Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session F60: Topological Materials: Magnetism and Hybrid StructuresFocus

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Sponsoring Units: DMP Chair: Jisoo Moon, US Naval Research Laboratory Room: Mile High Ballroom 4A 
Tuesday, March 3, 2020 8:00AM  8:36AM 
F60.00001: Topological phenomena in magnetic topological insulator thin films Invited Speaker: Wenbo Wang Quantum anomalous Hall effect (QAHE), which is related to the momentumspace Berry phase, was experimentally realized in Crdoped (Bi,Sb)_{2}Te_{3} (BST) and Vdoped BST thin films. The full quantization of anomalous Hall effect (AHE) in these two systems requires extremely low temperature (<50mK)[1][2]. Magnetic inhomogeneity has been proposed as one of the main reasons that limit the temperature for the realization of QAHE. In the first part, we will present direct visualization of longrange ferromagnetism in the Cr,V codoped BST films, by using the MFM and insitu transport measurement[3]. The magnetization reversal process reveals typical ferromagnetic domain behavior, i.e. domain nucleation and domain wall propagation. In the second part, we will discuss topological Hall effect(THE), a realspace Berry phase effect that originates from the noncoplanar spin textures[4]. Intrinsic THE was revealed at different temperatures in the magnetic topological insulator heterostructure Sb_{2}Te_{3}/Vdoped Sb_{2}Te_{3}, after a backgate voltage was applied to zero the AHE. The hightemperature THE persists above the Curie temperature, which originates from the spin chirality fluctuations[5]. At temperatures well below the Curie temperature, THE emerges around the coercive fields, when the ferromagnetic bubble domains form. The lowtemperature THE likely results from the static topological charges at the chiral domain walls. 
Tuesday, March 3, 2020 8:36AM  8:48AM 
F60.00002: Magnetotransport in Pb_{x}Sn_{1x}Se Quantum Well Jiashu Wang, Xinyu Liu, Logan S Riney, Jacek K. Furdyna, Badih A Assaf Pb_{x}Sn1_{x}Se (0.16<x<0.4) is a narrow gap semiconductor that exhibits a topological phase transition to a topological crystalline insulator (TCI) when the concentration of Sn increases. It is a valley degenerate topological system that can exhibit exotic quantum Hall effects and strong coupling between the crystal and electronic structures. Here we investigated the transport properties of PbEuSe/Pb_{x}Sn_{1x}Se/PbEuSe quantum wells, of which the thickness is 30 and 50nm and x=0.3. By doping Bismuth during the growth, we can efficiently tune the carrier type from p type to n type with carrier densities as low as 1E18cm^{3} and high mobility (>10000cm^{2}/ (Vs)). We found out that the sample showed linear magnetoresistance starting at relatively low field (<1T). The slope of ΔR/R_{0} is proportional to the mobility. We applied several model to explain this phenomenon. Further process like gating is also used to investigate the mechanism. With high mobility and low carrier density in the quantum well, it will be a great platform to study 2D quantum phenomenon in the landau quantized regime. 
Tuesday, March 3, 2020 8:48AM  9:00AM 
F60.00003: Magnetotransport properties in antimony single crystals Paul C. W. Chu, Zheng Wu, Liangzi Deng, Melissa Gooch, Trevor Bontke The magnetoresistance of antimony single crystals has been measured with the measuring current perpendicular and parallel to the magnetic field, respectively. Extremely large positive transverse magnetoresistance (TMR) up to 1,000,000% was obtained in a magnetic field of 7 T. Field dependence of TMR followed the power law MR ∝ H^{2a} in all measured temperatures from 2 K to 300 K. The extremely large TMR is attributed to the well compensated electron and hole carriers with high mobility. A very pronounced negative longitudinal magnetoresistance (LMR) at high field and a valley at zero field was observed. The general behavior appears to be consistent with the chiral magneto effect predicted in quantum solids. Moreover, a very unusual negative potential from voltage leads was detected by IV curve, dc, and ac resistance measurements at high magnetic field. This observation cannot be understood in terms of available theories and may be related to some exotic topological characteristics predicted for antimony. 
