Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session C44: Dirac and Weyl Semimetals: Theory IFocus

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Sponsoring Units: DMP Chair: Rahul Roy, University of California, Los Angeles Room: 391 
Monday, March 13, 2017 2:30PM  2:42PM 
C44.00001: Fillingenforced semimetals in magneticordered materials Xu Yang, Ying Ran Semimetals with pointlike or linelike Fermi surfaces are under intensive studies recently. Among them, Dirac semimetals and nodalline semimetals are usually found in timereversal symmetric system. Here we report two kinds of semimetals at certain electron filling fractions in materials with buildin magnetic order. The first example is a material in which timereversal symmetry T and inversion symmetry I are broken due to the magnetic moment but their combination IT is preserved. This symmetry IT, together with other space group symmetries, protects Dirac points at certain highsymmetry points. The second example is a material with certain magnetic space group symmetry, which, at certain filling, must have nodal rings around high symmetry points. Our results extend the search for semimetals in Tinvariant materials to that in magnetic materials, and provide a platform for the interesting interplay between magnetism and exotic phases. [Preview Abstract] 
Monday, March 13, 2017 2:42PM  2:54PM 
C44.00002: Dirac Fermions in an Antiferromagnetic Semimetal Peizhe Tang, Quan Zhou, Gang Xu, ShouCheng Zhang Analogues of the elementary particles have been extensively searched for in condensed matter systems for both scientific interest and technological applications. Recently, massless Dirac fermions were found to emerge as low energy excitations in materials now known as Dirac semimetals. All the currently known Dirac semimetals are nonmagnetic with both timereversal symmetry $\mathcal{T}$ and inversion symmetry $\mathcal{P}$. Here we show that Dirac fermions can exist in one type of antiferromagnetic systems, where both $\mathcal{T}$ and $\mathcal{P}$ are broken but their combination $\mathcal{PT}$ is respected. We propose orthorhombic antiferromagnet CuMnAs as a candidate, analyze the robustness of the Dirac points under symmetry protections, and demonstrate its distinctive bulk dispersions as well as the corresponding surface states by \emph{ab initio} calculations. Our results provide a possible platform to study the interplay of Dirac fermion physics and magnetism. [Preview Abstract] 
Monday, March 13, 2017 2:54PM  3:30PM 
C44.00003: Topological Weyl Semimetal Materials: Charge and Spin Transport in the Bulk Invited Speaker: Binghai Yan Thus far Weyl semimetals have been discovered in many materials such as TaAs (typeI) and MoTe2 (typeII). In this talk, I will first introduce themagnetotransport properties of TaAs and MoTe2type Weyl materials, where large magnetoresistance with strong quantum oscillations commonly exists [1,2]. We have reconstructed the 3D bulk Fermi surfaces from the quantum oscillations and band structure calculations [3,4,5], so that their magnetotransport behaviour can be furtherunderstood. Based on the band structure of Weyl materials, I will demonstrate the large spin Hall effect in both typeI and typeII Weyl semimetals [6]. The spin Hall effect, which can convert the charge current to spin current efficiently, not only paves a way for the application in spintronics, but also indicates a new guideline to design Weyl and Dirac semimetals from the pool of spintronic materials [7]. References: [1] Nature Phys. 11, 645 (2015).[2] Nature Comm. 7, 11038 (2016). [3] Phys. Rev. B 93, 121105 (2016). (2016). [4] Phys. Rev. Lett. 117, 146401 (2016). [5] Nature Comm. 7, 11615 (2016). [6] Phys. Rev. Lett. 117, 146403 (2016). [7] arXiv:1608.03404 (2016). [Preview Abstract] 
Monday, March 13, 2017 3:30PM  3:42PM 
