Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session C13: 2D Materials (General) -- Magnetic and Thermal PropertiesFocus
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Sponsoring Units: DMP GMAG DCOMP Chair: Michael McGuire, Oak Ridge National Lab Room: BCEC 153B |
Monday, March 4, 2019 2:30PM - 2:42PM |
C13.00001: Magnetic order in the cleavable van der Waals crystal Fe5GeTe2 Andrew May, Dmitry Ovchinnikov, Qiang Zheng, Raphael Hermann, Stuart Calder, Bevin Huang, Zaiyao Fei, Yaohua Liu, Xiaodong Xu, Michael A McGuire Van der Waals bonded materials with intrinsic magnetism play a key component in several aspects of present-day materials physics. In this study, we report the structure and properties of Fe5-xGeTe2. The average crystal structure obtained from single crystal x-ray diffraction contains disorder associated with occupation of Fe/Ge split sites. Atomic-resolution STEM has been utilized to inspect the local crystal structure and has revealed local ordering. The magnetism has been characterized using magnetization measurements on single crystals, as well as powder neutron diffraction and Mössbauer spectroscopy on a polycrystalline sample. The crystals have been successfully exfoliated, and Hall effect measurements on nanoflakes confirm magnetic order near room temperature. These results demonstrate that Fe5-xGeTe2 will be a promising material for incorporation into van der Waals heterostructures and devices. |
Monday, March 4, 2019 2:42PM - 2:54PM |
C13.00002: Single Crystal Neutron Diffuse Scattering on Nonstoichiometric Quasi-2D Ferromagnetic Fe3-xGeTe2 Yaohua Liu, Stuart Calder, Andrew May, Yawei Hui Two-dimensional (2D) van der Waals magnetic materials hold great potential for spintronics. Particularly, the recent discovery of gate-tunable room-temperature ferromagnetism in atomically thin Fe3GeTe2 makes it a promising candidate toward voltage-controlled low-dispassion devices. However, the magnetic properties of Fe3-xGeTe2 show a considerable dependence on the stoichiometry. To better understand the role of the Fe-deficiency, we have performed single crystal neutron diffuse scattering at the CORELLI spectrometer at the Spallation Neutron Source to investigate the local structure. The nonstoichiometric sample shows highly structured diffuse scattering patterns in the HK plane with a weak q dependence along the L direction. As revealed from the cross-correlation analysis with the statistical chopper, the observed diffuse scattering is mainly of a static origin. The L dependence can be partially attributed to the stacking faults. To produce the HK plane diffuse scattering features, we have performed Monte Carlo simulations to model the short-range order of the Fe vacancies and considered the lattice relaxation around the Fe vacancies. |
Monday, March 4, 2019 2:54PM - 3:06PM |
C13.00003: Skyrmions in the Moire of van der Waals 2D Magnets Qing Tong, Fei Liu, Jiang Xiao, Wang Yao We explore the skyrmion formation and control possibilities in 2D magnets from the ubiquitous moiré pattern in vdW heterostructures. Using the example of a ferromagnetic monolayer on an antiferromagnetic substrate, we demonstrate a new origin of skyrmions in the 2D magnets, from the lateral modulation of interlayer magnetic coupling by the locally different atomic registries in moiré. The moiré skyrmions are doubly degenerate with opposite topological charge, and trapped at an ordered array of sites with the moiré periodicity that can be dramatically tuned by strain and interlayer translation. At relatively strong interlayer coupling, the ground states are skyrmion lattices, where magnetic field can switch the skyrmion vorticity and location in the moiré. At weak interlayer coupling limit, we find metastable skyrmion excitations on the ferromagnetic ground state that can be deterministically moved between the ordered moiré trapping sites by current pulses. |
Monday, March 4, 2019 3:06PM - 3:42PM |
