Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session P10: Type II Weyl SemimetalsFocus

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Sponsoring Units: DMP Chair: Gavin Osterhoudt, Boston Coll Room: LACC 301B 
Wednesday, March 7, 2018 2:30PM  3:06PM 
P10.00001: Te vacancy driven superconductivity in type II Weyl semimetal MoTe2 Invited Speaker: Suyeon Cho Twodimensional (2D) transition metal dichalcogenides (TMDs) have received great attentions because of diverse quantum electronic states such as topological insulating (TI), Weyl semimetallic (WSM) and superconducting states. Recently, the superconducting states emerged in pressurized semimetallic TMDs such as MoTe2 and WTe2 have become one of the central issues due to their predicted WSM states. However, the difficulty in synthetic control of chalcogen vacancies and the ambiguous magneto transport properties have hindered the rigorous study on superconducting and WSM states. Here, we report the emergence of superconductivity at 2.1 K in Tedeficient orthorhombic TdMoTe2−x with an intrinsic electrondoping, while stoichiometric monoclinic 1T′MoTe2 shows no superconducting state down to 10 mK, but exhibits a large magnetoresistance of 32,000% at 2 K in a magnetic field of 14 T originating from nearly perfect compensation of electron and hole carriers. Scanning tunnelling spectroscopy and synchrotron xray diffraction combined with theoretical calculations clarify that Te vacancies trigger superconductivity via intrinsic electron doping and the evolution of the Td phase from the 1T′ phase below 200 K. Unlike the pressureinduced superconducting state of monoclinic MoTe2, this Te vacancyinduced superconductivity is emerged in orthorhombic MoTe2, which is predicted as Weyl semimetal, via electrondoping. This chalcogen vacancy inducedsuperconductivity provides a new route for cultivating superconducting state together with WSM state in 2D van der Waals materials. 
Wednesday, March 7, 2018 3:06PM  3:18PM 
P10.00002: Origin of Structural Phase Transition and Superconductivity in Different Phases of MoTe_{2}; A FirstPrinciples Study Taner Yildirim, Linda Hung MoTe_{2} has recently attracted much attention due to its rich properties including Weyl Semimetallic (WS) behaviour and superconductivity [1,2]. MoTe_{2} exhibits three different phases; 2H, 1T', and T_{d}. While it is a semiconductor with an indirect band gap of 1.2 eV in the 2H phase, it shows semimetallic and superconducting properties in 1T' and T_{d} phases. In this talk we discuss the origin of the observed phase transition between monoclinic 1T' phase with inversion symmetry and orthorhombic T_{d} phase without inversion symmetry (WS candidate) within temperature and pressure (TP) plane. Our calculations show that these phases are almost degenerate in energy with a very low energy barrier for the transition. The effect of phonon energies and entropy contributions to the phase stability will be presented. We will discuss the electronphonon coupling and superconductivity in both Weylsemimetal phase (T_{d}) and 1T' phases with and without the spinorbit coupling. A detailed understanding the origin of phase transition between T_{d} and 1T' phases will allow us to tune the final structure in TP plane and, in turn, to control the physical properties of these interesting systems. 
Wednesday, March 7, 2018 3:18PM  3:30PM 
P10.00003: Correlating the crystal structure and electronic ground state of typeII Weyl semimetal MoTe$_2$: structural measurements Colin Heikes, ILin Liu, Linda Hung, Tristin Metz, Chris Eckberg, Yan Wu, Huibo Cao, Johnpierre Paglione, Taner Yildirim, Nicholas Butch, William Ratcliff The reported pressure dependent superconductivity of the typeII Weyl semimetal MoTe$_2$ has led to speculation about the possibility of a topologically nontrivial superconducting state in this material. A key question about the topology of this superconducting state is the evolution of the crystal structure of MoTe$_2$ at pressures and temperatures relevant for superconductivity, as the noncentrosymmetric T$_d$ orthorhombic phase is needed for the Weyl state. Semimetallic MoTe$_2$ has two metastable polymorphs, with a first order structural transition between the centrosymmetric monoclinic 1T’ and the orthorhombic T$_d$ structure near 270 K. This structural transition has a demonstrated pressure dependence, with a suppression of the T$_d$ phase at increased pressures. We have grown single crystals of MoTe$_2$, and using a combination of elastic neutron scattering, magnetotransport, and spectroscopic techniques we have investigated the nature of this transition. We report pressure and temperature dependent neutron scattering measurements which elucidate the nature of this phase transition at temperature and pressures relevant for the previously reported superconductivity. 
