Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session G59: Ferromagnetic Kagome Metals 
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Sponsoring Units: DMP Room: Mile High Ballroom 3C 
Tuesday, March 3, 2020 11:15AM  11:27AM 
G59.00001: Spin dynamics of the isotropic kagome ferromagnet Fe_{3}Sn_{2} Jeffrey Lynn, Rebecca Dally, Nirmal Ghimire, Nishchal Thapa Magar, Daniel Phelan, John Mitchell The giant anomalous Hall effect in the itinerant kagome magnet Fe_{3}Sn_{2} was recently attributed to massive Dirac fermions resulting in a Berry curvature in the ferromagnetic phase [1], with a topologically nontrivial fielddependent spin texture phase discovered[2], demonstrating an intimate relationship between the electronic and magnetic properties in this strongly correlated electron material. Here we present a new perspective on Fe_{3}Sn_{2} by studying the magnetic excitations near q=0 using inelastic neutron scattering. The results on polycrystalline samples show this material to be an isotropic ferromagnet near its T_{C} of 660 K, where we find a negligible spin gap, and large spinwave stiffness. The moments initially point along the caxis and slowly rotate over the course of several hundred degrees towards the abplane. This indicates the “soft” nature of the ferromagnet, which is at odds with the argument that anisotropy is responsible for some the materials’ properties elsewhere in phase space. Additionally, magnetic susceptibility and magnetization measurements agree with our neutron diffraction results showing this sample does not undergo a reentrant spinglass phase as reported by some. 
Tuesday, March 3, 2020 11:27AM  11:39AM 
G59.00002: Scanning Tunneling Microscopy Study of Fe_{3}Sn_{2} Jorge Olivares Rodriguez, Anuva Aishwarya, Lin Jiao, Vidya Madhavan The peculiar geometry of the Kagome lattice, composed of cornersharing triangles, has been known to be a fertile ground for exotic quantum phases since theory predicts the possible coexistence of topological and frustrated magnetic states stemming from this structure. The recently rediscovered metallic ferromagnet Fe_{3}Sn_{2}, consisting of two kagome bilayers (Fe_{3}Sn) layers separated by a honeycomb Sn layer, is seemingly a good candidate to explore the interplay between frustrated magnetism and topological band structure due to recent reports which include the observation of massive dirac fermions and the anomalous Hall effect. In addition, it has also been reported that its electronic structure can be tuned by changing its magnetic moments or temperature, making it a viable candidate for spintronic applications. Here we present a scanning tunneling microscopy/spectroscopy study of Fe_{3}Sn_{2} at low temperatures and under a magnetic field. 
Tuesday, March 3, 2020 11:39AM  11:51AM 
G59.00003: Defects in magnetic Weyl semimetal Co3Sn2S2 Qiang Zou, Mingming Fu, Mina Yoon, Rui Xue, David Mandrus, Zheng Gai In Co3Sn2S2, a magnetic Weyl semimetal with kagomelattice, the existence of bulk Weyl nodes, which are formed under broken inversion or timereversal symmetry, creates nontrivial topological properties, for example, robust Giant anomalous hall effect. The surface–bulk correspondence ensures the bulk bands related topological “Fermi arc” surface bands dispersion. In this presentation, we use low temperature high magnetic field scanning tunneling microscope, spin polarized STM, and quasiparticle interference (QPI) to study the influence of local defects including magnetic and nonmagnetic vacancies and adatoms to the Weyl nodes movement. Co and Sn vacancies in the Kagomelattice are identified, their behavior under magnetic field are studied. S adatoms in 1D forms are compared with individual adatoms. The interplay among topology, defects and magnetism are discussed for the understanding of the involved quantum phenomena. 
Tuesday, March 3, 2020 11:51AM  12:03PM 
G59.00004: Temperature dependent conductivity in magnetic Weyl semimetal Co3Sn2S2with terahertz spectroscopy Elizabeth Fuller, Evan Jasper, YUFEI Li, Rolando Valdes Aguilar, Rui Xue, David Mandrus The recently discovered Weyl semimetal, Co3Sn2S2, is attractive for its intrinsic ferromagnetic behavior below Tc ~ 175 K with almost full spin polarization, i.e. halfmetallicity. Previous studies^{1,2} report evidence of a highly tunable, large anomalous Hall effect (AHE) with change in temperature. We utilize spectroscopic methods in the terahertz range to measure the optical conductivity in a single crystal Co3Sn2S2within the 8 K to 290 K temperature range. We observe a clear downward shift in the reflectance of the crystal with increasing temperatures near the ferromagnetic transition and a similar trend for low temperatures. We will discuss these measurements in the context of the Anomalous Hall conductivity that varies with shifts in Weyl node position. 