C13.00004: Design and discovery of van der Waals magnets toward room temperature devices Invited Speaker: Fazel Fallah Tafti Chromium trihalides were recognized as magnetic Van der Waals materials since 1920s. They were studied vigorously during 1950s to 70s when fundamental theories of magnetic ordering, exchange interactions, and magnetic anisotropy were developing. Recent advances in device fabrication and magneto-optical measurements lead to the discovery of 2D magnetic ordering in CrI3 monolayers with great potential for atomically thin devices. CrCl3, CrBr3, and CrI3 are stable in air and easy to exfoliate, therefore ideal for applications. Here, we present the chemical pathway to tune all the physical properties of these materials by mixing different halide species. We use CrCl3-xBrx as a benchmark to show the design principles of mixed halides where isovalent substitution of Br for Cl occurs during a chemical vapor transport process. Remarkably, all physical properties including the van der Waals gap, the magnetic ordering temperature, the ferromagnetic exchange coupling, the optical gap, and the magnetic anisotropy are tuned linearly as a function of x. Our results demonstrate unprecedented control over the magneto-optical properties of CrCl3-xBrx required for a range of applications from spintronics to biomedical industries. |
Monday, March 4, 2019 3:42PM - 3:54PM |
C13.00005: Synthesis and Characterization of quasi 2D-Chromium Sulfide Furkan Muhammet Altincicek, Soumya Banerjee, Xiaodong Hou, Michael Mann, Nuri Oncel Transition metal dichalcogenides (TMDs) are atomically thin semiconductors of the type MX2 with M being a transition metal (such as Mo, W etc.) and X being a chalcogen atom (such as S, Se and Te). TMDs have recently attracted considerable attention because of their novel electronic and optical properties. Little attention is given to the synthesis and characterization of chromium dichalcogenides. Here, we introduce a new method for obtaining Cr2S3 via vapor-solid growth mechanism. Polycrystalline Cr-sulfide sample was synthesized by heating a mixture of Cr and S in a sealed and evacuated silica tube (<10-5 mbar) at 500°C for 24 hours and then 800°C for another four days. At 800 °C, with the help of Ar as carrier gas, we deposited Cr2S3 on sapphire samples. AFM images showed only quasi-two-dimensional triangular islands on otherwise flat samples. The phase and structural analysis of as synthesized powder were examined by a powder X-ray diffractometer. Some of the XRD peaks match well with the calculated XRD data for Cr2S3 confirming the quasi-2D nature of the islands observed in the AFM images. |
Monday, March 4, 2019 3:54PM - 4:06PM |
C13.00006: MBE Synthesis and STM Characterization of 2D Magnets on Topological Insulator Tiancong Zhu, Dante O'Hara, Sisheng Yu, Jacob J Repicky, Brenton Noesges, Perry Corbett, Leonard Brillson, Fengyuan Yang, Jay A Gupta, Roland Kawakami Integrating two-dimensional(2D) magnet with topological insulator is an exciting topic. Other than the possible proximity induced magnetic ordering inside topological insulator, the 2D magnet/ topological insulator heterostructure can also lead to more efficient spin orbit torque switching, or the formation of magnetic skyrmions. The recent discovery of room temperature ferromagnetic ordering in 2D material MnSe2 and VSe2 further brings more potential in such heterostructure systems. In this talk, we report the synthesis and characterization of 2D magnet MnSe2 and VSe2 on topological insulator Bi2Se3. Monolayer and few layers of MnSe2 and VSe2 are grown on Bi2Se3 with molecular beam epitaxy. The material quality and magnetic property are characterized with RHEED, XRD, XPS, and SQUID. Furthermore, the 2D magnet/ topological insulator heterostructures are characterized with STM to study their atomic structure and local electronic properties. Finally, recent progress of measuring the local magnetic properties of these films with spin-polarized STM will also be discussed. |
Monday, March 4, 2019 4:06PM - 4:18PM |
