Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session C04: Dirac/Weyl Semimetals -- Nodal-line Semimetals and BeyondFocus
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Sponsoring Units: DMP Chair: Lu Li, University of Michigan Room: BCEC 107C |
Monday, March 4, 2019 2:30PM - 3:06PM |
C04.00001: Electronic structure of topological nodal semimetals Invited Speaker: Tay-Rong Chang Topological phases in condensed matter physics have attracted intense worldwide interest in the past decade because these phases lie outside the framework of Landau’s spontaneous symmetry breaking paradigm. The early period of the field explored topological insulators (TIs). Three-dimensional (3D) TIs are distinct from the conventional band insulators in that they feature a bulk energy gap driven by spin-orbit coupling effects, and gapless surface states protected by time-reversal symmetry. In the last few years, the focus has shifted from insulators to topological semimetals (TSMs). Three types of TSMs have been identified, namely, Dirac, Weyl, and nodal-line semimetals. Dirac and Weyl semimetals are 3D analogs of graphene in which the bulk band structure disperses linearly from the nodal point in all three momentum directions, while the nodal-line semimetals display 1D nodal loops in the 3D Brillouin zone. In addition to the graphene-type (type-I) TSMs, the novel type-II TSMs where the band dispersion strongly violates Lorentz symmetry have also been studied extensively. In this talk, we will review a range of materials predictions including type-II Dirac semimetals [1], tunable Weyl semimetals [2], and nodal-line semimetals [3-5] by using first-principles calculations, and discuss various exotic phenomena observed experimentally in these compounds. |
(Author Not Attending)
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C04.00002: Topological nodal line semimetals without drumhead surface states Qiu-Bo Cheng, Ching-Kai Chiu, Hongming Weng Topological nodal lines are protected by reflection symmetry or space-time inversion symmetry. The symmetries can quantize Berry phase in a proper basis and the π Berry phase stems from the presence of the protected topological lines. However, it has been shown that the only π Berry phase cannot lead to the presence of the stable drumhead surface states, which connect the bulk nodal lines. The other essential condition for the drumhead surface states is that no atoms should be located at the symmetry centers in the crystal structure. However, in condensed matter systems most of the topological nodal line semimetals fail this requirement and possess the drumhead surface states. In this talk, we show several concrete topological nodal line materials not possessing any drumhead surface states. The examples demonstrate that the bulk-boundary correspondence has very strict conditions for crystalline symmetries. |
Monday, March 4, 2019 3:18PM - 3:30PM |
C04.00003: ABSTRACT WITHDRAWN
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Monday, March 4, 2019 3:30PM - 3:42PM |
C04.00004: Nodal surface semimetals: Theory and materials realization Weikang Wu, Ying Liu, Si Li, Chengyong Zhong, Zhi-Ming Yu, Xian-Lei Sheng, Yuxin Zhao, Shengyuan Yang We theoretically study the three-dimensional topological semimetals with nodal surfaces protected by crystalline symmetries. Different from the well-known nodal-point and nodal-line semimetals, in these materials, the conduction and valence bands cross on closed nodal surfaces in the Brillouin zone. We propose different classes of nodal surfaces, both in the absence and in the presence of spin-orbit coupling (SOC). In the absence of SOC, a class of nodal surfaces can be protected by spacetime inversion symmetry and sublattice symmetry, and characterized by a Z_2 index, while another class of nodal surfaces are guaranteed by a combination of nonsymmorphic two-fold screw-rotational symmetry and time-reversal symmetry. We show that the inclusion of SOC will destroy the former class of nodal surfaces but may preserve the latter provided that the inversion symmetry is broken. We further generalize the result to magnetically ordered systems and show that protected nodal surfaces can also exist in magnetic materials without and with SOC, given that certain magnetic group symmetry requirements are satisfied. Several concrete nodal-surface material examples are predicted via the first-principles calculations. The possibility of multi-nodal-surface materials are discussed. |
Monday, March 4, 2019 3:42PM - 3:54PM |