Tuesday, March 3, 2020 9:00AM  9:12AM 
F60.00004: Thickness Dependence of Electronic Properties of Topological Antiferromagnet MnBi_{2}Te_{4} Films Yifan Zhao, Guang Wang, lingjie zhou, Dmitry Ovchinnikov, Fei Wang, ling zhang, Hemian Yi, baitao zhang, Moses HungWai Chan, Xiaodong Xu, CuiZu Chang The prerequisite for the realization of the quantum anomalous Hall (QAH) state is an interplay between topology and magnetism. More recently, MnBi_{2}Te_{4} was experimentally demonstrated to be a material in which the topological and magnetic states intrinsically coexist. The QAH and axion insulator states were predicted in thin films of MnBi_{2}Te_{4} when the number of layers is odd and even, respectively. Here, we synthesized MnBi_{2}Te_{4} thin films with thicknesses down to one layer using molecular beam epitaxy (MBE) and systematically studied the thickness dependence of electronic properties. Both Hall and MagnetoOptical Kerr Effect measurements show a clear ferromagnetic hysteresis in the onelayer sample. For films thicker than one layer, similar to the exfoliated MnBi_{2}Te_{4} devices, the samples show a small ferromagnetic hysteresis loop at the low magnetic field and spin flop transitions at the high magnetic field. We also found the Hall resistance under zero magnetic field displays an odd/even oscillation, i.e., the samples with odd (even) layers exhibit an anomalous Hall (AH) hysteresis loop with a negative (positive) sign. 
Tuesday, March 3, 2020 9:12AM  9:24AM 
F60.00005: Discovery of a quantum topological kagome magnet by vector magnetic fieldbased STM Jiaxin Yin, Wenlong Ma, Songtian Sonia Zhang, Tyler Cochran, Xitong Xu, Nana Shumiya, Biao Lian, Zhida Song, Kun Jiang, TayRong Chang, Titus Neupert, Ziqiang Wang, Nan Yao, Shuang Jia, Zahid Hasan Kagome magnets have been known to exhibit rich topologically based vector magnetic field responses, including giant and anisotropic spinorbit tunability (J.X. Yin et al. Nature 562, 9195 (2018)) and negative flatband magnetism (J.X. Yin et al. Nature Physics 15, 443448 (2019)). Here we use the same vector magnetic fieldbased STM technique combined with Berry curvature sensitive transport to discover a new quantum topological kagome magnet. We observe striking Landau quantization of the kagome lattice, dissipationless chiral edge state, and quantumlimit Hall response. These combined results point to the realization of a hightemperature quantum topological kagome magnet. 
Tuesday, March 3, 2020 9:24AM  9:36AM 
F60.00006: Quantum oscillations in thin films of antiferromagnetic kagome metal FeSn Minyong Han, Hisashi Inoue, Mun K. Chan, David E Graf, Linda Ye, Takehito Suzuki, Joseph G Checkelsky FeSn is an antiferromagnetic metal, consisting of twodimensional layers of cornersharing triangles of Fe, separated by honeycomb Sn spacer layers. This geometrical arrangement of Fe atoms, known as the kagome lattice, is predicted to give rise to Dirac fermions and topological flat bands. Recent photoemission and quantum oscillation studies on FeSn bulk single crystals have experimentally confirmed the existence of these features in their electronic structures [1]. Here we report the quantum oscillation studies on thin films of FeSn. The observed oscillation frequency reproduces the previous measurements on FeSn bulk single crystals. Temperature and fieldangle dependent oscillation frequencies respectively reveal the scattering timescale and morphology of the observed Fermi pocket. The realization of high quality FeSn thin films offers a promising platform to explore the interplay of magnetism and electronic topology via quantum confinement or electrostatic gating [2]. 
Tuesday, March 3, 2020 9:36AM  9:48AM 
F60.00007: DiracSurfaceStateModulated Bilinear Magnetoresistance in Topological Insulators Fei Wang, ling zhang, Ziqiao Wang, Yifan Zhao, Run Xiao, Guang Wang, Hemian Yi, lingjie zhou, Ruoxi Zhang, baitao zhang, Moses HungWai Chan, Nitin Samarth, Qi Li, Weiwei Zhao, CuiZu Chang Topological insulators (TIs) are a class of quantum materials in which the interior is insulating but electrons can travel through the twodimensional (2D) conducting surfaces known as Dirac surface states. The Dirac surface states harbor a helical spin texture and thus provide a good platform for potential spintronic applications. Prior experiments have shown that the helical spin texture of the Dirac surface states can be resolved using bilinear magnetoresistance (BMR) measurements. The observation of BMR in previous studies was attributed to the presence of the hexagonally warped Fermi surface at the Dirac surface state in samples with heavy doping. Here, we fabricated a series of (Bi_{1x}Sb_{x})_{2}Te_{3} samples with different x and systematically studied their BMR. When the chemical potential is near the Dirac point and the warping effect of the Dirac surface state disappears, the BMR reaches a maximum rather than vanishes. This observation is further confirmed in a dualgatetuned device. Our work indicates that BMR has more complex origins than the warping effect of the Dirac surface state alone. 
