Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session S10: New Phenomena In Dirac and Other Topological SemimetalsFocus

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Sponsoring Units: DMP Chair: Zhijun Wang, Princeton University Room: LACC 301B 
Thursday, March 8, 2018 11:15AM  11:51AM 
S10.00001: Fermi arc mediated entropy transport in topological semimetals Invited Speaker: Nandini Trivedi Topological Weyl semimetals can be viewed as three –dimensional analogs of graphene but richer. They have three main characteristics: (a) monopoles of Berry curvature in momentum space at nodal points with opposite chirality; (b) linearly dispersing fermions emanating from these nodal points; (c) Fermi arcs that terminate on the projections of Weyl nodes on the two surfaces. I will discuss how Fermi arcs on the surface couple with the bulk to create a channel for entropy transport driven by circulating electronic currents in an externally applied magnetic field but with no net charge transport. This work provides the first proposed signature of Fermi arcmediated thermal transport and sets the stage for utilizing and manipulating the topological Fermi arcs in experimental thermal applications. 
Thursday, March 8, 2018 11:51AM  12:03PM 
S10.00002: Spin Pumping and Inverse Spin Hall Effect in Topological Dirac Semimetals Takahiro Misawa, Kentaro Nomura We study the timedevelopment processes of spin and charge transport phenomena in a topological Dirac semimetal attached to a ferromagnetic insulator with a precessing magnetization. Topological Dirac semimetals manifest a large inverse spin Hall effect when a spin current is pumped from the attached ferromagnetic insulator, compared to conventional normal metals. While the pumped spin current is affected by disorder, the induced charge current hardly depends on the disorder strength. This result indicates that the robustness originates to topological properties of the topological Dirac semimetals, such as the helical Fermi arc states at the interface. 
Thursday, March 8, 2018 12:03PM  12:15PM 
S10.00003: Timeevolution, dynamics and control of edge states in laserdriven graphene nanoribbons Matteo Puviani, Franca Manghi, Andrea Bertoni An intense laser field in the highfrequency regime drives carriers in graphene nanoribbons out of equilibrium, creating topologically protected edge states. Based on a solution of the Floquet Hamiltonian we have studied these states in different regimes of intensity and polarization, showing the emergence of timedependent edge states responsible for charge oscillations across the ribbon. Furthermore, they exhibit a robust dynamics also in the presence of localized atomic vacancies and we reveal how it is possible to control them by a modulated electrostatic potential in a way similar to Hall edge states, making them promising candidates for flying qubits architectures [1]. 
Thursday, March 8, 2018 12:15PM  12:27PM 
S10.00004: Magnifying the Landau level transition via resonant coupling in topological nodeline semimetal Xiang Yuan, Zhongbo Yan, Chaoyu Song, Cheng Zhang, Awadhesh Narayan, Stefano Sanvito, Hugen Yan, Faxian Xiu Since the discovery of Dirac and Weyl semimetals, topological properties in semimetals have stimulated unprecedented research interests as the extension of topological insulators. Among them, nodeline semimetals remain largely untapped in experiments, in spite of the numerous progresses made in theory. The difficult lies in the fact that the nodeline semimetals lack a distinct signature for photoemission such as Fermi arcs, and typically show very high carrier density and complex band structure, which hinders the detection of Landau levels. Here we report a new way to visualize the Landau level transition in nodeline semimetal InBi by utilizing the strong resonant coupling with the gap from another band in infrared spectroscopy. Combining with the density functional calculations and model calculations, we find that this dip corresponds to the gap induced by spinorbit coupling in another unprotected Dirac band. Interestingly, the observed nodal lines are found to be close to the Fermi level, but cannot be clearly resolved through conventional transport or photoemission experiments. Our result demonstrates InBi as a promising platform for studying nodeline fermions and provides an effective experimental approach to magnify the Landau level transition via resonant coupling. 