C04.00005: Possible pressure-induced topological quantum phase transition in the nodal line semimetal ZrSiS Derrick VanGennep, Tiffany Paul, Chase W. Yerger, Samuel T Weir, Yogesh Kumar Vohra, James J. Hamlin ZrSiS has recently gained attention due to its unusual electronic properties: nearly perfect electron-hole compensation, large, anisotropic magneto-resistance, multiple Dirac nodes near the Fermi level, and an extremely large range of linear dispersion of up to ∼2 eV. We have carried out a series of high pressure electrical resistivity measurements on single crystals of ZrSiS. Shubnikov-de Haas measurements show two distinct oscillation frequencies. For the smaller orbit, we observe that the phase of the oscillations changes by ∼0.5 between ∼0.16-0.5 GPa. This change in phase is accompanied by an abrupt decrease of the cross-sectional area of this Fermi surface. We attribute this change in phase to a possible topological quantum phase transition. The phase of the larger orbit exhibits a Berry phase of π and remains roughly constant up to ∼2.3 GPa. Resistivity measurements to higher pressures show no evidence for pressure-induced superconductivity to at least 20 GPa. |
Monday, March 4, 2019 3:54PM - 4:06PM |
C04.00006: Observation of Dirac nodal lines in rutile oxide IrO2 Jocienne Nelson, Luca Moreschini, Jason Kawasaki, Eli Rotenberg, Darrell G. Schlom, Kyle M Shen The complex oxide IrO2 is predicted to be a dirac nodal line semimetal protected by nonsymmorphic symmetry. Some of its intriguing physical properties include a large spin hall effect, significant magnetoresistance, and a hall effect where the sign of the carrier may be controlled by an external magnetic field. Previously there has been no direct evidence of the topological features of the band structure. This is a non trivial prediction because the relevant orbitals Ir 5d and O 2p are known to often host strong electron correlations and spin orbit coupling. Using a combination of reactive oxide molecular beam epitaxy and Angle-Resolved Photoemission Spectroscopy we show that IrO2 is a Dirac nodal line semimetal in agreement with the predictions of Sun et. al. This is the first experimental observation of nodal lines protected by nonsymmorphic symmetry. Significantly the crossing points are protected even in the case of large spin orbit coupling characteristic of iridates. Furthermore they cross the fermi level making IrO2 an ideal system to study the low energy properties of Dirac nodal line materials. |
Monday, March 4, 2019 4:06PM - 4:18PM |
C04.00007: Multiple topologically non-trivial bands in non-centrosymmetric YSn2 Yanglin Zhu, Tiantian Zhang, Jin Hu, Jamin Kidd, David E Graf, Xin Gui, Weiwei Xie, Mengze Zhu, Xianglin Ke, Huibo Cao, Zhong Fang, Hongming Weng, Zhiqiang Mao We show that, in non-centrosymmetric compound YSn2, the slightly distorted square lattice of Sn generates multiple topologically non-trivial bands, one of which likely hosts nodal line and tunable Weyl semimetal state induced by the Rashba spin-orbit coupling (SOC) and proper external magnetic field. The quasiparticles described as relativistic fermions from these bands are manifested by nearly zero mass and non-trivial Berry phases probed in de Haas–van Alphen (dHvA) oscillations. The dHvA study also reveals YSn2 has a complicated Fermi surface (FS), consisting of several 3D and one 2D pockets. Our first principle calculations show the point-like 3D pocket at Y point on the Brillouin zone boundary hosts the possible Weyl state. Our findings establish YSn2 as a new interesting platform for observing novel topological phases and studying their underlying physics. |
Monday, March 4, 2019 4:18PM - 4:30PM |
C04.00008: Unraveling the Topological Nature of ZrSiTe using Scanning Tunneling Microscopy Brandon Stuart, Jisun Kim, Seokhwan Choi, Leslie Schoop, Douglas Bonn, Sarah Burke 3D topological Dirac semimetals have been a topic of immense study since their experimental realization in 2014. The family of materials ZrSiX (X = S, Se, Te) are topological nodal-line Dirac semimetals, which exhibit a nodal-line band crossing encircling the Γ-point, and two Dirac cones at the X-point protected by a non-symmorphic symmetry. From this family, ZrSiS was the first material to be experimentally studied, and was shown to have two Dirac cones at ± 0.5 V [1]. It was predicted that due to the size difference, substituting S with Te would result in a uniaxial strain along the c-axis, shifting the upper Dirac cone towards the Fermi level [2]. |
Monday, March 4, 2019 4:30PM - 4:42PM |
