Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session G10: Interacting Dirac and Parabolic Semimetals 
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Sponsoring Units: DCMP Chair: Igor Herbut, Simon Fraser University Room: 007A 
Tuesday, March 3, 2015 11:15AM  11:27AM 
G10.00001: Influence of electronic band topology on phonon properties in Dirac materials Ion Garate, Kush Saha, Katherine L\'egar\'e In Dirac materials, the interaction between electrons and longwavelength phonons has been shown to induce and stabilize topological insulation [12]. Here report on a theoretical study of the converse effect, namely the influence of band topology on phonon properties. We calculate how electronphonon interactions change the bulk phonon dispersion as a function of pressure and temperature, in both trivial and topological phases. We find that (i) topological insulators are more prone to lattice instabilities than trivial insulators, and (ii) Raman and neutron scattering measurements can be used to determine the electronic band topology. \\[4pt] [1] I. Garate, PRL 110, 046402 (2013).\\[0pt] [2] K. Saha and I. Garate, PRB 89, 205103 (2014). [Preview Abstract] 
Tuesday, March 3, 2015 11:27AM  11:39AM 
G10.00002: Weyl Semimetal in the Limit of Strong Coulomb Interactions Akihiko Sekine, Kentaro Nomura Weyl semimetals have a topological property such that an energy gap opens only if the Weyl nodes with opposite chirality meet and annihilate each other. Then it is expected that Weyl semimetals are stable against perturbations. Motivated by this, we study the stability of a timereversal symmetry broken Weyl semimetal with two nodes against strong $1/r$ longrange Coulomb interactions. We consider the case where magnetic impurities are doped into a 3D topological insulator, and take into account the $1/r$ Coulomb interactions between the bulk electrons. In this case, the system can be described by the U(1) lattice gauge theory. With the use of the strong coupling expansion of the lattice gauge theory and the meanfield approximation, we analyze the system from the strong coupling limit. It is shown that parity (spatial inversion) symmetry of the system is spontaneously broken in the strong coupling limit, and a different type of the Weyl semimetal, in which timereversal and parity symmetries are broken, appears in the strong coupling limit. [Preview Abstract] 
Tuesday, March 3, 2015 11:39AM  11:51AM 
G10.00003: Topological Mott Insulator in ThreeDimensional Systems with Quadratic Band Touching Lukas Janssen, Igor Herbut We will discuss the effects of the longrange Coulomb interaction in threedimensional systems in which conduction and valence bands touch quadratically at the Fermi level. Such band structure is realized in various strongly spinorbitcoupled materials, such as HgTe, $\alpha$Sn, and some pyrochlore iridates. We will argue that these systems may be unstable towards spontaneous formation of the strong topological Mott insulator already at weak longrange Coulomb interaction. The mechanism of the instability can be understood as the collision of a nonFermiliquid fixed point, discovered by Abrikosov in the '70s and revisited recently, with another, critical, fixed point, which approaches it in the coupling space as the system's dimensionality approaches a certain ``critical dimension'' from above. Some universal characteristics of this scenario, the width of the nonFermiliquid crossover regime, and the observability of the topological Mott phase will be discussed. Reference: I. F. Herbut and L. Janssen, Phys. Rev. Lett. 113, 106401 (2014). [Preview Abstract] 
Tuesday, March 3, 2015 11:51AM  12:03PM 
G10.00004: Interacting Weyl semimetals: characterization via the topological Hamiltonian and its breakdown William WitczakKrempa, Michael Knap, Dmitry Abanin Weyl semimetals (WSMs) have robust linearlydispersing excitations. Unusual properties arise from the latter, such as anomalous electrodynamic responses and open Fermi arcs on boundaries. We derive a simple criterion to identify and characterize WSMs in an interacting setting using the exact electronic Green's function at zero frequency, which defines a topological Bloch Hamiltonian. We apply this criterion by numerically analyzing, via cluster and other methods, interacting models with and without timereversal symmetry. We thus identify mechanisms for how interactions move and renormalize Weyl fermions. Our methods remain valid in the presence of longranged Coulomb repulsion. Finally, we introduce a WSMlike phase for which our criterion breaks down, due to fractionalization of the electron.\\[4pt] W.~WitczakKrempa, M.~Knap, D.~Abanin, Phys.~Rev.~Lett.~113, 136402 (2014) [Preview Abstract] 