Thursday, March 8, 2018 12:27PM  12:39PM 
S10.00005: Coherent helix vacancy phonon and its ultrafast dynamics waning in topological Dirac semimetal Cd_{3}As_{2} Fei Sun, Qiong Wu, Yanling Wu, Hui Zhao, changjiang yi, Yichao Tian, Hongwen Liu, Youguo Shi, Hong Ding, Xi Dai, Pierre Richard, Jimin Zhao We report an ultrafast lattice dynamics investigation of the topological Dirac semimetal Cd_{3}As_{2}. A coherent phonon beating among three evenly spaced A_{1}_{g} optical phonon modes (of frequencies 1.80, 1.96, and 2.11 THz, respectively) is unambiguously observed. The two side modes originate from the counter helixes composing Cd vacancies. Significantly, such helix vacancyinduced phonon (HVP) modes experience prominent extra waning in their ultrafast dynamics as temperature increases, which is immune to the central mode. Above 200 K, the HVP becomes inactive, which may potentially affect the topological properties. Our results in the lattice degree of freedom suggest the indispensable role of temperature in considering topological properties of such quantum materials. 
Thursday, March 8, 2018 12:39PM  12:51PM 
S10.00006: Coherent phonon oscillations and the lowenergy band structure of the Dirac semimetal SrMnSb_{2} Madison Masten, Christopher Weber, Thomas Ogloza, Jinyu Liu, Zhiqiang Mao, Dennis Klug, Adebayo Adeleke, Yansun Yao In the threedimensional Dirac semimetal Sr_{1y}Mn_{1z}Sb_{2}, Dirac fermions derived from the Sb plane coexist with breaking of timereversal symmetry by the Mn plane. A Peierls distortion causes the Sb squarenet to form zigzag chains. Quantum oscillations show the persistence of Dirac fermions, despite the distortion, while calculations for stoichiometric SrMnSb_{2} predict that the distortion should gap the Dirac cones. We have previously shown that photoexcitation of SrMnSb_{2} with a short optical pulse launches the coherent oscillation of several phonon modes. This suggests that, by using phonons to coherently control the atoms’ positions, one could open or close a gap at a Dirac point on a subpicosecond timescale. Here we combine firstprinciples calculations of the electronic structure and of the phonon spectrum with timeresolved and Raman spectroscopy to identify the atomic displacements of several of the coherentlycontrolled phonon modes. We show that some of these modes oscillate in a way that periodically strengthens and relaxes the Peierls distortion. We calculate the effect of largeamplitude motions in these modes on the electronic structure. For one particular A_{g} mode, a largeamplitude motion is predicted to close the gap by restoring the Dirac cone near the Ypoint. 
Thursday, March 8, 2018 12:51PM  1:03PM 
S10.00007: Magnetic response in topological Dirac semimetals Yuya Ominato, Shuta Tatsumi, Kentaro Nomura In recent condensed matter physics, topological semimetals attract many researchers. Topological Dirac semimetals are one of them and realized in Na_{3}Bi and Cd_{3}As_{2}. The topological Dirac semimetals have pairs of band touching points called Dirac points. We theoretically study the spinorbital susceptibility of the topological Dirac semimetal. In strong spinorbit coupling systems, orbital magnetization is induced by Zeeman coupling. We find the spinorbital susceptibility of the topological Dirac semimetal can be comparable to the conventional spin magnetic susceptibility. 
Thursday, March 8, 2018 1:03PM  1:15PM 
S10.00008: Mirror anomaly in Dirac semimetals Anton Burkov We demonstrate that, apart from the chiral anomaly, Dirac semimetals possess another quantum anomaly, which we call mirror anomaly, and which manifests in a singular response of the Dirac semimetal to an applied magnetic field. Namely, the anomalous Hall conductivity exhibits stepfunction singularities when the field is rotated. We show that this behavior may be understood as quantum mechanical violation of the mirror symmetry, which emerges near any Dirac point at a time reversal invariant momentum. 