C04.00009: Topological Phases of Zintl Compounds Ba3X2Y4 (X=Zn, Cd; Y=As, Sb) Tan Zhang, Si-Min Nie, Hongming Weng, Zhong Fang Based on first-principles calculations and effective k.p model analysis, we have studied three Zintl compounds, including Ba3Zn2As4, Ba3Cd2As4 and Ba3Cd2Sb4, to reveal their electronic band topology under different conditions, such as including and not including spin-orbit coupling (SOC) and different external strains. We find that these family materials are close to the boundaries of several topological phases. When SOC is ignored, there are topological nodal lines around Fermi energy due to band inversion between valence and conduction bands at one or two time-reversal invariant momenta, which can be tuned by proper strain. When SOC is further included, these nodal lines are gaped and these materials become weak or strong topological insulators (TIs). They are suitable for studying topological phase transition. |
Monday, March 4, 2019 4:42PM - 4:54PM |
C04.00010: Topological hourglass Dirac semimetal in Ag2BiO3 Bahadur Singh, Barun Ghosh, Chenliang Su, Hsin Lin, Amit Agarwal, Arun Bansil Materials with tunable charge and lattice degrees of freedom provide excellent platforms for investigating multiple phases that can be controlled via external stimuli. We show how the charge-ordered ferroelectric oxide Ag2BiO3, which has been realized experimentally, presents a unique exemplar of a metal-insulator transition under an external electric field. Our first-principles calculations combined with a symmetry analysis, reveal the presence of a nearly ideal hourglass-Dirac-semimetal state in the nonpolar structure of Ag2BiO3. The low-energy band structure consists of two hourglass-like nodal lines located on two mutually orthogonal glide-mirror planes in the absence of spin-orbit coupling (SOC) effects. These lines cross at a common point and form an interlinked chain-like structure, which extends beyond the first Brillouin zone. Inclusion of the SOC opens a small gap in the nodal lines and results in two symmetry-enforced hourglass-like Dirac points on the C2y screw rotation axis. Our results indicate that Ag2BiO3 will provide an ideal platform for exploring ferroelectric-semiconductor to Dirac-semimetal transition by the application of an external electric field. |
Monday, March 4, 2019 4:54PM - 5:06PM |
C04.00011: ABSTRACT WITHDRAWN
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Monday, March 4, 2019 5:06PM - 5:18PM |
C04.00012: Exceptional lines in topological semimetals and superconductors Alexander Zyuzin, Kristof Moors, Rakesh P. Tiwari, Thomas L Schmidt We consider the impact of the disorder on the spectrum of three-dimensional Weyl and nodal-line semimetals. We show that the combination of disorder and a tilted spectrum leads to a non-Hermitian self-energy contribution that can split a Weyl node and nodal line into a single nodal ring and pair of exceptional lines, respectively. In nodal-line semimetals, these exceptional lines form the boundary of an open and orientable bulk Fermi ribbon in reciprocal space on which the energy gap vanishes. We find that the surface of such a disorder-induced bulk Fermi ribbon in general lies orthogonal to the direction of the tilt, which can be exploited to realize a bulk Fermi ribbon with nontrivial topology by means of a tilt vector that twists along a nodal loop. |
Monday, March 4, 2019 5:18PM - 5:30PM |
C04.00013: Two-dimensional nonsymmorphic Dirac semimetal in chemically modified group-VA Monolayer with Black Phosphorene structure Kyung-Hwan Jin, Huaqing Huang, Zhengfei Wang, Feng Liu The symmetry-protected 2D Dirac semimetals have attracted intense interest due to their intriguing properties. Here, we investigate the 2D nonsymmorphic Dirac semimetal state by chemically modified group-VA 2D puckered structure. Based on first-principles calculations, we demonstrated 2D Dirac fermionic states not only exist, but in fact occur with two different types: one is a Dirac nodal line (DNL) structure in one-side modified phosphorene structure with negligible SOC; the other is a hourglass fermion protected by nonsymmorphic symmetry for the heavy elements with strong SOC. In the absence of SOC, the DNL exhibits anisotropic behavior and unique electronic properties, such as constant density of states. The Dirac node is protected from gap opening by nonsymmorphic symmetry. With the inclusion of SOC, the DNL states are split to form the hourglass dispersion due to broken inversion symmetry and Rashba SOC interaction. Moreover, around certain high symmetry points in the Brillouin zone, the spin orientation is enforced to be along a specific direction. The essential physics of 2D nonsymmorphic Dirac states is further analyzed by effective tight-binding models and group theory. |
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