C44.00004: Correlation and transport phenomena in topological nodalloop semimetals Jianpeng Liu, Leon Balents We theoretically study the unique physical properties of topological nodalloop semimetals protected by the coexistence of timereversal and inversion symmetries with negligible spinorbit coupling. We argue that strong correlation effects occur at the surface of such systems for relatively small Hubbard interaction $U$, due to the narrow bandwidth of the ``drumhead'' surface states. Our HartreeFock and RPA calculations indicate that surface ferromagnetic and surface chargeordered phases appear at small interactions, whose order parameters are exponentially localized at the surface. The transition from a nonordered to a surface ferromagnetic phase is characterized by the surfacemode divergence of spin susceptibility. The quantum critical behavior of the surface ferromagnetic transition is nontrivial in the sense that the surface spin order parameter couple to Fermisurface excitations from both surface and bulk states, leading to unconventional Landau damping and consequently a naive dynamical critical exponent $z\approx 1$. We also show that, quantum oscillations arise due to bulk states. The bulk magnetic susceptibility diverges logarithmically whenever the nodal loop exactly overlaps with a quantized magnetic orbit in the bulk Brillouin zone. [Preview Abstract] 
Monday, March 13, 2017 3:42PM  3:54PM 
C44.00005: Strategies for Designing Magnetic Weyl Semimetals Guoqing Chang, SuYang Xu, Hao Zheng, Bahadur Singh, ChuangHan Hsu, ShinMing Huang, Guang Bian, Ilya Belopolski, Daniel S. Sanchez, Nasser Alidoust, TayRong Chang, Hong Lu, Xiao Zhang, Yi Bian, ZhiMing Yu, Shengyuan A. Yang, HorngTay Jeng, Titus Neupert, Shuang Jia, Arun Bansil, Hsin Lin, M. Zahid Hasan Weyl semimetals are novel topological conductors that host Weyl fermions as emergent quasiparticles. Weyl quasiparticles can arise through the breaking of either the inversion or timereversal symmetry. Although the first inversionbreaking Weyl semimetal was discovered recently in TaAs, its magnetic counterpart has remained elusive. The timereversal breaking Weyl phase is predicted to exhibit exotic properties distinct from the inversionbreaking phases. Here we propose and compare different strategies for designing Weyl semimetals, and identify a large class of magnetic Weyl semimetals in RAlGe[1 2] and Co2TiX[3] families. We will also illustrate our approach for generating magnetic Weyl nodes from Nexus fermions[4]. \\1 SY. Xu et al, arXiv: 1603.07318 \\2 G. Chang et al, arXiv: 1604.02124 \\3 G. Chang et al, arXiv: 1603.01255 \\4 G. Chang et al, arXiv: 1605.06831 [Preview Abstract] 
Monday, March 13, 2017 3:54PM  4:06PM 
C44.00006: Space group protection of Dirac manifolds: a roadmap towards topological semimetal materials Adrien Bouhon, Annica BlackSchaffer Combining space group representation theory and multiband Berry phase arguments we derive simple algebraic rules that can be used to predict global protection of Dirac points, Dirac lines, and Dirac surfaces as a function of the space group. This approach leads to many robust predictions concerning real material candidates. [Preview Abstract] 
Monday, March 13, 2017 4:06PM  4:18PM 
C44.00007: Cubic Dirac fermions in quasionedimensional transitionmetal chalcogenide semimetals immune to Peierls distortion Qihang Liu, Alex Zunger A Cubic Dirac Fermion in condensedmatter physics refers to a band crossing in periodic solids that has 4fold degeneracy with cubic dispersions in certain directions. Such a crystalline symmetry induced fermion is composed of 6 Weyl fermions where 3 have lefthanded and 3 have righthanded chirality, and constitutes one of the ``new fermions'' that have no counterpart in highenergy physics. However, no prediction has yet pointed to a plausible example of a material candidate hosting such a cubicallydispersed Dirac semimetal (CDSM). Here we establish the design principles for CDSM finding that only 2 out of 230 space groups possess the required symmetry elements. Adding the required band occupancy criteria, we conduct a material search using density functional band theory identifying a group of quasionedimensional molybdenum chalcogenide compounds A(MoX)$_{\mathrm{3}}$ (A $=$ Na, K, Rb, In, Tl; X $=$ S, Se, Te) with space group P6$_{\mathrm{3}}$/m as ideal CDSM candidates. Studying the stability of the A(MoX)$_{\mathrm{3}}$ family towards a Peierls distortion reveals a few candidates such as Rb(MoTe)$_{\mathrm{3}}$ and Tl(MoTe)$_{\mathrm{3}}$ that are resilliant to Peierls distortion, thus retaining the metallic character. [Preview Abstract] 
Monday, March 13, 2017 4:18PM  4:30PM 
