Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session A57: 2D Semiconductors: Transport and MagnetismFocus Live
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Sponsoring Units: DMP GMAG Chair: Xia Hong, University of Nebraska - Lincoln |
Monday, March 15, 2021 8:00AM - 8:12AM Live |
A57.00001: Cr-based van der Waals compounds as a platform to reach two dimensional ferromagnets. A comparative study between CrBr3 and Cr2Ge2Te6 Adolfo Fumega, Santiago Blanco-Canosa, Francisco Rivadulla, Victor Pardo Two dimensional ferromagnets (2DFM) have been recently synthesized for the first time causing a great excitement in the community due to their potential applications. Cr-based van der Waals ferromagnets have been identified to be the parent platform to build many of these 2DFM. In this talk, we present a pressure-dependent comparative study, both theoretical and experimental, between CrBr3 and Cr2Ge2Te6. We analyze the relationship between their electronic structure and the evolution of their magnetic properties with pressure. While CrBr3 is found to be a robust Mott-Hubbard-like insulator in which the magnetic exchange constants evolve monotonically, Cr2Ge2Te6 has a charge transfer character which makes the in-plane exchange constant to reach a constant value with pressure. This dissimilar microscopic behavior leads to a different evolution of the transition temperature for each compound. Evidence of a possible phase transition around 7 GPa is also provided for Cr2Ge2Te6 [1]. |
Monday, March 15, 2021 8:12AM - 8:24AM Live |
A57.00002: Electrical-Current-Tuned Ferromagnetism in 2D-van der Waals System Cr2Ge2Te6 Yifei Ni, Hengdi Zhao, Bing Hu, Sebastian Selter, Saicharan Aswartham, Bernd Büchner, Gang Cao Two-dimensional (2D) magnetic systems exhibit intriguing physical properties and technological significance. Cr2Ge2Te6, a ferromagnetic van der Waals material with a Curie temperature at 66 K, is a candidate platform for studying 2D magnetism. The ground state of Cr2Ge2Te6 is a delicate balance among magnetic anisotropy, dimensionality and exchange interactions, making the system tunable under external perturbations. Here, we report our investigation on the electrical-current-tuned structural and physical properties of Cr2Ge2Te6. In particular, we find that the small applied current density effectively reduces the Curie temperature and suppresses it at 0.52 A/cm2. The current-induced changes in the magnetic state closely track those of the lattice, indicating a close correlation between the lattice and the magnetic state. The results of Cr2Ge2Te6 will be discussed with a comparison drawn with other related 2D-van der Waals materials. |
Monday, March 15, 2021 8:24AM - 8:36AM Live |
A57.00003: Room-Temperature Ferromagnetism, Perpendicular Anisotropy, and Reverse Magnetostriction in 2D MnGaN Yingqiao Ma, Diego Hunt, Kengyuan Meng, Tyler Erickson, Fengyuan Yang, Maria Andrea Barral, Valeria Ferrari, Arthur Smith MnGaN-2D has been discovered as an ultimately-thin (2D) DMS material showing 300 K ferromagnetism as demonstrated using spin-polarized scanning tunneling microscopy and confirmed by density functional theory revealing highly spin-split and spin-polarized manganese states.[1] SQUID magnetometry confirms these results and finds a high spin-polarization of ∼79% at room temperature and perpendicular magnetic anisotropy.[2] Spin-orbit coupling is included in the first-principles theoretical calculations, which indicate either in-plane or out-of-plane anisotropy, depending on the type of strain. Clear evidence for both compressive and tensile local lattice strains is also found by detailed analysis of atomic resolution STM images. Furthermore, scanning tunneling spectroscopy finds fluctuations in the electronic position of the filled states manganese peak, which is also found to depend on lattice strain in the theoretical calculations, thus indicating a connection between electronic states and magnetic anisotropy. |
Monday, March 15, 2021 8:36AM - 8:48AM Live |
A57.00004: First-principles study on correlations and contact barriers
in magnetic Fe3GeTe2/CrGeTe3 heterobilayer Eunjung Ko, Young-Woo Son Recently the novel magnetic properties of van der Waals ferromagnetic layered metallic Fe3GeTe2 and semiconducting CrGeTe3 materials have drawn attention in their mono- or few-layers compounds. Based on a first-principles computational method treating the local Coulomb repulsion of U and Hund’s coupling of J separately, we present the effects of these important interactions on the electronic and magnetic structures of a layered ferromagnetic semiconductor CrGeTe3. With the proper U and J values for CrGeTe3, we also show the structural, electronic, and magnetic structures of a layered magnetic van der Waals Fe3GeTe2/CrGeTe3 heterobilayer. We discuss the possible stacking structures and the band alignment including the contact barrier height with and without the electrostatic gating in the heterobilayer. |
