Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
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 semi-metals and their symmetry-protected 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 symmetry-enforced semi-metals 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 Shan-Shan Wang, Shengyuan Yang Nonsymmorphic symmetries, which involve fractional lattice translations in crystalline materials, can generate exotic types of fermionic excitations that are robust against spin-orbit coupling. Here we report on a new topological phase arising from nonsymmorphic symmetries—the hourglass Dirac chain metal, and demonstrate its realization in three-dimensional 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 two-dimensional surface in momentum space that carries non-zero total Berry flux. We develop a Hamiltonian for such charged nodal surface, and show that such a Hamiltonian can be implemented in a tight-binding model as well as in an acoustic meta-material. 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 Non-Symmorphic Compounds Ding-Fu Shao, Ling-Ling Tao, Xiaoqian Dang, Evgeny Tsymbal Recently, a number of new semimetals with non-trivial energy bands crossings have been predicted. Such bands crossings are considered as new quasiparticles with no counterparts in high-energy 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 non-symmorphic compounds under spin-orbit coupling, supported by two or more nodal lines protected by different symmetry-invariant transformations. Using first-principles 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 non-symmorphic 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 half-metallic 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, half-metallic GdN, where the splitting of the triple points comes from the spin-orbit coupling. The size of the splitting ranges from 15 to 150 meV depending on the magnetization orientation, enabling a transition between a Weyl-point phase and a “nearly triple point” phase which exhibits very similar surface spectra and transport properties compared to a true triple-point system. The rich topological surface states, manipulable via the orientation of the magnetization, make half-metallic GdN a promising platform for future investigations and applications. |
Thursday, March 8, 2018 3:54PM - 4:06PM |
V10.00006: Saddle-like 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 first-principles calculations and an effective model analysis, we identify the presence of topological nodal-line semimetal states in the low crystalline symmetric TT'X family of compounds (T, T' = transition metal, X= Si, or Ge) in the absence of spin-orbit 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 ky=0 plane with large energy dispersion and a unique drumhead surface state with a saddle-like energy dispersion. When the SOC is included, the nodal line gaps out and the system transitions to a strong topological insulator state with Z2=(1;000). The topological surface state evolves from the drumhead surface state via the sharing of its saddle-like 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 Bi2Se3 with Dirac-cone-like 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, Shin-Ming Huang, Ilya Belopolski, Tay-Rong 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 first-principles 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 filling-enforced semimetal that features near its Fermi surface a chiral double sixfold-degenerate spin-1 Weyl node at R and a previously uncharacterized fourfold-degenerate 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 two-dimensional noncentrosymmetric systems Sungjoon Park, Bohm-Jung Yang Through a group theoretical analysis, we provide the complete list of nodal semimetals that can be created by accidental band crossing in two-dimensional noncentrosymmetric systems with time-reversal 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, two-fold rotation about the z-axis (normal to the system) combined with time reversal always stabilizes two-dimensional Weyl nodes. Second, two-fold rotation (mirror) symmetry with the rotation (normal) axis lying in the two-dimensional 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 dx2-y2 orbitals in a magnetic layer group, and are topologically equivalent to recently presented spinless, flux-based models of insulators with quantized quadrupole moments. We show that these "hinge-arc" semimetals can be realized with or without time-reversal 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 Co2MnGa Daniel Multer, Guoqing Chang, Suyang Xu, Xiaoting Zhou, Shin-Ming 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 nodal-line 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 coupled-chain, 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 first-principles band structure calculations, we predict the ferromagnetic full Heusler compound Co2MnGa as a candidate. Both Hopf link and chain-like 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, Ching-Kai 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 time-reversal and a spatial symmetry, such as an antiferromagnetic translation. We consider mirror, twofold rotation, and parity symmetry combined with one of the three non-spatial symmetries (time-reversal, particle-hole 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, topological-index 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 mid-gap 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 momentum-locations 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 type-II Weyl semimetals. Unexpectedly, an intriguing node structure with twelve Weyl points and a pair of nodal lines protected by mirror symmetry was found by first-principle calculations, with its complex signatures such as the topologically non-trivial band crossings and topologically trivial Fermi arcs cross-validated by angle-resolved 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 transition-metal tellurides as a unique test ground for topological state characterization and engineering. |
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