C13.00007: Synthesis, Raman scattering and transport studies of quasi-two-dimensional magnetic Cr2S3 nanoplates Wencao Yang, Amanda Coughlin, Lucas G Webster, Gaihua Ye, Krystyna Lopez, Herbert Fertig, Rui He, Jia-An Yan, Shixiong Zhang The recent discovery of intrinsic ferromagnetism in atomically thin crystals has sparked a growing effort in synthesizing two-dimensional magnetic materials for new spintronic applications. In this work, we synthesized quasi-two-dimensional Cr2S3 nanoplates via a facile sulfurization approach, in which Cr metal was annealed in sulfur atmosphere. Depending on the thickness of Cr layers, the Cr2S3 nanoplates were grown either epitaxially or non-epitaxially (i.e. randomly oriented) on the sapphire substrates. Raman scattering studies suggest that both types of nanoplates have a rhombohedral R3 structural phase, which in its bulk form shows weak ferromagnetism below a metal-insulator transition (MIT) temperature (~120K). Density-functional calculations clearly identified most of the Raman-active peaks and their vibrational patterns. The epitaxially grown nanoplates exhibit an enhanced MIT up to ~260 K, while the non-epitaxial sample shows an insulating behavior that can be described by the variable-range-hopping model. |
Monday, March 4, 2019 4:18PM - 4:30PM |
C13.00008: Electron Spin Resonance of Magnetic Two-Dimensional Covalent Organic Frameworks Abigail Firme, Valerie A. Kuehl, Joseph Murphy, John O. Hoberg, William Rice Two-dimensional (2D) materials have generated significant interest because of their unique electrical, optical, and magnetic single-layer behaviors. However, the inability to add electrical or magnetic dopants to a substantial set of 2D materials hinders their ability to be incorporated into device architectures. Here, we synthesize 2D covalent organic frameworks (COFs) that have a lattice of nanopores, which we are able to synthetically fill with magnetic ions (Mn2+) that are hexagonally arranged. We use a host of characterization techniques, such as x-ray diffraction, TEM, NMR, and FTIR, to demonstrate that our COFs are ordered, nanoporous, and 2D. Magnetic ion filling is empirically shown through electron spin resonance measurements. Unlike the unfilled COFs, which have a single peak at g=2.0, the Mn-filled COFs show a hyperfine-split, sextet of peaks with a spin relaxation time of ~0.5 ns and a 19 G exchange splitting. The ability to chemically change the nanopore spacing and the inter-ion distance, combined with our confirmation of the hexagonal Mn2+ ion arrangement, suggests the possibility of using these 2D COFs for potential quantum spin liquids or in magneto-optical devices. |
Monday, March 4, 2019 4:30PM - 4:42PM |
C13.00009: Anisotropic structural dynamics of monolayer crystals revealed by femtosecond surface x-ray scattering I-Cheng Tung, Aravind Krishnamoorthy, Sridhar Sadasivam, Hua Zhou, Qi Zhang, Kyle Seyler, Genevieve Clark, Ehren Mannebach, Clara M Nyby, Friederike Ernst, Diling Zhu, James M Glownia, Michael E Kozina, Sanghoon Song, Silke Nelson, Hiroyuki Kumazoe, Fuyuki Shimojo, Rajiv Kalia, Priya Vashishta, Pierre Darancet, Tony F Heinz, Aiichiro Nakano, Xiaodong Xu, Aaron M Lindenberg, Haidan Wen The direct structural characterization of nonequilibrium processes within a monolayer crystal remains a challenge due to significantly reduced scattering volume. In this work, we demonstrate femtosecond surface x-ray diffraction in combination with crystallographic model-refinement calculations to measure the ultrafast structural dynamics of monolayer WSe2 crystals on a substrate. We found the absorbed photon energy is preferably coupled to the in-plane lattice vibrations within 2 picoseconds while the out-of-plane lattice vibration amplitude remains unchanged during the first 10 picoseconds. The model-assisted fitting suggests an asymmetric intralayer spacing change upon excitation. The demonstrated methods unlock the benefit of x-ray scattering to quantitatively measure ultrafast structural dynamics in atomically thin materials and across interfaces. |
Monday, March 4, 2019 4:42PM - 4:54PM |
