Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session V10: Topological Semimetals Beyond Weyl And DiracFocus

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Sponsoring Units: DMP Chair: Barry Bradlyn, Princeton University Room: LACC 301B 
Thursday, March 8, 2018 2:30PM  3:06PM 
V10.00001: From local symmetry to band structure topology Invited Speaker: Jennifer Cano Weyl semimetals and their symmetryprotected generalizations are sought after for their exotic Fermi arcs and unconventional transport properties. Symmetry can force these degeneracies to lie on special points or lines. However, it can be a challenge to find materials that realize these band crossings near the Fermi level. Using the theory of topological quantum chemistry, we describe how the local symmetry properties of a chemical compound (i.e., its orbitals and crystal structure) determine which symmetry representations appear at each point in the Brillouin zone and, ultimately, the topology of the band structure. This theory unites symmetryenforced semimetals and topological crystalline insulators. We describe algorithms to search for materials that realize these phases. 
Thursday, March 8, 2018 3:06PM  3:18PM 
V10.00002: Hourglass Dirac Chain metal in Rhenium Dioxide ShanShan Wang, Shengyuan Yang Nonsymmorphic symmetries, which involve fractional lattice translations in crystalline materials, can generate exotic types of fermionic excitations that are robust against spinorbit coupling. Here we report on a new topological phase arising from nonsymmorphic symmetries—the hourglass Dirac chain metal, and demonstrate its realization in threedimensional rhenium 
Thursday, March 8, 2018 3:18PM  3:30PM 
V10.00003: Topologically Charged Nodal Surface Meng Xiao, Shanhui Fan We report the existence of topologically charged nodal surface – a band degeneracy on a twodimensional surface in momentum space that carries nonzero total Berry flux. We develop a Hamiltonian for such charged nodal surface, and show that such a Hamiltonian can be implemented in a tightbinding model as well as in an acoustic metamaterial. We also identify a topological phase transition, through which the charges of the nodal surface changes by absorbing or emitting an integer number of Weyl points. Our result indicates that in the band theory, topologically charged objects are not restrict to zero dimension as in a Weyl point, and thus pointing to previously unexplored opportunities for the design of topological materials. 
Thursday, March 8, 2018 3:30PM  3:42PM 
V10.00004: Tunable Nodal Nets in NonSymmorphic Compounds DingFu Shao, LingLing Tao, Xiaoqian Dang, Evgeny Tsymbal Recently, a number of new semimetals with nontrivial energy bands crossings have been predicted. Such bands crossings are considered as new quasiparticles with no counterparts in highenergy physics and might lead to novel physical properties. Particular attention has been attracted by the nodal line semimetals; in some of them the exotic nodal net/chain patterns are found, formed from multiple nodal lines. In this work, we predict the emergence of stable nodal nets in nonsymmorphic compounds under spinorbit coupling, supported by two or more nodal lines protected by different symmetryinvariant transformations. Using firstprinciples density functional calculations, we present a few examples of real materials with multiple nodal lines. We show that strain and pressure can control the size and shape of the nodal lines, and even produce additional nodal lines. Such tunable nodal lines result in novel nodal nets with interesting characteristics, for example, a nodal net with a quantized size. Due to the tunable nature of the nodal nets, such nonsymmorphic compounds can be a viable platform to explore potential new quasiparticles and their physical properties. 
Thursday, March 8, 2018 3:42PM  3:54PM 
V10.00005: Nearly triple point topological phase in halfmetallic GdN Jinwoong Kim, Heung Sik Kim, David Vanderbilt Recent developments in topological semimetals open a way to realize relativistic dispersions in condensed matter systems. One recently studied topological features is the “triple point” where three bands become degenerate. In contrast to Weyl and Dirac nodes, triple points, which are protected by a rotational symmetry, have nodal lines attached, so that a characterization in terms of a chirality is not possible. Previous studies of triple points considered nonmagnetic systems, although an artificial Zeeman splitting was used to probe the topological nature. Here instead we treat a ferromagnetic material, halfmetallic GdN, where the splitting of the triple points comes from the spinorbit coupling. The size of the splitting ranges from 15 to 150 meV depending on the magnetization orientation, enabling a transition between a Weylpoint phase and a “nearly triple point” phase which exhibits very similar surface spectra and transport properties compared to a true triplepoint system. The rich topological surface states, manipulable via the orientation of the magnetization, make halfmetallic GdN a promising platform for future investigations and applications. 