Tuesday, March 3, 2020 9:48AM  10:00AM 
F60.00008: Dirac Band Engineering of Topological Insulator/Graphene Hybrid Structures Guang Wang, Yifan Zhao, Huixia Fu, Zengle Huang, Fei Wang, Hemian Yi, Timothy Pillsbury, lingjie zhou, Chengye Dong, Joshua Robinson, Nitin Samarth, Weida Wu, binghai yan, CuiZu Chang Dirac materials such as graphene and topological insulators (TI) have attracted much attention because of their unique electrical properties. Stacking these two Dirac materials to form a heterostructure provides a platform for the exploration of the interesting physics of coupled Dirac fermions. We recently used molecular beam epitaxy to fabricate the (Bi_{1x}Sb_{x})_{2}Te_{3}/graphene hybrid structures and systematically studied the electronic band structures as a function of x and thickness. Because the ratio between the lattice constants of Sb_{2}Te_{3} and graphene is √3, the Dirac cone of graphene can be folded to the Γ point of the Brillouin zone of Sb_{2}Te_{3}. The coexistence of two different Dirac lowenergy states is indeed observed in angleresolved photoemission spectroscopy measurements. This phenomenon is further demonstrated by scanning tunneling microscopy results and firstprinciples calculations. Our work is of great importance for understanding the interplay of Dirac fermions across the TI/graphene interface and paves a new way to study band engineering in Dirac materials. 
Tuesday, March 3, 2020 10:00AM  10:12AM 
F60.00009: Topological Properties of SnSe/EuS and SnTe/CaTe Heterostructures Shuyang Yang, Chunzhi Wu, Noa Marom We use density functional theory (DFT) to study epitaxial interfaces of the topological crystalline insulators SnSe and SnTe with the ferromagnetic semiconductor EuS and the nonmagnetic semiconductor CaTe, respectively. We consider both surface slab models and periodic "sandwich" heterostructures. We find that a gapped surface state arises in the SnSe/EuS interface due to the magnetic proximity effect. By comparison, the gapless topological surface state at the interface of SnTe/CaTe is unperturbed. 
Tuesday, March 3, 2020 10:12AM  10:24AM 
F60.00010: Synthesis of hybrid oxide/chalcogenide interfaces to explore topological crystalline insulator properties Stephen D Albright, Ke Zou, Shuhang Pan, Frederick J Walker, Charles H Ahn The class of topological crystalline insulators (TCIs), including SnTe, provides new opportunities to explore topological behaviors. Surface carriers in TCIs experience the same spinmomentum locking as conventional topological insulators, but are protected by crystalline symmetry, which makes possible controlling topological states in SnTe through electric field gating. This work presents growth of SnTe thin enough to achieve topological state control on a substrate, SrTiO_{3}, easily integrated into fieldeffect devices. We grow SnTe films by cosublimationdeposition, where a highquality thick film is controllably thinned by elevating the substrate temperature in constant flux. Xray diffraction and atomic force microscopy confirm SnTe films grown by this process are uniform down to 7 nm. Carrier density over a range of film thickness, extracted from Hall measurements, reveals surfacedominated transport in SnTe films less than 16 nm. Weak antilocalization (WAL) behavior observed in magnetotransport is consistent with topological conduction through these surface states, and we fit WAL to theory to extract details of the topological transport. The growth method presented here opens up future opportunities to design heterostructures to further explore topological behavior. 
Tuesday, March 3, 2020 10:24AM  10:36AM 
F60.00011: Modular Arithmetic with Nodal Lines: Drumhead Surface States in ZrSiTe Lukas Muechler, Andreas Topp, Raquel Queiroz, Maxim Krivenkov, Andrei Varykhalov, Jennifer Cano, Christian R Ast, Leslie Schoop We study the electronic structure of the nodal line semimetal ZrSiTe both experimentally and theoretically. 
Tuesday, March 3, 2020 10:36AM  10:48AM 
F60.00012: NMR investigation of topological quantum material ZrTe_{5} Yefan Tian, Nader Ghassemi, Joseph Hansbro Ross In this work, we performed ^{125}Te NMR measurements on the topological quantum material ZrTe_{5}. The measured spinlattice relaxation rates (1/T_{1}) can be wellexplained by a theoretical model for Dirac electron systems, which reveals that the topological characteristic of ZrTe_{5} is Tdependent. Combined with DFT calculations, the topological transition ordering can be confirmed from weak topological state to strong topological state with temperature increasing, the reverse of what has been proposed. The Tdependence of Dirac band gap can be derived from NMR results, demonstrating a gapless Dirac semimetal state occurring at a Lifshitz transition temperature, T_{c} = 85 K in our crystals. We also show that the changes in NMR shift at the Lifshitz transition provide direct evidence of band inversion when the topological phase transition occurs. 

F60.00013: Heteroepitaxial growth of single layer ferromagnetic VSe_{2} on graphene and Bi_{2}Se_{3} Mengchen Li, Ying Liu

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