Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session T53: 2D Magnetism in the van der Waals WorldFocus Recordings Available
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Sponsoring Units: GMAG DMP Chair: Leah Weiss, Oak Ridge National Laboratory Room: McCormick Place W-475B |
Thursday, March 17, 2022 11:30AM - 12:06PM |
T53.00001: Axial Higgs Mode Detected by Raman Scattering in RTe3 Invited Speaker: Yiping Wang The observation of the Higgs boson solidified the standard model of particle physics. However, explanations of anomalies (e.g. dark matter) rely on further symmetry breaking calling for a yet-to-be-discovered axial Higgs mode. In condensed matter, the Higgs mode has been observed in magnetic, superconducting, and charge density wave systems (CDW), and is typically assumed to have a scalar representation. Uncovering the vector properties of a low energy mode is extremely challenging, requiring going beyond typical spectroscopic or scattering techniques to reveal the hidden aspects of their wavefunctions. Here we set up a simple optical scheme and discover an unconventional axial Higgs mode in the 2D material GdTe3, which also has high mobility and antiferromagnetic order. The Axial Higgs mode is revealed using the interference of excitation quantum pathways in Raman scattering. I will discuss how the Axial Higgs mode emerges from the combination of the quantum geometry of the Fermi surface and the charge density wave, opening opportunities for new topological correlated states in 2D systems. |
Thursday, March 17, 2022 12:06PM - 12:18PM |
T53.00002: Anisotropic gigahertz frequency antiferromagnetic resonance in layered van der Waals semiconductor Thow Min Jerald Cham, Yunqiu Kelly Luo, Saba Karimeddiny, Avalon H Dismukes, Xavier Roy, Kin Fai Mak, Jie Shan, Daniel C Ralph Antiferromagnetic spintronics in layered van der Waals magnetic systems provides unique advantages in manipulating magnon modes and provides emerging device concepts for microwave electronics. CrSBr, an air-stable vdW antiferromagnetic semiconductor, exhibits intralayer ferromagnetic order and interlayer antiferromagnetic coupling with in-plane magnetic easy axis. In this talk, we report anisotropic gigahertz frequency antiferromagnetic resonance of bulk CrSBr using broadband microwave absorption spectroscopy. We observe a rich spectrum of resonances that reflect crystalline uniaxial anisotropy, with signatures of the spin-flop transition. The role of interlayer antiferromagnetic exchange will also be discussed. |
Thursday, March 17, 2022 12:18PM - 12:30PM |
T53.00003: Magnon phonon hybridization in 2D antiferromagnet Jiaming Luo, Gaihua Ye, Zhipeng Ye, Rui Xu, Lebin Chen, Shuyi Li, Xiaokun Teng, Pengcheng Dai, Andriy H Nevidomskyy, Rui He, Hanyu Zhu Magnons in magnetically ordered low-dimensional materials potentially enable novel nano-spintronics. The lifetime of magnons often depends on the strength of spin-lattice coupling and scattering with phonons. Here, we found coherent magnon phonon hybridization in the class of transition metal phosphorous trichalcogenides (MPX3, M = Fe, Mn, Ni and X = S, Se). From bulk to ultrathin layer, the antiferromagnetic magnons are identified by Zeeman splitting with applied magnetic field. A model is developed to quantitatively explain the nature of the emergent quasiparticles, as well as their center frequencies, scattering intensities, and polarization selection. This strong coupling may result in finite Berry curvature in the hybridization bands and possibly non-trivial topology. |
Thursday, March 17, 2022 12:30PM - 12:42PM |
