Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session F51: Magnetism and Magnetic Phenomena in 2D Materials |
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Sponsoring Units: DCMP Chair: Te-Yu Chien, University of Wyoming Room: Mile High Ballroom 1D |
Tuesday, March 3, 2020 8:00AM - 8:12AM |
F51.00001: Unconventional anomalous Hall effect in Fe3GeTe2 and Fe5GeTe2 Juan Macy, Yu-Che Chiu, Ashley Weiland, Gregory McCandless, Julia Chan, Luis Balicas Fe3GeTe2 and Fe5GeTe2 are layered and exfoliable compounds that display what seemingly are ferromagnetic (FM) transitions below Tc ~ 220 and 280 K, respectively. At low fields, both compounds display an anomalous Hall effect (AHE) that scales with the magnetization. For FM materials, this AHE has been described using Berry phase concepts or argued to be topological in nature [1, 2], with a specific scenario [3] proposed for Fe3GeTe2. However, measurements of the magnetic torque reveal a second metamagnetic transition observed in both compounds in fields exceeding 10 T. These indicate that neither compound can be considered as a simple collinear ferromagnet [4], in agreement with Ref. [5], and hence that their AHE is not intrinsic. For both compounds, the AHE remains oblivious with respect to the occurrence of these metamagnetic transitions, indicating that an unconventional scenario is required to explain it. |
Tuesday, March 3, 2020 8:12AM - 8:24AM |
F51.00002: Gate-tunable magnetism in vanadium-doped tungsten diselenide monolayer Dinh Loc Duong, Seok Joon Yun, Young-Hee Lee Controlling magnetic properties by tuning carrier concentration is a key feature of diluted ferromagnetic semiconductors, giving possibilities for multifunctional spintronic devices. Here, we investigate the gate-modulation of the long-range magnetic order in p-type vanadium-doped WSe2 monolayer by both experiments and simulations. We found that a transition from the long-range ferromagnetic order to the short-range antiferromagnetic order can be tuned by injecting electrons to compensate for the p-doping effect of vanadium by density functional calculations. The magnetic domains observed by magnetic force microscopy are strongly modulated by applying gate biases. Our findings propose the strong coupling between charge and spin order in vanadium-doped WSe2, opening possibilities for using two-dimensional semiconductors for spintronic devices. |
Tuesday, March 3, 2020 8:24AM - 8:36AM |
F51.00003: Proximity effect between graphene and 2D anti-ferromagnetic insulator α-RuCl3 Edward Seifert, Devashish P Gopalan, Zaiyao Fei, Xiaodong Xu, David Mandrus, Benjamin Hunt Proximity effects attempting to combine electronic properties of graphene with exotic phenomena in other 2D van der Waals materials have been a central focus of condensed matter research for the past decade. To date, much work has been done studying proximity effects between graphene and ferromagnetic insulators, but little is known experimentally about the coupling between graphene and anti-ferromagnetic insulators. We report transport results from proximity devices consisting of graphene and α-RuCl3, a layered anti-ferromagnetic Mott insulator predicted to host a so-called Kitaev quantum spin liquid. Upon cooling the device, the derivative of resistance with temperature shows a peak-dip feature. Depending on the gate voltage, this feature is seen anywhere from 35-50K, much higher than the Neel temperature of 14K. This could indicate a magnetic phase transition in the heterostructure. |
Tuesday, March 3, 2020 8:36AM - 8:48AM |