Monday, March 15, 2021 8:48AM - 9:00AM Live |
A57.00005: Electron transport in dual-gated three-layer MoS2 Michele Masseroni, Tim Davatz, Riccardo Pisoni, Thomas Ihn, Klaus Ensslin We investigate the conduction band of a dual-gated three layer MoS2 by means of magnetotransport experiments. The overall carrier density is tuned by both top and bottom gates. By applying an asymmetric gate voltage configuration, electrons accumulate in the layer closest to the positively biased electrode, and the three-layer MoS2 behaves electronically like a monolayer. In the situation where a positive voltage is applied to both gates leads to the occupation of all three layers. Shubnikov-de Haas oscillations originating from several bands are separately attributed to carrier densities in the top and bottom layers. We find a two-fold Landau level degeneracy for each band, suggesting that the minima of the conduction band lie at the K points of the first Brillouin zone, in contrast to band structure calculations, which report minima at the Q points. Even though the inter-layer tunnel coupling seems to leave the low energy conduction band unaffected, we observe scattering between the outermost layers, while the middle layer remains decoupled due to the inverted spin/valley symmetry. |
Monday, March 15, 2021 9:00AM - 9:12AM Live |
A57.00006: First principles investigation of structural and magnetic properties of the bulk layered compound Fe5GeTe2 Jonathan Reichanadter, Jeffrey Neaton The room-temperature van der Waals itinerant ferromagnet Fe5GeTe2 (TC=270K) has become an increasingly attractive material for spintronic and valleytronic applications, owing to recent work [1,2] detailing its magnetic and structural properties doping on the Fe sites. The influence of intra-layer exchange, interlayer exchange, stacking faults, and intra-layer registry on the compound’s magnetic phase transitions is not yet rigorously understood. Here we perform ab-initio density functional theory calculations of the structural, electronic, and magnetic properties of pristine and doped Fe5GeTe2. In particular, we compute symmetric and antisymmetric Heisenberg exchange Hamiltonian coupling parameters in the pristine and doped cases, and identify magnetic phases, assess their stability and dynamics, and compare with experiments. |
Monday, March 15, 2021 9:12AM - 9:24AM Live |
A57.00007: Creating half-metallicity in two-dimensional magnetoelectric semiconductors Cheng Gong, Shi-Jing Gong, Geunsik Lee, Andrew Marshall Rappe, Xiang Zhang Magnetic two-dimensional (2D) semiconductors [1, 2] opened up unprecedented opportunities for nanoscale spintronic devices. For the spintronics field, two long-sought goals are the device downscaling and the high energy efficiency [3]. For the efficient spintronics, the realization of high spin polarization via minimal amount of electrical input is a critical prerequisite. In this talk, I will progressively introduce an appealing approach [4, 5] to creating half-metallicity in 2D magnetoelectric semiconductors. First [4], the electric field is applied across the bilayer A-type antiferromagnetic 2D materials to create half-metallicity. Then [5], a multiferroic superlattice consisting of alternative layers of ferroelectrics and A-type antiferromagnets is constructed for the ferroelectric control of half-metallicity. These material systems are experimentally relevant, and the ferroelectric spintronics introduced here will provide valuable contributions to the developments of miniaturized high-efficiency spintronic devices. |
Monday, March 15, 2021 9:24AM - 9:36AM Live |
A57.00008: Magnetic-Field-Induced Quantum Phase Transitions in bulk CrI3* Gaihua Ye, Zhipeng Ye, Rui He, Hyun Ho Kim, Bowen Yang, Adam W Tsen, Siwen Li, Xiangpeng Luo, Kai Sun, Liuyan Zhao Using magnetic field and temperature dependent circularly polarized Raman spectroscopy of phonons and magnons (spin waves), we reveal a novel mixed state of layered AFM and FM in 3D CrI3 bulk crystals where the layered AFM survives in the surface layers and the FM appears in deeper bulk layers. We then show that the surface layered AFM transits into the FM at a critical magnetic field of 2 T, similar to what was found in the few layer case. Concurrent with this magnetic phase transition, we discover a first-order structural phase transition that alters the crystallographic point group from C3i (rhombohedral) to C2h (monoclinic). Our result reveals the intimate relationship between the layered AFM-to-FM and the crystalline rhombohedral-to-monoclinic phase transitions. |
Monday, March 15, 2021 9:36AM - 9:48AM Live |
