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 C51: Topological Materials: Lines, Loops and BeyondFocus Live

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Sponsoring Units: DMP Chair: Jed Pixley, Rutgers University, New Brunswick 
Monday, March 15, 2021 3:00PM  3:36PM Live 
C51.00001: Discovery of topological magnetic Weyl loops by spectroscopy and transport Invited Speaker: Ilya Belopolski There is a great zoo of topological electronic states in quantum matter, comprising the quantum Hall state, topological insulators, Weyl semimetals and other exotic states. In this talk, I'll introduce a new animal in this zoo: the magnetic Weyl loop [1,2]. I'll describe the electronic dispersion of this object, discuss its π Berry phase topological invariant and argue that it should arise naturally in ferromagnets with mirror symmetry. Then I'll present our recent experiments using angleresolved photoemission spectroscopy (ARPES) on two such ferromagnets, Co_{2}MnGa and Co_{3}Sn_{2}S_{2}. We directly observe the characteristic cone dispersions of Weyl loops on the bulk Brillouin zone mirror planes. We can further measure the energy dispersions of the Weyl loops, as well as map out their trajectories in momentum space and even observe topological drumhead surface states. Weyl loops are typically expected to gap out under finite spinorbit coupling, leaving behind a concentrated Berry curvature distribution which can drive anomalous transport. I'll provide evidence that Weyl loops drive the giant anomalous Hall and Nernst effects in Co_{2}MnGa and Co_{3}Sn_{2}S_{2}. I'll also comment on the relationship of the Weyl loop with the Weyl points expected in Co_{3}Sn_{2}S_{2}. Broadly, I'll argue that Weyl loops naturally provide a powerful mechanism for generating large, robust topological response. Time permitting, I'll comment on related exotic phenomena in other topological magnets. 
Monday, March 15, 2021 3:36PM  3:48PM Live 
C51.00002: Modular Arithmetic with Nodal Lines: Drumhead Surface States in ZrSiTe Lukas Muechler, Andreas Topp, Raquel Queiroz, Maxim Krivenkov, Andrei Varykhalov, Jennifer Cano, Christian Ast, Leslie M Schoop We study the electronic structure of the nodal line semimetal ZrSiTe both experimentally and theoretically. 
Monday, March 15, 2021 3:48PM  4:00PM Live 
C51.00003: Complex Diraclike Electronic Structure in Atomic Site Ordered Rh_{3}In_{3.4}Ge_{3.6} Aikaterini Flessa Savvidou, Judith K. Clark, Hua Wang, Kaya Wei, Eun Sang Choi, Shirin Mozaffari, Xiaofeng Qian, Michael Shatruk, Luis Balicas We report the synthesis via an indium flux method of a novel singlecrystalline compound Rh_{3}In_{3.4}Ge_{3.6} that belongs to the cubic Ir_{3}Ge_{7} structure type. In Rh_{3}In_{3.4}Ge_{3.6}, the In and Ge atoms choose to preferentially occupy, respectively, the 12d and 16f sites of the Im3m space group, thus creating a colored version of the Ir_{3}Ge_{7} structure. Like the other compounds of the Ir_{3}Ge_{7} family, Rh_{3}In_{3.4}Ge_{3.6} shows potential as a thermoelectric displaying a relatively large power factor, PF ~ 2 mW/cmK^{2}, at a temperature T ~ 225 K albeit showing a modest figure of merit, ZT = 8x10^{4}, due to the lack of a finite band gap. Remarkably, band structure calculations reveal that this compound displays a complex Diraclike electronic structure relatively close to the Fermi level. The electronic structure is composed of several Dirac typeI and typeII nodes, and even Dirac typeIII nodes that result from the crossing between a flat band having nearly zero group velocity and a linearly dispersing band. 
Monday, March 15, 2021 4:00PM  4:12PM Live 
C51.00004: Magnus Hall effect Michal Papaj, Liang Fu A new type of a linear response Hall effect is predicted in timereversalinvariant systems with a builtin electric field at zero magnetic field [1]. The Hall response results from a quantum Magnus effect where a selfrotating Bloch electron wave packet moving under an electric field develops an anomalous velocity in the transverse direction. We show that in the ballistic limit the Magnus Hall conductance measures the distribution of the Berry curvature on the Fermi surface. 