C44.00008: TypeII SymmetryProtected Topological Dirac Semimetals TayRong Chang, SuYang Xu, Daniel S. Sanchez, WeiFeng Tsai, ShinMing Huang, Guoqing Chang, ChuangHan Hsu, Guang Bian, Ilya Belopolski, ZhiMing Yu, Shengyuan A. Yang, Titus Neupert, HorngTay Jeng, Hsin Lin, M. Zahid Hasan The discoveries of Dirac and Weyl semimetal states in real materials led to the realizations of elementary particle analogs in tabletop experiments. Recently, a new type of Weyl fermion attracted interest because it strongly violates Lorentz symmetry whose analog does not exist in the Standard Model. While this state has been dubbed the typeII Weyl semimetal and predicted in a number of materials, its Dirac counterpart has remained elusive. In this work, we propose the concept of the typeII Dirac fermion and theoretically identify this new state in MA$_3$ (M=V, Nb, Ta; A=Al, Ga, In) [1]. We show that the VAl$_3$ family features a pair of typeII Dirac nodes and that each Dirac node consists of four typeII Weyl nodes via symmetry breaking. Furthermore, we predict the Landau level spectrum arising from the typeII Dirac fermions in VAl$_3$ that is distinct from that of known Dirac/Weyl semimetals. We also show a topological phase transition from a typeII Dirac to a quadratic Weyl or a topological crystalline insulator via crystalline distortions.\\ $[1]$ T.R. Chang et al., arXiv:1606.07555 [Preview Abstract] 
Monday, March 13, 2017 4:30PM  4:42PM 
C44.00009: Hourglass semimetals Luyang Wang, ShaoKai Jian, Hong Yao It was recently found that nonsymmorphic space group symmetries can protect surface states with hourglasslike dispersion. Here, we show that such dispersion can also appear in the bulk of systems which have nonsymmorphic symmetries. We construct lattice models with hourglasslike band structures in the bulk of systems in one, two, and three dimensions, which are protected by nonsymmorphic symmetries and time reversal symmetry. We name such materials as hourglass semimetals, since they all have point or line node degeneracies due to the hourglasslike dispersion. In three dimensions, the hourglass nodal lines in high symmetry planes are protected by glide reflection symmetry, while the hourglass Weyl points at high symmetry axes are protected by screw rotation symmetry. In the latter case, the hourglass Weyl semimetals host four Weyl points at each screw invariant axis, which can collectively disappear and reemerge when tuning spinorbit couplings. These Weyl points are stable even if perturbations that break all the symmetries are turned on, but their locations shift away from the high symmetry axes. This scenario also provides a systematic way to find new nodal line and Weyl semimetals. [Preview Abstract] 
Monday, March 13, 2017 4:42PM  4:54PM 
C44.00010: Disorderinduced transport properties in Weyl semimetals Caio Lewenkopf, Bruno Rizzo, Alexis Hernandez We study the transport properties of Weyl semimetals in the presence of generic disorder. We propose a discretization scheme of the Weyl Hamiltonian that avoids the fermion doubling problem and allows to include an external magnetic field, making possible a direct calculation of spectral and magnetotransport properties. We show the efficiency of the method by calculating the density of states near the Weyl point and of quantum transport properties at the Weyl point in a variety of situations and comparing with the literature results. We consider external magnetic fields to study the effect of disorder in the magnetoconductivity near a Weyl node, analysing the possibility of a disorderinduced phase transition from a pseudoballistic to a diffusive regime. [Preview Abstract] 
Monday, March 13, 2017 4:54PM  5:06PM 
C44.00011: Hyperbolic Weyl point in reciprocal chiral metamaterial. meng xiao, qian lin, shanhui fan Weyl point is a topological singular point in the momentum space. There are two types of Weyl points, typeI and typeII, both are topologically nontrivial but exhibit very different physical properties. In this work, we report the existence of Weyl points in a class of noncentral symmetric metamaterials which preserves time reversal symmetry. We break inversion symmetry utilizing chiral coupling between the electric and magnetic fields. The exploration of Weyl point in metamaterials as described by homogeneous effective material parameters is of fundamental interest since the wavevector space of such metamaterial is noncompact, which is in contrast with the wavevector space of periodic systems which is always topologically compact. This class of metamaterial exhibits either typeI or typeII Weyl points depending on its nonlocal response. We also provide a physical realization of such metamaterial consisting of an array of metal wires in the shape of elliptical helixes which exhibits typeII Weyl points. Such metamaterial should be relatively straightforward to construct experimentally in both microwave and nearinfrared region. [Preview Abstract] 
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