F51.00004: Emergent quantum Hall ground states driven by proximity exchange coupling in 2D material/ferromagnet interfaces Dhavala Suri, Andrew Saydjari, Lingyi Zhong, Siddharth Omar, Zhongxun Guo, Mason Gray, Mirko Rocci, Kenneth Burch, Wenzhong Bao, Bart Van Wees, Patrick A Lee, Amir Yacoby, Peng Wei, Jagadeesh Moodera In hBN/Graphene/EuS layer system we observe phase transitions spanning various quantum Hall regimes from quantum spin Hall edge state to anti-ferromagnetic state via an intermediate canted anti ferromagnetic (CAF) state. This is first demonstration of quantum Hall ferromagnetic ground states driven by proximity coupling with a ferromagnetic insulator (resulting in a huge exchange field) and achieved at a much lower applied magnetic field than any study in the past. The results clearly demonstrate the opening of edge state corresponding to CAF gap, while at much higher fields we observe reentrant insulating phase corresponding to the bulk gap as predicted by theory [1]. We also investigate the effects with WS2/Graphene/EuS stack. This led to interesting ground states emerging as a result of ferromagnet and SOC coupling induced by proximity effects. In this talk we will discuss the details of emergent ground state in 2D material/Ferromagnet interface effects along with future direction. |
Tuesday, March 3, 2020 8:48AM - 9:00AM |
F51.00005: Observation of Exchange bias effect in ferro-/antiferromagnetic van der Waals heterostructures Changgu Lee, Pawan Kumar Srivastava, Yasir Hassan, Hyobin Ahn, Soon-Gil Jung, Yisehak Gebredingle, Minwoong Joe, Abbas Sabbtain, Byunggil Kang, Tuson Park, Je-Guen Park, Kyung-Jin Lee Recent discovery of magnetic van der Waals (vdW) materials offers an intriguing possibility for two-dimensional (2D) spintronics. An essential phenomenon for realizing 2D spintronics is the exchange bias effect in ferromagnet (FM)/antiferromagnet (AFM) vdW heterostructures. In a metallic FM /semiconducting AFM vdW heterostructure, we experimentally demonstrate that a finite exchange bias is maintained down to bilayer AFM and it vanishes at monolayer. Moreover, the semiconducting property of AFM allows us to electrically control the exchange bias, providing an energy-efficient knob for spintronic devices. |
Tuesday, March 3, 2020 9:00AM - 9:12AM |
F51.00006: Edge Effects in Transition Metal Dichalcogenides on Antiferromagnetic Substrates Natalia Cortés, Oscar Avalos-Ovando, Sergio E Ulloa We explore proximity-induced antiferromagnetism (AFM) on transition metal dichalcogenides (TMD), focusing on molybdenum ditelluride (MoTe2) ribbons with zigzag and/or armchair edges, deposited on AFM substrates with diverse AFM orders, such as prospective candidates MnO, CoO or MnPX3 (X = S; Se) substrates. We model the heterostructure in real space with a tight-binding model, incorporating the exchange and Rashba fields induced by proximity from the substrate. For the zigzag terminated TMDs, we find augmented Rashba SOC and Rashba type spin splittings for midgap states. For the armchair terminated TMDs, we find propagating pseudohelical edge modes, as well as either gapped or gapless edge modes depending on the ribbon’s width. We also explore the creation of spin polarized currents on these edges. These hybrid structures can serve as building blocks for spintronic devices and provide versatile platforms to further understand proximity effects in diverse materials systems. |
Tuesday, March 3, 2020 9:12AM - 9:24AM |
F51.00007: Partial Flatbands in a Spin-Orbit Coupled Twisted Bilayer Lieb Lattice Avadh Saxena, Saikat Banerjee Topological classification of band-structure in various solid state materials has become one of the centerpieces of research in modern condensed matter physics. Typically, the specific topological nature of a dispersive band in a lattice system originates from a particular discrete symmetry present in the crystal. In this work, we analyze the topological properties of a partial/complete flatband in an apparently not so well known system -- a bilayer Lieb lattice. We demonstrate the emergence of flatbands with large Chern numbers in the presence of spin-orbit coupling. Our results are then contrasted with the flatbands in bilayer graphene and bilayer kagome lattice. We also consider a multi-layer Lieb lattice with even larger Chern numbers and speculate on the possibility of a superfluid phase in the presence of certain types of interactions. |
Tuesday, March 3, 2020 9:24AM - 9:36AM |