C13.00010: Vibrational Properties of van der Waals Materials Sabine Neal, Heung Sik Kim, Amanda Haglund, Kevin Smith, Michael Crocker Martin, Hans Bechtel, G Lawrence Carr, David Vanderbilt, Kristjan Haule, David George Mandrus, Janice Lynn Musfeldt Single sheet van der Waals materials have become increasingly important over the last decade because of their broad range of applications in areas such as catalysis, energy storage, and microelectronics. These single/few layer constructs have been thoroughly analyzed by Raman spectroscopy but all infrared spectroscopy to date has been unsuccessful due to the inability to overcome the diffraction limit. The investigation of characteristic ungerade infrared modes is, however, crucially important in understanding material functionality. Synchrotron infrared nanospectroscopy, a fusion of near-field optical microscopy with high brightness infrared synchrotron radiation, has overcome this fight for photons and enabled a better understanding of size-induced effects, including quantum confinement and symmetry breaking which lead to distinctive chemical, electronic, optical, and thermal properties that are quite different from the single crystal. This approach will be illustrated with the complex magnetic semiconductor, MnPS3. |
Monday, March 4, 2019 4:54PM - 5:06PM |
C13.00011: Phonons and thermal transport in hafnium disulphide (HfS2) Jie Peng, Peter Chung, Sina Najmaei, Madan Dubey Recent studies have shown that hafnium disulphide (HfS2) exhibits many desirable properties such as high room-temperature mobility, finite bandgap, and high on-off current ratio that are favored by electronic devices. Despite the large number of existing studies of HfS2, thermal transport properties have not been fully explored and the related question of how temperature affects the thermal properties remain unanswered. |
Monday, March 4, 2019 5:06PM - 5:18PM |
C13.00012: Anisotropic thermal conductivity of printed films made of 2D-materials based inks Mizanur Rahman, Khaled Parvez, Chaochao Dun, Cinzia Casiraghi, Simone Pisana Graphene and other 2-dimensional (2D) materials are the subject of intense research due to their distinct properties, particularly for electronic and optoelectronic applications. Solution-processed 2D materials can accelerate the progress even further due to their compatibility with flexible substrates, large scale and low-cost device fabrication. In addition, printed films of such inks, could have potential use in thermal and thermoelectric applications. However, very little is known about the thermal properties of 2D-materials based inks due to the challenge and complexity associated with measuring their thermal properties, which are highly anisotropic. To develop applications based on the thermal properties of 2D-materials based inks, it is of fundamental importance to understand the relation between structure and property, and to determine how electron and heat transport relate in 2D materials. |
Monday, March 4, 2019 5:18PM - 5:30PM |
C13.00013: Temperature Dependent Phonons of Transition- Metal Dichalcogenides Calculated from Spectral Energy Density: Thermal Lattice Conductivity as an Application Arash Mobaraki, Cem Sevik, Deniz Çakir, Oguz Gulseren Predicting the mechanical and thermal properties of quasi two dimensional transition metal dichalcogenides (TMDs) is an essential task necessary for their implementation in device applications. Although, rigorous density functional theory (DFT) based calculations are able to predict mechanical and electronic properties, mostly they are limited to zero temperature. Classical molecular dynamics (MD) facilitates the investigation of temperature dependent properties, but its performance is highly depend on potential used for defining interactions between atoms. In this study, we calculated temperature dependent phonon properties of single layer TMDs , namely MoS2, MoSe2, WS2, and WSe2, by utilizing Stillinger-Weber (SW) type potentials with optimized sets of parameters with respect to the first principle results. The phonon lifetimes and contribution of each phonon mode in thermal conductivities in monolayer MoS2, MoSe2, WS2, and WSe2 are systematically investigated by means of spectral energy density (SED) method. The obtained results from this approach are in good agreement with previously available results from Green-Kubo (GK) method. |
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