T53.00004: Observation of strain-locked in-plane spin orders in a 2D XY antiferromagnet Zhuoliang Ni, Amanda V Haglund, Hua Wang, Bing Xu, Christian Bernhard, David G Mandrus, Xiaofeng Qian, Eugene J Mele, Charles L Kane, Liang Wu It is believed that no long-range magnetic order exists in a 2D magnet with the XY anisotropy under Mermin–Wagner theorem. Surprisingly, we directly observe the long-range antiferromagnetic (AFM) N\'eel orders in an XY magnet MnPSe$_3$ down to monolayer by optical second-harmonic generation (SHG) microscopy. We further demonstrate the directions of the in-plane spins are decoupled to the crystalline axes and actually locked by the external strains. A simple Ginzburg–Landau theory is applied to explain our observation, where an inevitable tiny strain in the real world would drive the system from XY Universality class to Ising Universality class and help stabilize the long-range orders. Our work also demonstrate the SHG microscopy as a powerful tool to imaging AFM spin structures in 2D magnets. |
Thursday, March 17, 2022 12:42PM - 12:54PM |
T53.00005: Real-space visualization of temperature- and layer-dependent susceptibility in the layered antiferromagnet CrSBr Daniel J Rizzo, Alexander S McLeod, Caitlin Carnahan, Avalon H Dismukes, Ren A Wiscons, Evan J Telford, Yinan Dong, Abhay N Pasupathy, Xavier Roy, Di Xiao, Dmitri N Basov Layered magnetic materials have recently garnered substantial interest as platforms for realizing tunable magnetic devices and studying fundamental two-dimensional (2D) magnetic properties. A growing body of research on chromium-based van der Waals (vdW) materials demonstrates the existence magnetism in 2D films down to single atomic layers. Among these, CrSBr has emerged as an air-stable layered antiferromagnetic (AFM) semiconductor with a high transition temperature (TN ≈ 132 K) and gate-tunable magnetic ordering. However, the direct real-space visualization and temperature evolution of magnetic domains in CrSBr remains unexplored, and requires a sensitive local probe. To achieve this, we conducted a variable-temperature magnetic force microscopy (MFM) study under ultrahigh vacuum (UHV) conditions, revealing incipient magnetism well above TN associated with the onset of in-plane magnetic correlations which eventually give way to layer-dependent magnetization in the low-temperature AFM phase (T < TN). In addition, we observe temperature-dependent magnetic susceptibility and switching with a high degree of spatial inhomogeneity arising, in part, from the parity of the underlying layer number. |
Thursday, March 17, 2022 12:54PM - 1:06PM |
T53.00006: Room-temperature ferromagnetism in Co-doped 2D graphitic ZnO Rui Chen, Jie Yao Long-range ferromagnetic ordering has been successfully observed in 2D materials, displaying novel quantum phases and spintronic phenomena. Although it is a giant step forward in fundamental physics to realize 2D ferromagnetism, most 2D magnets still suffer from multiple challenges including Curie temperatures below room temperature, chemical instability, and so on, which significantly limit their emergent applications in computing and storage. Recently, we successfully observed 2D ferromagnetism above room temperature, by introducing Co atoms into a non-magnetic 2D vdW system, graphitic ZnO. Room-temperature ferromagnetism has been verified in 2D Zn1-xCoxO (ZCO) by means of superconducting quantum interference device, magneto-optic Kerr effect, and X-ray magnetic circular dichroism measurements. ZCO also shows exceptional stability in ambient conditions as well as at elevated temperatures. Our work indicates that the one-atom-thick ZCO can serve as a promising platform to explore new magnetic and topological phases with above-room-temperature TC, environmental stability, and dopant-tunability. |
Thursday, March 17, 2022 1:06PM - 1:18PM |
T53.00007: ARPES Study of Ni-Doped Fe5GeTe2 with Tc exceeding 450K Tyler L Werner, Yu He, Xiang Chen, Donghui Lu, Makoto Hashimoto, Elio Vescovo, Turgut Yilmaz, Robert J Birgeneau The vdW metallic ferromagnet Fe5GeTe2 has one of the highest Curie temperatures of any 2D ferromagnet at 270K. The Curie temperature can be further enhanced up to >450K with nickel doping of iron sites. Further doping induces a structural instability concomitant with a ferromagnetic to paramagnetic transition. Here we report the evolution of the electronic structure by ARPES measurement as a function of Ni-doping. Based on core level and valence electron photoemission, we discuss the implication of phase separation and domain pinning to the > 50% enhancement in Tc. |