Monday, March 15, 2021 4:12PM  4:24PM Live 
C51.00005: Topological phonons in oxide perovskites controlled by light Bo Peng, Yuchen Hu, Shuichi Murakami, Tiantian Zhang, Bartomeu Monserrat Oxide perovskites have received widespread attention ever since their discovery due to the multiple physical properties they exhibit, including ferroelectricity and superconductivity. We find that topological phonons – nodal rings, nodal lines, and Weyl points – are ubiquitous in oxide perovskites in terms of structures (tetragonal, orthorhombic, and rhombohedral), compounds (BaTiO_{3}, PbTiO_{3}, and SrTiO_{3}), and external conditions (photoexcitation, strain, and temperature). In addition, photoexcitation can be used to control the emergent topological states, for example driving the creation/annihilation of Weyl points and switching between nodal lines and nodal rings. Overall, we propose oxide perovskites as a versatile platform in which to study topological phonons and their manipulation with light [1]. Our discovery opens the door for studying topologyrelated phenomena in this family of materials, which may facilitate integration of topologydriven phenomena in technological devices based on oxide perovskites. 
Monday, March 15, 2021 4:24PM  4:36PM Live 
C51.00006: Fractional Corner Charge of Sodium Chloride Hoi Chun Po, Haruki Watanabe Recent developments in higherorder topological phases have elucidated on the relationship between nontrivial multipole moments in the bulk and the emergence of fractionally quantized charges at the boundary. Here, we put on the table a proposal of the threedimensional octupole insulator with fractionally quantized corner charges ±e/8: sodium chloride, commonly known as table salt. We further show that the electrostatic signature of the fractional charge is wellpreserved even when the ideal crystal is sprinkled with defects. Our results suggest that the observation of corner charges from quantized multipole moments in solids might be around the corner. 
Monday, March 15, 2021 4:36PM  4:48PM Live 
C51.00007: LaN Structural and Topological Phase Transitions Driven by Pressure and Temperature WeiChih Chen, ChiaMin Lin, Joseph Maciejko, ChengChien Chen Lanthanum monopnictides (LaX, X = N, P, As, Sb, Bi) have attracted much attention due to their unusual electronic and topological properties. In this study, we examine the stabilities of all rocksalt structured LaX, and show that LaN exhibits soft or negative phonon modes, and is thereby dynamically unstable. Using evolutionary algorithm, we predict a stable structure of LaN with lower symmetry and lower energy, compared to the facecentered cubic phase. The lowersymmetry phase possesses spontaneous polarization, and therefore LaN can be a potential ferroelectric material at low temperature. We also study how temperature and pressure change the structural and electronic properties of LaN, and demonstrate that topological phase transitions in this material can be driven by both effects. 
Monday, March 15, 2021 4:48PM  5:00PM Live 
C51.00008: Anisotropic optical response in quasionedimensional topological insulators Bi_{4}Br_{4} and Bi_{4}I_{4} Spenser Talkington, Fan Zhang Quasitwodimensional materials like graphite exhibit an anisotripic optical response that depends on the direction of polarization of the incident light. This anisotropy manifests as a relatively large inplane optical conductivity, and a relatively small outofplane optical conductivity. In a similar way, the recently predicted topological insulators Bi_{4}Br_{4} and Bi_{4}I_{4} (which can be both first and second order) are quasionedimensional; atomic chains with relatively large optical conductivity stack to form sheets with moderate transverse optical conductivity which pile into a threedimensional material with relatively small outofplane optical conductivity. By using tightbinding models derived from firstprinciples calculations, we provide quantitative predictions for the optical properties. Additionally, we report refractive indices, absorbance coefficients, and the optical properties of surface states and of finitelayer systems. 
Monday, March 15, 2021 5:00PM  5:12PM Live 
C51.00009: Epitaxial HfTe_{2}, ZrTe_{2} and typeIII Dirac fermions in Hf_{x}Zr_{x1}Te_{2} topological semimetal candidate Sotirios Fragkos, Polychronis Tsipas, Dimitra Tsoutsou, Evangelia Xenogiannopoulou, Roberto Sant, Carlos Alvarez, Gilles Renaud, Hanako Okuno, Yerassimos Panayiotatos, Athanasios Dimoulas Topological semimetals are hosts of interesting types of lowenergy quasiparticles such as typeI and typeII Dirac and Weyl fermions. Yet a typeIII^{1,2,3} emerges as a theoretical possibility exactly at the border between typeI and II, characterized by a linelike Fermi surface and a flat energy dispersion along one direction in the Brillouin Zone. A typeIII fermion could undergo an electronic (Lifshitz) phase transition^{1}. We theoretically predict that 1THfTe_{2} and 1TZrTe_{2} transition metal dichalcogenides are typeI and typeII Dirac semimetals, respectively. By alloying the two materials, a new Hf_{x}Zr_{x1}Te_{2} alloy with typeIII Dirac cone emerges at x=0.2. We also provide experimental evidence that by using MBE, single and few layers of HfTe_{2}^{4}, ZrTe_{2}^{5} and Hf_{0.2}Zr_{0.8}Te_{2} can be grown on InAs(111) substrates, and by using insitu ARPES, that the Dirac point lies at or very close to the E_{F}. 
