oC) in a relatively weak magnetic field (B = 4 T). In addition, the oscillations are periodic in 1/B with a fundamental frequency which is independent on carrier density, and that is governed only by the magnetic field value (B0) when one flux quantum pierces the superlattice unit cell. The effect originates from the magnetic translation group, first proposed by Brown and Zak in the 60’s, which states that translational symmetry is restored in the electron wave function for particular values of B, when the quantum magnetic length is commensurable with the lattice periodicity. The BZ oscillations are also used as a probe to study self-similarity and the zero-effective magnetic field associated with different fractals of the corresponding magnetic Bloch states. |
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