Thursday, March 17, 2022 1:18PM - 1:30PM |
T53.00008: The effect of high pressures on the quantum limit of graphite Leah Snyder Graphite is a semi-metal with a low carrier density that can be tuned to the quantum limit with a comparatively small magnetic field of ~ 7.5 T. Below this quantum limit, a magnetic field applied perpendicular to the layers exhibits Shubnikov-de Haas oscillations of two different frequencies, corresponding to the hole and electron fermi surfaces, each of which has a different effective mass. Previous measurements suggest that the carrier imbalance remains constant up to 25 T, above which multiple field induced phase transitions are observed. While much attention in recent years has focused on these high field states, we seek to better understand how the low temperature, low field ground state of the material is altered with the application of pressure and determine what effect if any that has on the carrier imbalance and the quantum limit. We will discuss the results of these studies in fields below 15 T and to pressures in excess of 20 kbar. |
Thursday, March 17, 2022 1:30PM - 1:42PM |
T53.00009: Magnetic proximity effect in CrPS4 heterostructure Suhan Son, Joolee Son, Jieun Lee, Je-Geun Park Recently studied 2D ferromagnetic materials including CrI3 show the intriguing phenomena. However, there remains a challenge for the application due to the chemical instability under atmosphere. Here, we report the observation of an Cr-based air-stable ferromagnetism in magnetic van der Waals material CrPS4, down to monolayer, and its layer-dependent ferromagnetism using magneto-optic Kerr effect microscopy. Besides those ferromagnetism, we also observed the strong proximity effect to superconducting NbSe2 layer and suppression of superconductivity in vicinity of ferromagnetic layer. For the fabriation, strongly adhesive polymer based fabrication, PCL method, should be introduced, which is also briefly discussed. |
Thursday, March 17, 2022 1:42PM - 1:54PM |
T53.00010: Strain engineering and the hidden role of magnetism in monolayer VTe2 Do Hoon Kiem, Min Yong Jeong, Hongkee Yoon, Myung Joon Han Two-dimensional transition-metal dichalcogenides have attracted lots of attention. Motivated by a recent study of crystalline bulk VTe2 [1], we theoretically investigated the spin-charge-lattice interplay in monolayer VTe2. To understand the controversial experimental reports on several different charge density wave (CDW) ground state, we paid a special attention to the ‘hidden’ role of antiferromagnetism as its direct experimental detection may be challenging. Our first-principles calculations show that the 4x1 charge density wave and the corresponding lattice deformation are accompanied by the ‘double-stripe’ AFM spin order in its ground state. This phase has not only the lowest total energy but also the dynamical phonon stability, which supports a group of previous experiments. Interestingly enough, this ground state is stabilized only by assuming the underlying spin order. Other previously reported phases were found to have either significantly larger total energy or unstable phonon profile. By noticing this intriguing and previously unknown interplay between magnetism and other degrees of freedom, we further suggest a possible strain engineering. By applying tensile strain, monolayer VTe2 exhibits phase transition first to a different charge density wave phase and then eventually to a ferromagnetically ordered one. |
Thursday, March 17, 2022 1:54PM - 2:06PM |