Monday, March 15, 2021 5:12PM  5:24PM Live 
C51.00010: Fillingenforced Dirac nodal loops in the nonmagnetic systems and their evolutions under various perturbations Dexi Shao, Chen Fang Based on symmetry analysis, we propose that fillingenforced Dirac nodal loops (FEDLs) in nonmagnetic systems exist and only exist in only five space groups (SGs), namely, SG.57, SG.60, SG.61, SG.62 and SG.205. We explore all possible configurations of the FEDLs in these SGs, and classify them accordingly. Furthermore, we study the evolutions of the FEDLs under various types of symmetrybreaking perturbations, such as an applied strain or an external field. The results show that FEDL materials can serve as parent materials of both topological semimetals hosting nodal points/loops, and topological insulators/topological crystalline insulators. By means of firstprinciples calculations, almost all materials possessing FEDLs are predicted. 
Monday, March 15, 2021 5:24PM  5:36PM Live 
C51.00011: Highharmonic generation in Dirac metals: Application of the semiconductorBloch formalism to topological systems Jack Crewse, Patrick Groessing, Jan Wilhem, Ferdinand H Evers The generation of highorder harmonic radiation from the interaction of intense light waves with electrons in a solid has been a key observation coming from recent advancements in timeresolved spectroscopy. These highharmonics provide a window into subcycle dynamics of the system and can provide insights to material properties. The semiconductorBloch formalism has seen success in describing highharmonic generation (HHG) theoretically in semiconductor materials but topological effects are still not fully understood. Here we investigate these topological effects via a simulation of the semiconductorBloch equations. We compare the emission spectra of typical bulk and surface state Hamiltonians and find that the properties of the HHG are in good agreement with analytic considerations as well as recent experimental evidence. 
Monday, March 15, 2021 5:36PM  5:48PM Live 
C51.00012: Corner charge and bulk multipole moment in periodic systems Haruki Watanabe, Seishiro Ono Recently, various kind of insulators that feature electric charges localized to their hinges and corners have been proposed and studied. In this talk, we discuss a formula for the boundary charges in terms of the bulk quadrupole moment of the insulator. This is an analog of the "modern theory" formula for the surface charge density in terms of the bulk polarization. In two dimensional systems with nfold rotation symmetry (n = 3, 4, and 6), the quadrupole moment is quantized and is independent of the spread or shape of Wannier orbitals, depending only on the location of Wannier centers of filled bands. In this case, our formula predicts the fractional part of the quadrupole moment purely from the bulk property. The system can contain manybody interactions as long as the ground state is gapped and topologically trivial in the sense it is smoothly connected to a product state limit. In three dimensions, in general, even the fractional part of the corner charge is not fully predictable from the bulk perspective even in the presence of point group symmetry. 
Monday, March 15, 2021 5:48PM  6:00PM Live 
C51.00013: Synthesis and Characterization of the Kagome metal LuMn_{6}Sn_{6} Rui Xue, Brianna Musicó, Candice KinslerFedon, Nan Huang, Takahiro Matsuoka, David George Mandrus Kagome metals have attracted interest as they can host both nontrivial band topologies and flat bands, leading to the interplay of topology, magnetism, and strong correlations [1]. Recent studies of YMn_{6}Sn_{6} have shown that it has multiple nontrivial magnetic phases, and that one of them hosts a large topological Hall effect near room temperature [2]. Here we report the crystal growth, magnetization, and electrical transport properties of a related compound, LuMn_{6}Sn_{6}. LuMn_{6}Sn_{6} is an antiferromagnet with Néel temperature of 400 K. Magnetization and resonant ultrasound spectroscopy indicate a spin reorientation at 258 K. LuMn_{6}Sn_{6} also displays an anomalous Hall effect when temperature is under 250K. 
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