Tuesday, March 3, 2020 12:03PM  12:15PM 
G59.00005: Fermiarc diversity on surface terminations of the magnetic Weyl semimetal Co_{3}Sn_{2}S_{2} Nurit Avraham, Noam Morali, Pranab Kumar Nag, Rajib Batabyal, Liu Enke, Qiunan Xu, Yan Sun, binghai yan, Claudia Felser, Haim Beidenkopf Bulk–surface correspondence in Weyl semimetals ensures the formation of topological “Fermi arc” surface bands whose existence is guaranteed by bulk Weyl nodes. By investigating three distinct surface terminations of the ferromagnetic semimetal Co_{3}Sn_{2}S_{2}, we verify spectroscopically its classification as a timereversal symmetrybroken Weyl semimetal. We show that the distinct surface potentials imposed by three different terminations modify the Fermiarc contour and Weyl node connectivity. On the tin (Sn) surface, we identify intra–Brillouin zone Weyl node connectivity of Fermi arcs, whereas on cobalt (Co) termination, the connectivity is across adjacent Brillouin zones. On the sulfur (S) surface, Fermi arcs overlap with nontopological bulk and surface states. We thus resolve both topologically protected and nonprotected electronic properties of a Weyl semimetal. 
Tuesday, March 3, 2020 12:15PM  12:27PM 
G59.00006: Large anomalous Hall and planar Hall effect in magnetic Weyl semimetal Co_{3}Sn_{2}S_{2} ShuoYing Yang, Jonathan Noky, Jacob D Gayles, Fasil Kidane Dejene, Yan Sun, Enke Liu, Mazhar Ali, Claudia Felser, Stuart Parkin Weyl fermions are chiral massless fermions manifested in crystalline solids by spin split conduction and valence bands crossing at discrete points. Timereversalsymmetrybroken Weyl semimetals (WSMs) have attracted particular attention because of their interesting interplay between intrinsic magnetism and topologically nontrivial electrons. In this work, we perform detailed transport studies on a magnetic Weyl semimetal, Co_{3}Sn_{2}S_{2}. Nanoplates as thin as 180 nm were grown via chemical vapor transfer methods. Through magnetotransport measurements, we report a large intrinsic anomalous Hall conductivity generated by a large Berry curvature from the Weyl nodes. The anomalous Hall conductivity is robust against both increased temperature and charge conductivity, reaching ~1420 ohm^{1} cm^{1}. In addition, we discuss the observation of a large planar Hall effect (PHE) in Co_{3}Sn_{2}S_{2}_{. }We carefully examined all possible origins of the PHE including ferromagnetism, orbital magnetoresistance and the chiral anomaly. Our analysis reveals that even though negative magnetoresistance (NMR) was not seen, the observed PHE is chiral anomaly dominated. Our results show how multiple PHE contributions can be disentangled in a magnetic WSM system and suggest that PHE is a sensitive probe of Weyl transport. 
Tuesday, March 3, 2020 12:27PM  12:39PM 
G59.00007: Exchange biased Anomalous Hall Effect driven by frustration in a magnetic Kagome lattice Ella Lachman, Ryan Murphy, Nikola Maksimovic, Robert Kealhofer, Shannon C Haley, Ross McDonald, Jeffrey R Long, James Analytis Co_{3}Sn_{2}S_{2} is a ferromagnetic Weyl semimetal that has been the subject of intense scientific interest due to its large anomalous Hall effect (AHE). We show that the coupling of this material's topological properties to its magnetic texture leads to a strongly exchange biased AHE, and argue that this is likely caused by coexistence of ferromagnetism and spin glass phases. The spin glass is being driven not by disorder, but by the geometric frustration intrinsic to the Kagome network of magnetic ions. Both phases are thought to originate from the Co spin system, in an interesting display of Exchange Bias eminating from a single magnetic phase. Magnetism plays an important role in the robustness of the QAHE in magnetically doped topological insulators, and may play a crucial role in unlocking the possibility of a QAHE in lowdimensional structures of Co_{3}Sn_{2}S_{2}. 