Thursday, March 8, 2018 1:15PM  1:27PM 
S10.00009: Surface states in Dirac metals and topological crystalline insulators Grigory Bednik We reconsider the problem of surface states spectrum in type Ι Dirac metal. We find that its surface states, despite being gapped, always form branches terminating at Dirac points. Furthermore, we consider evolution of the surface states in the case, when rotational symmetry is broken, and as a result, Dirac points are gapped. We find, that in this case, special role is played by mirror symmetry relative to the plane connecting the Dirac points. When it is present, the resulting gapped state is a topological crystalline insulator, which surface spectrum can contain either one or three Dirac points, two of which are protected solely by the mirror symmetry. The Dirac metal can be viewed as a topological phase transition between two phases with different mirror Chern numbers. 
Thursday, March 8, 2018 1:27PM  1:39PM 
S10.00010: Ferroelectric Phase Transition in LiOsO_{3}: Transformation from a Multitype Dirac Semimetal into a Crossednodalring Semimetal Xiaoting Zhou, Wing Chi Yu, Fengchuan Chuang, Shengyuan Yang, Hsin Lin, Arun Bansil LiOsO_{3} is a ferroelectric metal. Upon lowering temperature, LiOsO_{3 }undergoes a structural phase transition from the nonferroelectric R3c phase to the ferroelectric R3c phase with spontaneously broken inversion symmetry. Here we show that this transition also presents the first case of a topological phase transition between a multitype Dirac semimetal and a crossednodalring semimetal. In the hightemperature nonferroelectric phase, the nonsymmorphic glide mirrors help to stabilize multiple Dirac points close to the Fermi level, including three linear Dirac points at L, and one cubic and two linear Dirac points at T. Across the phase transition into the ferroelectric phase, the broken inversion symmetry transforms each linear Dirac point at L into a nodal ring, and interestingly, transforms the cubic Dirac point at T into three mutuallycrossed nodal rings. 
Thursday, March 8, 2018 1:39PM  1:51PM 
S10.00011: Crystalline symmetry protected Majorana mode in numberconserving Dirac semimetal nanowires RuiXing Zhang, ChaoXing Liu

Thursday, March 8, 2018 1:51PM  2:03PM 
S10.00012: Model Hamiltonian for hexagonal ABC topological semimetals J.W.F. Venderbos, Heng Gao, Youngkuk Kim, Eugene Mele, Andrew Rappe We introduce a lattice regularized k.p model Hamiltonian for a broad class of hexagonal ABC materials with space group P6_{3}mc. Firstprinciples electronic band structure calculations have shown that specific members of this class realize Dirac, Weyl, or nodal line semimetals. By deriving a model Hamiltonian for the relevant lowenergy bands, formulated in terms of two effective j=3/2 quartets related by the screw rotation, we show that the nature of the electronic structure of these materials can be understood from a conceptually unified picture. In particular, we elucidate the origin of the topological semimetallic phases, and relate the parameters of our model to the properties of the ABC materials (e.g. crystal field splitting, spinorbit coupling, inversion asymmetry). Since the hexagonal ABC structures lack inversion symmetry, they are expected to show a nonlinear optical response. We employ our model Hamiltonian to address second harmonic generation and photovoltaic effects in ABC materials, and interpret the response in terms of band structure topology. 
Thursday, March 8, 2018 2:03PM  2:15PM 
S10.00013: Tunable Topological Materials from First Principles Sophie Weber, Sinead Griffin, Jeffrey Neaton The control of topological phases is the next step to harnessing their exotic electronic and transport properties. A promising route to achieving tunability is via a subclass of polar compounds that can be continuously deformed from a nonpolar reference phase via strain, pressure or chemical substitution. Using firstprinciples calculations we propose new polar topological materials which have been downselected from a highthroughput search for potential ferroelectrics. These compounds possess Dirac and Weyl nodes at or near the Fermi level. We investigate the capacity to induce and alter the topological properties in these materials by tuning with epitaxial strain, pressure and chemistry. 
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