Thursday, March 8, 2018 3:54PM  4:06PM 
V10.00006: Saddlelike topological surface states on the TT'X family of compounds (T, T' = Transition metal, X= Si, Ge) Bahadur Singh, Xiaoting Zhou, Hsin Lin, Arun Bansil Based on firstprinciples calculations and an effective model analysis, we identify the presence of topological nodalline semimetal states in the low crystalline symmetric TT'X family of compounds (T, T' = transition metal, X= Si, or Ge) in the absence of spinorbit coupling (SOC). Taking ZrPtGe as an exemplar system, we show that owing to lowered lattice symmetry this material harbors a single nodal line on the k_{y}=0 plane with large energy dispersion and a unique drumhead surface state with a saddlelike energy dispersion. When the SOC is included, the nodal line gaps out and the system transitions to a strong topological insulator state with Z_{2}=(1;000). The topological surface state evolves from the drumhead surface state via the sharing of its saddlelike energy dispersion within the bulk energy gap. These features differ remarkably from those of the currently known topological surface states in topological insulators such as Bi_{2}Se_{3} with Diracconelike energy dispersions. 
Thursday, March 8, 2018 4:06PM  4:18PM 
V10.00007: Unconventional Chiral Fermions and Large Topological Fermi Arcs in RhSi Hsin Lin, Guoqing Chang, Suyang Xu, Benjamin Wieder, Daniel Sanchez, ShinMing Huang, Ilya Belopolski, TayRong Chang, Songtian Sonia Zhang, Arun Bansil, Zahid Hasan The theoretical proposal of chiral fermions in topological semimetals has led to a significant effort towards their experimental realization. Recent theoretical works have proposed a number of unconventional chiral fermions beyond the standard model which are protected by unique combinations of topology and crystalline symmetries. However, materials candidates for experimentally probing the transport and response signatures of these unconventional fermions have thus far remained elusive. Using firstprinciples electronic structure calculations, we propose the RhSi family in space group No. 198 as the ideal platform for the experimental examination of unconventional chiral fermions. We find that RhSi is a fillingenforced semimetal that features near its Fermi surface a chiral double sixfolddegenerate spin1 Weyl node at R and a previously uncharacterized fourfolddegenerate chiral fermion at Γ. Each unconventional fermion displays C=4 at the Fermi level. We also show that RhSi displays the largest possible momentum separation of compensative chiral fermions, the largest proposed topologically nontrivial energy window, and the longest possible Fermi arcs on its surface. We conclude by proposing signatures of an exotic bulk photogalvanic response in RhSi. 
Thursday, March 8, 2018 4:18PM  4:30PM 
V10.00008: Classification of accidental band crossings and emergent semimetals in twodimensional noncentrosymmetric systems Sungjoon Park, BohmJung Yang Through a group theoretical analysis, we provide the complete list of nodal semimetals that can be created by accidental band crossing in twodimensional noncentrosymmetric systems with timereversal symmetry. Although a direct transition between two insulators is generally predicted in such a system, various stable semimetal phases with point or line nodes can also arise from accidental band crossing in the presence of additional crystalline symmetries. By analyzing the 49 noncentrosymmetric layer groups, we find that there are only three types of symmetries that can stabilize the semimetallic phase. First, twofold rotation about the zaxis (normal to the system) combined with time reversal always stabilizes twodimensional Weyl nodes. Second, twofold rotation (mirror) symmetry with the rotation (normal) axis lying in the twodimensional plane may stabilize Weyl nodes. Finally, mirror symmetry about the plane embracing the whole system may stabilize nodal lines. Our classification table can be used to help search for semimetals in two dimensional systems. 
Thursday, March 8, 2018 4:30PM  4:42PM 
V10.00009: Quadrupole Quantized Hinge Arcs in Crystalline Dirac Semimetals Benjamin Wieder, Jennifer Cano, Barry Bradlyn, Zhijun Wang, Xi Dai, Leslie Schoop, Andrei Bernevig Topological semimetals, from graphene in 2D to Weyl semimetals in 3D, have been shown to display topological surface states on boundaries with one fewer dimension than that of the bulk. In this talk, we present the discovery of the first 3D semimetal with topological surface states in two fewer dimensions than its bulk, such that it displays arcs on its 1D hinges connecting the projections of its bulk 3D Dirac points. Using the recently developed multipole generalization of topological electric moments, we use nested Wilson loops to show that 2D slices of this semimetal can be modeled using hybridized spinful s and d_{x2y2} orbitals in a magnetic layer group, and are topologically equivalent to recently presented spinless, fluxbased models of insulators with quantized quadrupole moments. We show that these "hingearc" semimetals can be realized with or without timereversal symmetry, and present ab initio calculations demonstrating the presence of topological hinge arcs in previously synthesized crystals. 