F51.00008: Yttrium-based Janus transition metal chalcogenides: Magnetic layered multiferroics Pankaj Kumar, Priyanka Manchanda, Pratibha Dev The discovery of graphene in 2004 heralded the further discovery of hundreds of other two-dimensional (2D) materials, with a wide range of electronic and optical properties. Till recently, however, it was believed that magnetism cannot exist in 2D materials, in keeping with the Mermin-Wagner Theorem. This changed with the observation of long-ranged magnetism in a few 2D materials, such as CrI3 and Cr2Ge2Te6. This has started an active search for additional magnetic 2D crystals. In this talk, we present our theoretical research on a new class of magnetic 2D materials - Yttrium-based Janus transition metal chalcogenide (TMD) semiconductors. As a prototype of these materials, we studied properties of the YSSe Janus structure. YSSe has a magnetic moment of 1μB/formula unit (f.u.), and an in-plane magneto-crystalline anisotropy energy of 162 μeV/f.u. Along with broken time reversal symmetry, a YSSe layer also lacks mirror symmetry. Hence, it exhibits ferroelectricity, with a polarization that can be switched under the application of strain, making it a promising candidate material for ultra-compact spintronic and multiferroics-based devices. |
Tuesday, March 3, 2020 9:36AM - 9:48AM |
F51.00009: Enhanced Zeeman splitting in MoS2 monolayers doped with transition metals. Arinjoy Bhattacharya, Mengying Bian, Arman Najafi, Peijian Wang, Alireza Jalouli, Jieqiong Wang, Jiadong Zhou, Sen Yang, Zheng Liu, Hao Zeng, Athos Petrou We developed an approach to dope MoS2 monolayers with controllable concentration of transition metal ions. In this approach, a single phase transition metal-doped MoO3 compound precursor was utilized to deposit ultrathin doped MoO3 film by electron beam evaporation, followed by sulfurization of the precursor oxide layer to achieve monolayer transition metal doped MoS2. |
Tuesday, March 3, 2020 9:48AM - 10:00AM |
F51.00010: Optical rotation in thin chiral/twisted materials and the gyrotropic magnetic effect Yanqi Wang, Takahiro Morimoto, Joel Moore The rotation of the plane of polarization of light passing through a non-magnetic material is known as natural optical activity or optical gyrotropy. The behavior of this effect in thin conductors is of current interest. For example, the low frequency limit of gyrotropy in chiral 3D crystals, known as the gyrotropic magnetic effect (GME), is controlled by the orbital magnetic moment of electrons, which has been proposed to be relevant to switching in twisted bilayer graphene. We show that the GME is not limited to bulk materials but also appears for quasi-2d systems with minimal structure incorporated in the third direction. Starting from multi-band Kubo formula, we derive a generic expression for GME current in quasi-2d materials induced by low-frequency light, and provide a Feynman diagrammatic interpretation. The relations between the 2d finite layered formula and 3d bulk formula are also discussed. |
Tuesday, March 3, 2020 10:00AM - 10:12AM |
F51.00011: Proximity exchange effects in excitons of TMDC/ferromagnet van der Waals heterostructures Paulo Faria Junior, Klaus Zollner, Jaroslav Fabian Proximity effects in two-dimensional van der Waals heterostructures are an efficient way to modify intrinsic electronic properties[1]. In particular, proximity exchange offers the possibility of inducing magnetic properties in nominally nonmagnetic materials. Furthermore, this induced synthetic Zeeman spltting exhibits strong signatures in the optical spectra. Combining ab initio calculations with tight-binding modeling and the effective Bethe-Salpeter equation for excitons, we investigate the proximity exchange in TMDC/ferromagnet systems: (i) (Mo,W)Se2 on the ultrathin van der Waals ferromagnet CrI3[2] and (ii) (Mo,W)S2/hBN on ferromagnets Co and Ni[3]. Since stacking different 2D materials requires adjusting the lattice parameters to obtain commensurate supercells, we also discuss the impact of biaxial strain in monolayer TMDCs[4]. We show the evolution of different optical transitions and the role of excitonic effects in the direct transitions. [1] Zutic et al., Mater. Today 22, 85 (2019). [2] Zollner, Faria Junior, Fabian, PRB 100, 085128 (2019). [3] Zollner, Faria Junior, Fabian, in preparation. [4] Zollner, Faria Junior, Fabian, arXiv:1909.10763 (2019). |