T53.00011: Escalating ferromagnetic order via Se-vacancy near vanadium in WSe2 Byeong Wook Cho Magnetic order has been proposed from a variety of defects including vacancies, anti-sites, and grain boundaries, which are relevant in numerous spintronics applications. Nevertheless, the origin of magnetism remains unelucidated, mainly due to the lack of structural analysis. We herein demonstrate the escalation of ferromagnetism in a vanadium (V)-doped WSe2 monolayer by tailoring Se-vacancy complexes via post heat-treatment. Structural analysis of atomic defects was systematically performed using scanning transmission electron microscopy (STEM). Temperature-dependent magnetoresistance hysteresis ensures enhanced magnetic order after high-temperature heat-treatment. The V – Se vacancy pairing is a key to promote ferromagnetism with additional Se vacancies around V via spin-flip by electron transfer from Se-vacancy complex to V atom, predicted from density-functional-theory (DFT) calculations. The elevated magnetic moment with heat-treatment was estimated by combining the Se-vacancy population extracted from STEM with DFT calculations, which is well corroborated by magnetic domain analysis. Our approach towards nano-defect engineering paves a way to promote weak magnetic order in diluted magnetic semiconductors for renovating spintronics with two-dimensional layered materials. |
Thursday, March 17, 2022 2:06PM - 2:18PM |
T53.00012: Large magneto-optical effect and magnetic anisotropy energy in two-dimensional metallic ferromagnet Fe3GeTe2 Ming-Chun Jiang, Guang-Yu Guo The discovery of ferromagnetic (FM) metal Fe3GeTe2 has opened up opportunities for studying two-dimensional (2D) magnetism, such as tunable Curie temperature and anomalous Hall effect. In this work, we study the magnetic anisotropy energy (MAE) and magneto-optical (MO) effects of multilayers and bulk Fe3GeTe2 based on first principle calculations. Firstly, all considered Fe3GeTe2 structures favor the out-of-plane magnetization with large MAEs of ~3.0 meV/f.u., being ~6 times larger than that of 2D FM semiconductors CrI3. This is attributed to the dominant Te px,y density of states near the Fermi level. Secondly, 2D and bulk Fe3GeTe2 also exhibit strong MO effects with their Kerr and Faraday rotation angles being larger than that of best-known MO materials such as Y3Fe5O12. The MO spectra are almost thickness-independent, indicating weak interlayer coupling. The MO conductivity spectra are analysed via the dipole selection rule at the Γ, K and K' points, which further reveal that odd layer-number Fe3GeTe2 films would exhibit anomalous ferrovalley Hall effect. All these interesting findings thus suggest that 2D and bulk ferromagnetic Fe3GeTe2 are promising materials for high density MO and spintronic nanodevices. |
Thursday, March 17, 2022 2:18PM - 2:30PM |
T53.00013: Ambient-Stable Two-Dimensional CrI3 via Organic-Inorganic Encapsulation Dmitry Lebedev, Tyler J Gish, Teodor Stanev, Shizhou Jiang, Leonidas Georgopoulos, Thomas W Song, Gilhwan Lim, Ethan S Garvey, Lukáš Valdman, Oluwaseyi Balogun, Zdeněk Sofer, Vinod K Sangwan, Nathaniel P Stern, Mark C Hersam 2D transition metal halides have recently attracted significant attention due to their thickness dependent magnetism and electrical control of magnetic order. However, this class of materials is extremely reactive chemically, leading to rapid irreversible degradation upon ambient exposure. Here we demonstrate long-term encapsulation of the prototypical 2D transition metal halide CrI3 by using atomic layer deposition (ALD) of alumina that is seeded with an organic buffer layer of perylenetetracarboxylic dianhydride (PTCDA). We show that this protocol allows for long-term encapsulation of CrI3 and preserves the structure and magnetic order down to monolayer thickness. In the absence of the PTCDA buffer layer, the ALD precursors damage the CrI3 top layer as revealed by X-ray photoelectron spectroscopy and thickness dependent magneto-optical Kerr-effect measurements. With this hybrid organic-inorganic encapsulation protocol, we demonstrate field-effect transistors, photodetectors, and optothermal measurements of CrI3 thermal conductivity in ambient conditions. Ongoing efforts to extend this methodology to charge transport measurements of additional 2D transition metal halides will also be discussed. |
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