G10.00005: ABSTRACT WITHDRAWN 
Tuesday, March 3, 2015 12:15PM  12:27PM 
G10.00006: Magnetotransport properties of threedimensional Weyl semimetals Navneeth Ramakrishnan, Mirco Milletari, Shaffique Adam We investigate theoretically the transport and magnetotransport properties of threedimensional Weyl semimetals. We consider the RPABoltzmann transport theory relevant for noninteracting electrons scattering off randomly distributed charged impurities, and employ an effective medium theory to average over the resulting spatially inhomogeneous carrier density profile. Our formalism allows us to smoothly connect results for the minimum conductivity near the Dirac point with known results for the conductivity at high carrier density. In the presence of a nonquantizing magnetic field, we predict that the magnetoresistance shows a transition from quadratic at low magnetic fields to linear at higher fields. In addition, our formalism can qualitatively explain some recent unexpected experimental results on the mixedchalcogenide compound TlBiSSe. This work is supported by the Singapore National Research Foundation NRFNRFF201201. [Preview Abstract] 
Tuesday, March 3, 2015 12:27PM  12:39PM 
G10.00007: Interactiondriven phase instabilities in twodimensional quadratic band touching systems James Murray, Kelly Pawlak, Oskar Vafek Quadratic band touching (QBT) systems, in which an upwarddispersing and a downwarddispersing energy band meet at a single point in momentum space, have emerged as an attractive arena in which to study multicriticality and intertwined orders driven by electron interactions. Analogous to the twovalley QBT occurring in bilayer graphene, singlevalley QBTs such as those arising on checkerboard and kagome lattices exhibit phase instabilties in multiple channels even for arbitrarily weak interactions. Using a Wilsonian renormalization group procedure, we show  without requiring spinorbit coupling or special values of the interaction  that the leading instabilities in the halffilled system are toward quantum anomalous Hall or quantum spin Hall phases. Upon doping away from halffilling, the repulsive interactions lead to superconductivity in swave or dwave channels. [Preview Abstract] 
Tuesday, March 3, 2015 12:39PM  12:51PM 
G10.00008: Calculation of the magnetooptical response of Kane fermions in HgCdTe John Malcolm, Elisabeth Nicol The concentration $x=x_{c}\approx 0.17$ in ${\rm Hg}_{x}{\rm Cd}_{1x}{\rm Te}$ describes a critical value in the phase transition between semimetal ($x 
Tuesday, March 3, 2015 12:51PM  1:03PM 
G10.00009: Topological property and phase transition in three dimensional Dirac semimetal Yongping Du, Bo Wan, Xiangang Wan Based on firstprinciples calculations and effective model analysis, we find a new three dimensional Dirac semimetal. This material has a Dirac point protected by crystal symmetry. It can be driven into various topological phases and Weyl semimetal state by breaking symmetries. This material may have linear quantum magnetoresistance, quantum spin Hall effect. [Preview Abstract] 
Tuesday, March 3, 2015 1:03PM  1:15PM 
G10.00010: Dirac Circles and Quantum Hall Effect in 3D InversionSymmetric Crystals Benjamin J. Wieder, Youngkuk Kim, C.L. Kane In the presence of inversion and timereversal symmetries, materials with weak spinorbit coupling may host topologically protected Dirac line nodes. A band inversion transition in these systems can produce a line node which closes on itself and forms a protected Dirac circle. The surfaces parallel to this circle host zeroenergy puddles in momentum space which are flat if the inverting bands have the same effective mass. In cases with differing effective masses, the surface modes disperse, but the bulk Dirac circle remains gapless. Adding an external magnetic field perpendicular to this circle creates surface Landau levels, whose number can be controlled by tuning the field strength. When a new level is created or destroyed, the bulk becomes gapless and the zerotemperature bulk conductivity displays a sharp peak. The sequence of conductivity peaks describes an unusual manifestation of the integer quantum hall effect. We characterize surface and bulk transport as a function of magnetic field strength and in the presence of disorder. [Preview Abstract] 