A57.00009: Polaronic character of excitons in a two-dimensional ferromagnet CrI3* Zhipeng Ye, Gaihua Ye, Laura Rojas, Rui He, Hyun Ho Kim, Bowen Yang, Fangzhou Yin, Adam W Tsen, Wencan Jin, Xiangpeng Luo, Jason Horng, Gongjun Xu, Hui Deng, Kai Sun, Liuyan Zhao Studies of exciton dynamics in two-dimensional (2D) transition metal dichalcogenide (TMDC) semiconductors have led to discoveries of a variety of fascinating properties for optoelectronic applications. It has been known that exciton dynamics can be strongly affected by lattice vibrations through electron-phonon (e-ph) coupling. The recently discovered 2D ferromagnetic semiconductor, CrI3, provides a new platform to explore exciton physics beyond the well-studied 2D TMDC semiconductors because of its localized orbitals, intrinsic long-range ferromagnetic order, and strong e-ph coupling. Focusing on bilayer CrI3, we first show the presence of strong electron-phonon coupling through temperature-dependent photoluminescence and absorption spectroscopy. We then report the observation of periodic broad modes up to the 8th order in Raman spectra, attributed to the polaronic character of excitons. We further show the temperature and magnetic field dependence of these periodic broad modes. Our study opens opportunities for tailoring light-matter interactions in 2D magnetic semiconductors. |
Monday, March 15, 2021 9:48AM - 10:00AM Live |
A57.00010: The surprising effect of electron correlations on exchange interactions and spin excitations in magnetic 2D van der Waals materials Liqin Ke, Mikhail I. Katsnelson Despite serious effort, the nature of the magnetic interactions and the role of electron correlations in magnetic two-dimensional (2D) van der Waals (vdW) materials remain elusive. Using CrI3 as a model system, we show that the calculated electronic structure including nonlocal electron correlations yields spin excitations consistent with inelastic neutron scattering measurements. Remarkably, this approach identifies a novel correlation-enhanced interlayer super-superexchange, which rotates the magnon Dirac lines off, and introduces a gap along, the Γ-K-M path. Our discovery provides a different perspective on the gap opening mechanism observed in CrI3, which was previously associated with spin-orbit coupling through the Dzyaloshinskii-Moriya interaction or Kitaev interaction. Our observation elucidates new concepts to describe spin ordering and dynamics in magnetic vdW materials and demonstrates the necessity of explicit treatment of electron correlation in the broad family of 2D magnetic materials. |
Monday, March 15, 2021 10:00AM - 10:12AM Live |
A57.00011: Anomalous Klein tunneling of optically controlled Dirac particles with elliptical dispersion Paula Fekete, Andrii Iurov, Liubov Zhemchuzhna, Godfrey Anthony Gumbs, Danhong Huang We investigate electron tunneling through an atomically smooth square potential barrier for the dice lattice and graphene under a linearly polarized, off-resonance, and high-frequency dressing field. We demonstrate anomalous Klein tunneling for nonzero angles of incidence when the dressed states of Dirac particles with optically controllable elliptical dispersion and the direction of incoming kinetic particles is misaligned. The nonzero angles of incidence are observed to depend on the light-induced anisotropy of the energy dispersion as well as the strength of the electron-light coupling. We observe larger off-peak transmission amplitudes in the dice lattice than in graphene. Our theoretical predictions could be applied to a wide range of Dirac materials and exploited in the control of coherent tunneling and ballistic transport in novel electronic, optical, and valleytronic switching devices. |
Monday, March 15, 2021 10:12AM - 10:48AM Live |
A57.00012: Quantum transport in magnetic two-dimensional materials Invited Speaker: Yujun Deng The advent of two-dimensional (2D) materials provides new opportunities at the frontier of material research. Our recent development of fabrication and doping techniques enables us to probe the quantum transport in novel 2D materials. In this talk, I will focus on two magnetic 2D materials, ferromagnetic metal Fe3GeTe2 and intrinsic magnetic topological insulator MnBi2Te4. In Fe3GeTe2, we observed intrinsic ferromagnetism in the 2D limit and demonstrated tunable room-temperature ferromagnetism by an ionic gate. In few-layer MnBi2Te4, we observed the quantum anomalous Hall effect together with the conventional quantum Hall effect. |
Monday, March 15, 2021 10:48AM - 11:00AM On Demand |
A57.00013: Flat band magnetism in electride monolayer LaBr Chengcheng Xiao, Arash A Mostofi, Nicholas Bristowe Bulk LaBr is a layered material that is weakly bounded by interlayer van der Waals interactions. It has been predicted to be a ferromagnetic topological material that is also an electride, i.e., has electrons localized at interstitial sites of the crystal lattice. |
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