Tuesday, March 3, 2020 12:39PM  12:51PM 
G59.00008: High field quantum oscillation studies of Dirac electrons in ironbased kagome lattice metals Linda Ye, Min Gu Kang, Mun K. Chan, Ross McDonald, David E Graf, Abraham L Levitan, Minyong Han, David Charles Bell, Madhav Prasad Ghimire, Shiang Fang, JhihShih You, Jorge I. Facio, Efthimios Kaxiras, Jeroen Van den Brink, Riccardo Comin, Joseph G Checkelsky The kagome lattice has long been theoretically known to harbor Dirac dispersions together with a dispersionless (flat) band. The relevance of the kagome lattice model in the context of electronic structures has recently been established in a class of binary hexagonal iron stannides, where Dirac dispersions derived from Fe 3d electrons are observed at Brillouin zone corners in photoemission studies [1,2]. Here we report high magnetic field quantum oscillation studies of Dirac electrons in these ironbased kagome metals including ferromagnetic Fe_{3}Sn_{2} [3] and antiferromagnetic FeSn [2]. In Fe_{3}Sn_{2} we observe a doublet of quasi2D bulk Dirac electrons while a single bulk Dirac pocket is identified in FeSn. We further discuss the impact of crystallographic stacking and magnetic order on the Dirac electronic states in Fe_{3}Sn_{2} and FeSn. References: [1] L. Ye, M. Kang et al., Nature 555 638642 (2018). [2] M. Kang, L. Ye et al., arXiv/1906.02167, Nat. Mater. (in press). [3] L. Ye et al., Nat. Comm. 10, 4870 (2019). 
Tuesday, March 3, 2020 12:51PM  1:03PM 
G59.00009: Topological Chern bands for electrons and magnons realized in one Kagome ferromagnet: CoCu_{3}(OH)_{6}Cl_{2} Zhuoran He, Gang Xu, Biao Lian We study the topological electron bands and spinon bands in the ferromagnetic phase of CoCu_{3}(OH)_{6}Cl_{2}. The Cu ions with fractional valence and magnetic moments form a 3D trigonal crystal stacked from 2D Kagome layers. A tJ model with spinorbit coupling is constructed from the hybridized Wannier orbitals and studied using the Schwinger bosons to calculate the spinon bands. The Chern numbers of the electron and spinon bands reveal their nontrivial topological properties and implications on the spintronic transport properties are discussed. Our work provides an example for the study of interactions between two topological systems and sheds light on the understanding of correlated electrons in magnetic topological materials. 
Tuesday, March 3, 2020 1:03PM  1:15PM 
G59.00010: Electronic structure of a metallic ferromagnetic pyrite Iñigo Robredo, Niels Schröter, Sebastian Klemenz, Robert Kirby, Andreas P Schnyder, Aitor Bergara, Vladimir Strokov, Jonas A. Krieger, Tianlun Yu, Fernando de Juan, Maia G Vergniory, Leslie Schoop In this work we present combined experimental and theoretical results to elucidate electronic and magnetic properties of metallic cobalt pyrite CoS_{2}. On one side we reveal that the Density Functional Theory predicted band structure is in good agreement with angular resolved photoemission spectroscopy (ARPES) measurements, performed for the first time. Further studying its band structure we discovered that this material exhibits topological behaviour. Close to the Fermi level, we found a potential 'New Fermion', a 4dimensional magnetic band crossing protected by cubic, nonsymmorphic magnetic symmetries, along with several neighbouring Weyl fermions. We finally analyze the surface states and identify surface Fermi arcs. 
Tuesday, March 3, 2020 1:15PM  1:27PM 
G59.00011: Systematic analysis of topological and magnetic properties of materials related to MnBi_{2}Te_{4} by substitution Sugata Chowdhury, Kevin Garrity, Francesca Tavazza The search for materials with axion insulator phases has motivated extensive research about interactions between topological surface states and symmetrybreaking magnetic ordering, with possible applications in spintronics and quantum information. Recently, experimental and theoretical works have focused on MnBi_{2}Te_{4}, which is predicted to display this phase. In this work, we have looked at alternative elements in the same septuple layer structure by considered a series of 3d transitions metal from V to Ni as the magnetic element, as well as Sb for Bi and Se for Te. We study the topological properties and magnetic properties of new candidate materials. Our calculations reveal several potential topological materials, with properties depending on the filling of d electrons and the magnetic ordering. We also discuss the optical properties and electron phonon interactions of those materials. These types of stoichiometric magnetic materials are an excellent candidate for future topological devices. 
Tuesday, March 3, 2020 1:27PM  1:39PM 
G59.00012: Phase transition from MnBi_{2}Te_{3} to MnBi_{2}Te_{4} Kejing Zhu, Menghan Liao, Yan Gong, Ke He, Qikun Xue The realization of MnBi_{2}Te_{4} film opens the vision to search rich topological effects in intrinsic magnetic topological insulator. In the cleaved single crystal samples, the high resolution angleresolved photoemission spectroscopy (ARPES) shows the clear topological electronic structures, and the transverse resistance reaches quantization in a high external magnetic field . Those results indicate that 124 family materials can be an ideal platform for further exploring various topological phenomena. But in MnBiTe system, 124 structure is not the solely stable phase. Mn atoms can replace the position of Bi atoms to form a Mn doped Bi_{2}Te_{3} phase. MnBi_{4}Te_{7} and Mn_{2}Bi_{2}Te_{5} are also predicted to be existed in this system. In order to give a clear phase diagram and give an instruction of growing MnBiTe material, we investigate the evolution of electronic energy band structures with increasing Mn doping level. We also study the temperature influence on the MnBiTe material. 

