Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session R19: 2D Antiferromagnets, Layers and Magnetic Thin FilmsFocus
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Sponsoring Units: GMAG DMP Chair: Tao Hong, Oak Ridge National Lab Room: LACC 308A |
Thursday, March 8, 2018 8:00AM - 8:36AM |
R19.00001: Exotic spin excitations in a two-dimensional quantum antiferromagnet near the quantum critical point Invited Speaker: Tao Hong The spin-1/2 antiferromagnetic Heisenberg two-leg ladder is a prototype system for low-dimensional magnetism. Recently, a novel spin-1/2 molecular compound C9H18N2CuBr4 (DLCB for short) was synthesized [1]. In contrast to most spin ladders reported to date, DLCB is a rare example in which the interladder coupling is very near the critical value required to drive the system to a Néel-ordered phase below 2 K without the assistance of a magnetic field. The size of the ordered moment is ~0.4µB, much smaller than 1µB of S=1/2 free ions, due to strong quantum fluctuations [2]. Our neutron scattering studies on DLCB in conjunction with theoretical calculations show evidences of the field-induced spontaneous (T=0 K) magnon decay in an applied transverse magnetic field [3] and existence of Higgs amplitude mode, which is analygous to the Higgs boson in particle physics and characterized by fluctuation of amplitude of the order parameter, near the quantum critical point in two dimensions [4]. Our experimental work on new example of the spin-1/2 molecular magnet with low dimensionality not only helps reveal the exotic quantum many-body effects experimentally but also challenge and advance the related theoretical studies. |
Thursday, March 8, 2018 8:36AM - 8:48AM |
R19.00002: Universal short time quantum critical dynamics of finite size systems Dao-Xin Yao, Shuai Yin, Yu-Rong Shu We investigate the short time quantum critical dynamics in the imaginary time relaxation processes of finite size systems. Universal scaling behaviors exist in the imaginary time evolution and in particular, the system undergoes a critical initial slip stage characterized by an exponent θ. We apply the method to the one- and two-dimensional transverse field Ising models using quantum Monte Carlo simulations. In the one-dimensional case, we locate the quantum critical point at (h/J)c=1.00003(8), and estimate the critical initial slip exponent θ=0.3734(2), static exponents β/ν=0.1250(2). For the two-dimensional square-lattice system, the critical coupling ratio is given by 3.04451(7) while the critical exponents are θ=0.209(3) and β/ν=0.5227(4). Remarkably, the critical initial slip exponents obtained in both models are notably distinct from their classical counterparts, owing to the essential differences between classical and quantum dynamics. The short time critical dynamics and the imaginary time relaxation QMC approach can be readily adapted to various models. Reference: Phys. Rev. B 96, 094304 (2017). |
Thursday, March 8, 2018 8:48AM - 9:00AM |
R19.00003: Electronic and magnetic properties of Nitrogen-doped silicene under biaxial strain Juan Hernández-Tecorralco, Miguel Eduardo Cifuentes Quintal, Lilia Meza-Montes, Romeo De Coss
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Thursday, March 8, 2018 9:00AM - 9:12AM |
R19.00004: Strain and point defects in MnSe2 ferromagnetic monolayers Tomas Rojas, Sergio Ulloa After the isolation of graphene in 2004, a vast amount of 2D materials have been studied. However, most 2D materials known are non-magnetic, which limits their potential technological applications. Although magnetic moments have been induced by defects, doping or interaction with a substrate, there was not a case of intrinsic ferromagnetism in a monolayer. This situation changed recently with the exfoliation of CrI3 and Cr2Ge2Te6 monolayers. Among these materials, MnSe2 and MnS2 have been predicted as stable ferromagnetic monolayers by first-principles calculations [1], and MnSe2 is currently a subject of experimental efforts to grow thin and monolayer samples [2]. |
(Author Not Attending)
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R19.00005: Magnetic Properties of Ultrathin Cobalt films on Si substrates Leonardo Rios E, Edgar Patino Magnetism in ultra-thin films has attracted many research efforts because of its practical applications in solid state devices. One of the most interesting properties of this area is that, contrary to the magnetism in bulk materials, it is possible to manipulate the magnetism by controlling the thickness of the sample. In this experimental work, we studied the variation of remanent and saturation magnetizations as well as the coercive fields for Cobalt films grown by Sputter deposition. The films were fabricated with thicknesses between 1 to 10nm and were characterized via VSM measurements. At 1.6nm results showed a deviation for the saturation magnetization and a significant change in coercive fields. |
Thursday, March 8, 2018 9:24AM - 9:36AM |
R19.00006: Intrinsic ferromagnetism in epitaxial MnSe2 van der Waals monolayers at room temperature Dante O'Hara, Tiancong Zhu, Amanda Trout, Adam Ahmed, Yunqiu (Kelly) Luo, Choong Hee Lee, Mark Brenner, David McComb, Siddharth Rajan, Roland Kawakami Monolayer van der Waals (vdW) magnets provide an exciting opportunity for exploring two-dimensional (2D) magnetism for scientific and technological advances, but the intrinsic ferromagnetism has only been observed at low temperatures. Here, we report the observation of room temperature ferromagnetism in the monolayer limit for epitaxial MnSe2 vdW layers. Since bulk Mn-Se compounds are not ferromagnetic and bulk van der Waals crystals of MnSe2 do not exist, we rely on molecular beam epitaxy to epitaxially stabilize MnSe2 layers. Magnetization measurements of monolayer films show ferromagnetic ordering with large saturation magnetization of ~ 4 Bohr magnetons per MnSe2 formula unit, which is comparable to previous density functional theory calculations. Continued growth produces epitaxial a-MnSe/MnSe2/GaSe heterostructures that have enhanced saturation magnetization and the emergence of a new x-ray diffraction peak consistent with expectations for vdW MnSe2 provides strong evidence for multilayers of vdW MnSe2 located at the interface between non-ferromagnetic a-MnSe and GaSe. These results establish the foundation for room temperature 2D ferromagnetism in large area epitaxial vdW magnets, which has potential application in energy efficient information storage and processing. |