Thursday, March 8, 2018 4:42PM  4:54PM 
V10.00010: Topological Hopf and Chain Link Semimetal States and Their Application to Co_{2}MnGa Daniel Multer, Guoqing Chang, Suyang Xu, Xiaoting Zhou, ShinMing Huang, Bahadur Singh, Baokai Wang, Ilya Belopolski, Jiaxin Yin, Songtian Sonia Zhang, Arun Bansil, Hsin Lin, Zahid Hasan Topological semimetals can be classified by the connectivity and dimensionality of the band crossing in momentum space. The band crossings of a Dirac, Weyl, or an unconventional fermion semimetal are 0D points, whereas the band crossings of a nodalline semimetal are 1D closed loops. Here we propose that the presence of perpendicular crystalline mirror planes can protect 3D band crossings characterized by nontrivial links such as a Hopf link or a coupledchain, giving rise to a variety of new types of topological semimetals. We show that the nontrivial winding number protects topological surface states distinct from those in previously known topological semimetals. We also show that these nontrivial links can be engineered by tuning the mirror eigenvalues associated with the perpendicular mirror planes. Using firstprinciples band structure calculations, we predict the ferromagnetic full Heusler compound Co_{2}MnGa as a candidate. Both Hopf link and chainlike bulk band crossings and unconventional topological surface states are identified. 
Thursday, March 8, 2018 4:54PM  5:06PM 
V10.00011: Classification of nodal topological band theories with combined twofold antiunitary spatial symmetries Meng Hua, Syed Raza, ChingKai Chiu, Jeffrey Teo A band theory that describes an electronic system can be invariant under a combined symmetry composed of two distinct operations. For example, a system can carry a magnetic symmetry, which is a combination of timereversal and a spatial symmetry, such as an antiferromagnetic translation. We consider mirror, twofold rotation, and parity symmetry combined with one of the three nonspatial symmetries (timereversal, particlehole and chiral symmetries) as composite symmetries. In the presence of a composite symmetry, topological phases can emerge, while the two symmetries constituting the composite symmetry are not necessarily preserved individually. In this work, we systemically classify topological (semi)metals and nodal superconductors with different composite symmetries in any dimensions. We also discuss model realization, topologicalindex characterization as well as potential material applications. 
Thursday, March 8, 2018 5:06PM  5:18PM 
V10.00012: Topological Quadrupolar Semimetals Mao Lin, Taylor Hughes In this work we predict several new types of topological semimetals that exhibit a bulk quadrupole moment. These semimetals are modeled with a 3D extension of the 2D quadrupole topological insulator. One type of semimetal has bulk nodes and gapped, topological surfaces. A second type, which we may call a higher order topological semimetal has a gapped bulk, but harbors a Dirac semimetal with an even number of nodes on one or more surfaces. The final type has a gapped bulk, but harbors half of a Dirac semimetal on multiple surfaces. Each of these semimetals gives rise to midgap hinge states and hinge charge, as well as surface polarization, which are all consequences of a bulk quadrupole moment. We show how the bulk quadrupole moments of these systems can be calculated from the momentumlocations of bulk or surface nodes in the energy spectrum. Finally, we illustrate that in some cases it is useful to examine nodes in the Wannier bands, instead of the energy bands, to extract the bulk quadrupole moment. 
Thursday, March 8, 2018 5:18PM  5:30PM 
V10.00013: Coexistence of Weyl Points and Topological Nodal Lines in Ternary Transition Metal Telluride TaIrTe4 Xiaoqing Zhou, Qihang Liu, QuanSheng Wu, Tom Nummy, Haoxiang Li, Justin Griffith, Stephen Parham, Justin Waugh, Eve Emmanouilidou, Bing Shen, Oleg Yazyev, Ni Ni, Daniel Dessau We report a combined theoretical and experimental study on TaIrTe4, a potential candidate of the minimal model of typeII Weyl semimetals. Unexpectedly, an intriguing node structure with twelve Weyl points and a pair of nodal lines protected by mirror symmetry was found by firstprinciple calculations, with its complex signatures such as the topologically nontrivial band crossings and topologically trivial Fermi arcs crossvalidated by angleresolved photoemission spectroscopy. Through external strain, the number of Weyl points can be reduced to the theoretical minimum of four, and the appearance of the nodal lines can be switched between different mirror planes in momentum space. The coexistence of tunable Weyl points and nodal lines establishes ternary transitionmetal tellurides as a unique test ground for topological state characterization and engineering. 
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