G59.00013: Abnormal Negative Magnetoresistance and Exceptional Hall Component in Magnetic Weyl Semimetal Co_{3}Sn_{2}S_{2} Nanodevices Qi Zeng, Gangxu Gu, Jianlei Shen, Enke Liu, Wenhong Wang, Yongqing Li Recently, the first magnetic Weyl semimetal Co_{3}Sn_{2}S_{2} has been discovered[13]. For topological related spintronics applications in low dimensions, the growth and physical researches of Co_{3}Sn_{2}S_{2} thin films are highly desired. In this work, we synthesized singlecrystalline Co_{3}Sn_{2}S_{2 }nanoflakes with thickness from 100 to 30 nm by chemical vapor method. The nanodevices were prepared by using microfabrication. The nanoflakes show stable Curie temperature around 181 K, high anomalous Hall conductivity of 800 S cm^{1}, large coercive force up to 5.5 T, and high quality with RRR up to 20. At low temperatures, the decreased resistance is observed when the external magnetic field is lower than the coercive field and antiparallel to the internal magnetization, showing an abnormal negative magentoresistance. Meanwhile, an exceptional Hall component, after subtracting the normal and anomalous Hall, is also found before the magnetic domain is switched. A value of this component is observed up to three times larger than the anomalous Hall contribution. Exotic behaviors were observed in magnetic Weyl semimetal Co_{3}Sn_{2}S_{2} nanodevices. 

G59.00014: Pressure Driven Topological and Quantum Phase Transition on Magnetic Weyl Kagome Lattice Qi Zeng, Qiushi Yao, Jianlei Shen, Hongyi Sun, Wenhong Wang, Yonggang Wang, Qihang Liu, Enke Liu Recently, the Shandite compound Co_{3}Sn_{2}S_{2} has been proved as a magnetic Weyl semimetal[13]. Co_{3}Sn_{2}S_{2} possesses the outofplane magnetic Kagome lattice formed by Co atoms, which dominates the behaviors of Weyl fermions in this system. In this work, we performed experiments and theoretical calculations under high pressures to study the evolution of the topological state in Co_{3}Sn_{2}S_{2}. Experimental results show that the Curie temperature, anomalous Hall conductivity, and coercitive force of Hall curves decrease with increasing pressure. The ordinary Hall coefficient changes sign at a critical pressure, producing a maximum of the carrier concentration and a switch of the carrier types. Around 40 GPa the magnetic order finally disappears and the system goes into the protection of time reversal symmetry, becoming a topological insulator phase above 40 GPa. The calculations further found that additional Weyl nodes emerge with increasing pressure, showing low and highpressure Weyl fermion states. The topological and quantum phase transition is observed under pressure in the magnetic Weyl semimetal Co_{3}Sn_{2}S_{2}. 

G59.00015: On the anisotropies of magnetization and electronic transport of magnetic Weyl semimetal Co_{3}Sn_{2}S_{2} Jianlei Shen, Qi Zeng, Enke Liu Co_{3}Sn_{2}S_{2}, a quasitwodimensional system with kagome lattice, has been found as the first magnetic Weyl semimetal recently[13]. In this work, the anisotropies of magnetization and transport properties of Co_{3}Sn_{2}S_{2} were investigated. High field measurement results indicate this semimetal shows a giant magnetocrystalline anisotropy with an outofplane saturation field of 0.9 kOe and an inplane saturation field of 230 kOe at 2 K, showing a magnetocrystalline anisotropy coefficient K_{u }up to 8.3 × 10^{5} J m^{3}, which indicates that it is extremely difficult to align the small moment of 0.29 μ_{B}/Co on the kagome lattice from c axis to ab plane. The outofplane angular dependences of Hall conductivity further reveal strong anisotropies in Berry curvature and ferromagnetism, and the vector directions of both are parallel with each other. For inplane situation, the longitudinal and transverse measurements for both I // a and I ⊥ a cases show that the transport on the kagome lattice is isotropic. These results provide essential understanding on the magnetization and transport behaviors for the magnetic Weyl semimetal Co_{3}Sn_{2}S_{2}. 
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