Tuesday, March 3, 2020 10:12AM - 10:24AM |
F51.00012: Influence of interface-induced spin-orbit interaction on transport in graphene-on-WSe2 heterostructures Muhammad Zubair, Muhammad Tahir, Panagiotis Vasilopoulos Proximity effects in graphene/transition-metal dichalcogenides heterostructures are expected to play a major role in the fields of spintronics and valleytronics. We investigate the electronic dispersion of such heterostructures in the presence of a proximity induced spin-orbit interaction (SOI) using a tight-binding (TB) model. The competition between different perturbation terms leads to inverted bands, when graphene is on WSe2, and to topologically trivial band structures when it is placed on MoS2, MoSe2, and WS2. In addition, we study the effect of symmetry breaking terms on ac and dc transport by evaluating the corresponding conductivities within linear response theory. The scattering-dependent diffusive conductivity increases linearly with electron density but weakens as the screening gets stronger. Further, we evaluate the power spectrum and assess its dependence on the spin and valley degrees of freedom and on the scattering which is essential at low frequencies. |
Tuesday, March 3, 2020 10:24AM - 10:36AM |
F51.00013: Flavor Quantum Dots and Artificial Quark Model in Transition Metal Dichalcogenides Zhi-qiang Bao, Patrick Cheung, Fan Zhang We show that the triply degenerate Q valleys in few-layer transition metal dichalcogenides provide a unique platform for exploring the rare flavor SU(3) symmetry in quantum dot geometry. The single and double dots are reminiscent of the quark model and eightfold way, and their many-body triplets and octets may be regarded as artificial quarks and hadrons. For the artificial quark transistor, each level hosts one central and two side Coulomb peaks of irrational height ratios, and flavor Kondo effects occur at 1/3 and 2/3 fillings with fractional conductance quantization in the unitary limit. |
Tuesday, March 3, 2020 10:36AM - 10:48AM |
F51.00014: Exchange Interaction Between Magnetic Adatoms in Irradiated Graphene Modi Ke, Mahmoud M. Asmar, Wang Kong Tse We present a theory for the non-equilibrium RKKY interaction in graphene irradiated by circularly polarized light. A k.p theory of the graphene Floquet Hamiltonian is developed, which provides a successive approximation scheme as the light-matter coupling strength is increased. Based on the Floquet k.p Hamiltonian, we calculate the real-space Floquet Green’s functions and the time-averaged RKKY interaction using Floquet-Keldysh formalism. We find that optical illumination generally suppresses the exchange interaction in undoped graphene, due to a band gap opened at the Dirac point. Results for doped graphene will also be presented. |
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F51.00015: Spin-Induced Optical Anisotropy on van der Waals Antiferromagnet Xingzhi Wang, Jun Cao, Hikari Kitadai, Tianshu Li, Xi Ling Two-dimensional (2D) magnetic systems have received growing interest revealing new physics of magnetism in the nanoscale limit. As an important family of van der Waals magnetic materials, metal phosphorus chalcogenides have attracted intense attention due to the intrinsic antiferromagnetism and the electronic bandgap in the visible range. Recently, we observe a strong anisotropy on optical absorption in 2D antiferromagnet, nickel phosphorus trisulfides (NiPS3), which is induced by its spin structure in the antiferromagnetic phase. The temperature and polarization dependent study indicates the strong correlation between electronic and magnetic structure. A control experiment has been conducted on the other compounds in this family, which further supports our conclusion. Our studies reveal the unique electronic and magnetic properties of NiPS3, paving the way for potential functionalities and applications of 2D magnetic materials. |
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