Thursday, March 8, 2018 9:36AM - 9:48AM |
R19.00007: Proximity effect induced magnetism in graphene Devashish Gopalan, Joe Seifert, Amanda Haglund, David Mandrus, Marek Skowronski, Benjamin Hunt Recent discovery of the 2D ferromagnetic insulators CrI3, Cr2Ge2Te6, and CrSiTe3 has spurred research in 2D magnetic materials. Van der Waals heterostructures of these materials with graphene can induce a nonzero magnetization in graphene by proximity. This also preserves the intrinsically high mobility of graphene, since no magnetic impurities are introduced to the system. Here, we employ nonlocal magneto-transport measurements to detect a magnetic proximity effect in graphene. Enhancement of this nonlocal signal indicates long-range magnetic order in graphene and provides a stepping stone towards observation of the quantum anomalous Hall effect at elevated temperatures. |
Thursday, March 8, 2018 9:48AM - 10:00AM |
R19.00008: Wafer-scale Two-dimensional Ferromagnetic Fe3GeTe2 Thin Films Grown by Molecular Beam Epitaxy Shanshan Liu, Xiang Yuan, Yichao Zou, Faxian Xiu Layered two-dimensional ferromagnetic materials (2D FMs) have attracted a great deal of interest for developing low-dimensional magnetic and spintronic devices. Mechanically exfoliated 2D FMs were discovered to possess ferromagnetism down to monolayer. It is therefore of great importance to investigate the distinct magnetic properties at low dimensionality. Here, we report the wafer-scale growth of 2D FM of Fe3GeTe2 via MBE, and their exotic magnetic properties can be manipulated via the film thickness, Fe composition and the interface coupling with antiferromagnetic MnTe. A 2D layer-by-layer growth mode has been achieved by in-situ RHEED oscillations, yielding a well-defined interlayer distance of 0.82 nm along {002} surface. The magnetic easy axis is oriented along c-axis with a Curie temperature (TC) of 216.4 K. Remarkably, the TC can be enhanced when reducing the thickness or rising the Fe composition. Upon coupling with MnTe, the coercive field dramatically increases 50 % from 0.65 to 0.94 Tesla. The large-scale layer-by-layer growth and controllable magnetic properties make Fe3GeTe2 a promising candidate for spintronic applications. It also opens up unprecedented opportunities to explore rich physics when coupled with other 2D superconductors and topological matters. |
Thursday, March 8, 2018 10:00AM - 10:12AM |
R19.00009: Engineering the Hamiltonian of coupled spin-1/2 atoms on a surface Kai Yang, Yujeong Bae, William Paul, Fabian Natterer, Philip Willke, Jose Lado, Alejandro Ferrón, Taeyoung Choi, Joaquin Fernandez-Rossier, Andreas Heinrich, Christopher Lutz Quantum spin networks having engineered geometries and interactions are eagerly pursued for quantum simulation and access to emergent quantum phenomena such as spin liquids. Spin-1/2 centers are particularly desirable because they readily manifest coherent quantum fluctuations and entanglement. Here we introduce a controllable spin-1/2 architecture consisting of 3d transition metal atoms on a magnesium oxide surface. We tailor the spin interactions by atomic-precision positioning using a scanning tunneling microscope (STM), and subsequently perform electron spin resonance (ESR) on individual atoms to drive transitions into and out of entangled states. Interactions between the atoms are mapped over a range of distances extending from highly anisotropic dipole coupling, to strong exchange coupling. The local magnetic field of the magnetic STM tip tunes the energy states of any selected atom, and serves to precisely adjust the level of entanglement of a pair of spins. The precise control of the spin-spin interactions and ability to probe the entangled states on individual spins demonstrated here will enable exploration of quantum many-body systems based on networks of spin-1/2 atoms on surfaces. |
Thursday, March 8, 2018 10:12AM - 10:24AM |
R19.00010: Possible stress-induced commensurate-to-incommensurate transition in the triangular antiferromagnet PdCrO2 Dan Sun, Pallavi Kushwaha, Jack Bartlett, Andrew Mackenzie, Clifford Hicks Antiferromagnetically interacting Heisenberg spins on a 2D triangular lattice generally order into a commensurate 120-degree phase below the Néel temperature. PdCrO2 is one such material. In our work, we study PdCrO2, in a piezo-actuated uniaxial stress cell, to probe the response of triangular antiferromagnetism to anisotropic lattice distortion. At a compression of about 0.4%, we find strong signatures of a first order transition in resistivity, magnetoresistivity and Hall effect, which may indicate a commensurate-to-incommensurate transition. |
Thursday, March 8, 2018 10:24AM - 10:36AM |
R19.00011: Spin-current order induced by a magnetic field Mengxing Ye, Andrey Chubukov We consider a 2D itinerant fermion system on a hexagonal lattice, with the Fermi surface consisting of well separated electron and hole pockets of similar sizes, centered at high-symmetry-points in the Brillouin zone: one hole pocket at the origin and two electron pockets at ± K. We perform RG analysis to select between superconductivity, charge and spin orders. Over some range of intra- and inter-pocket interactions, magnetism wins. We show that the magnetic order is either 120° spin-density-wave, or collinear order with magnetization on two-thirds of lattice sites, and no magnetization on another one-third. We further show that an application of a Zeeman field does not give rise to canting of SDW spin configuration, but rather triggers spin-current order. We discuss the structure of spin-current state and experimental implications of such an order. |
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