Tuesday, March 3, 2015 1:15PM  1:27PM 
G10.00011: Topological NodeLine Semimetal in Three Dimensional Graphene Networks Hongming Weng, Yunye Liang, Qiunan Xu, Rui Yu, Zhong Fang, Xi Dai, Yoshiyuki Kawazoe Graphene, a two dimensional (2D) carbon sheet, acquires many of its amazing properties from the Dirac point nature of its electronic structures with negligible spinorbit coupling. Extending to 3D space, graphene networks with negative curvature, called MackayTerrones crystals (MTC), have been proposed and experimentally explored, yet their topological properties remain to be discovered. Based on the firstprinciple calculations, we report an allcarbon MTC with topologically nontrivial electronic states by exhibiting nodelines in bulk. When the nodelines are projected on to surfaces to form circles, ``drumhead'' like flat surface bands nestled inside of the circles are formed. The bulk nodeline can evolve into 3D Dirac point in the absence of inversion symmetry, which has shown its plausible existence in recent experiments. [Preview Abstract] 
Tuesday, March 3, 2015 1:27PM  1:39PM 
G10.00012: Topological states of Sb thin films over doped graphene ChiHsuan Lee, ChihKai Yang Electronic properties of Sb thin films on pure, borondoped, and nitrogendoped graphene are investigated using density functional calculations. Various stacking configurations are taken into consideration. Sb films on borondoped graphene have stronger interaction than those on pure or nitrogendoped graphene. Dirac cones also occur in these systems and can serve as conduits for spinpolarized conduction. The results are useful for applications in topological transport and spintronics. [Preview Abstract] 

G10.00013: ABSTRACT WITHDRAWN 
Tuesday, March 3, 2015 1:51PM  2:03PM 
G10.00014: Phonon analogue of topological nodal semimetals Hoi Chun Po, Yasaman Bahri, Ashvin Vishwanath Recently, Kane and Lubensky proposed a mapping between bosonic phonon problems on isostatic lattices to chiral fermion systems based on factorization of the dynamical matrix [Nat. Phys. {\bf 10}, 39 (2014)]. The existence of topologically protected zero modes in such mechanical problems is related to their presence in the fermionic system and is dictated by a local index theorem. Here we adopt the proposed mapping to construct a twodimensional mechanical analogue of a fermionic topological nodal semimetal that hosts a robust bulk node in its linearized phonon spectrum. Such topologically protected soft modes with tunable wavevector may be useful in designing mechanical structures with faulttolerant properties. [Preview Abstract] 
Tuesday, March 3, 2015 2:03PM  2:15PM 
G10.00015: Photoinduced Topological phase Transition in a 2D Fermi System with a Quadratic Band Crossing Xiaoting Zhou, Gregory A. Fiete Recent years, physicists have been gripped by the topological phases in condensed matter systems, and many attentions have been concentrated on the pursuit of new topological matters. The achievement of the topological control by the external fields provides a new direction. Using Floquet theory, we demonstrate that, a topologically stable quadratic bandcrossing point (QBCP), carrying a Berry flux $\pm 2\Pi$ (or 2$\Pi$) , would be splitted into 2 Dirac points with Berry flux $\Pi$ (or $\Pi$), by the irradiation of a linearly polarized light. And the QBCP will either be lifted due to the breaking of the timereversal symmetry, or go to a trivial QBCP with distinct topological properties, by introducing a circularly polarized light. Therefore, the manipulation of band structure could be realized. [Preview Abstract] 
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