Wednesday, March 7, 2018 3:30PM  3:42PM 
P10.00004: Correlating the crystal structure and electronic ground state of typeII Weyl semimetal MoTe_{2}: Transport measurements ILin Liu, Colin Heikes, Taner Yildirim, Tristin Metz, Johnpierre Paglione, Chris Eckberg, Nicholas Butch, William Ratcliff The reported pressure dependent superconductivity of the typeII Weyl semimetal MoTe_{2} has led to speculation about the possibility of a topologically nontrivial superconducting state in this material. A key question about the topology of this superconducting state is the evolution of the crystal structure of MoTe_{2} at pressures and temperatures relevant for superconductivity, as the noncentrosymmetric T_{d }orthorhombic phase is needed for the Weyl state. Semimetallic MoTe_{2} has two metastable polymorphs, with a first order structural transition between the centrosymmetric monoclinic 1T’ and the orthorhombic T_{d} structure near 270 K. This structural transition has a demonstrated pressure dependence, with a suppression of the T_{d} phase at increased pressures. We have grown single crystals of MoTe_{2}, and using a combination of elastic neutron scattering, magnetotransport, and spectroscopic techniques we have investigated the nature of this transition. We report pressure and temperature dependent transport measurements which elucidate the nature of this phase transition at temperature and pressures relevant for the previously reported superconductivity. 
Wednesday, March 7, 2018 3:42PM  3:54PM 
P10.00005: Disorder and the 1T'T_{d} structural phase transition in MoTe_{2} John Schneeloch, Junjie Yang, Chunruo Duan, Despina Louca The transition metal dichalcogenide MoTe_{2} has been much studied recently due to a reported Weyl semimetal state in its lowtemperature T_{d} phase and very large magnetoresistance which onsets at low temperatures, but the structural transition, consisting of shifts of layers along a crystallographic axis in the 1T' phase relative to the T_{d} phase, has not received much experimental study since these phases were first reported several decades ago. We study the 1T'T_{d} transition in MoTe_{2} via single crystal neutron diffraction. In the vicinity of the transition, diffuse scattering rods are seen, indicating disorder in the displacement of layers along the shear direction. We determine the amount and kind of disorder by comparison with calculations, and discuss the nature of the structural phase transition in light of this disorder. 
Wednesday, March 7, 2018 3:54PM  4:06PM 
P10.00006: Disorderinduced phase transitions of typeII Weyl semimetals Moon Jip Park, Bora Basa, Matthew Gilbert Weyl semimetals are a newly discovered class of materials that host relativistic massless Weyl fermions as their lowenergy bulk excitations. Among this class of materials, there exist two types of semimetals that are of particular interest: typeI Weyl semimetals, which have broken inversion or timereversal symmetry, and typeII Weyl semimetals, which additionally break Lorentz invariance, hosting tilted Weyl cone. In this work, we use the Born approximation to demonstrate that the typeI Weyl semimetals undergo a quantum phase transition to typeII Weyl semimetals in the presence of the finite Anderson disorder when nonzero tilt of Weyl cone exists. The phase transition occurs when the disorder renormalizes the topological mass, thereby reducing the Fermi velocity near the Weyl cone below the tilt of the cone. We also confirm the presence of the disorderinduced phase transition in Weyl semimetals by using exact diagonalization of a threedimensional tightbinding model to calculate the resultant phase diagram of the 
Wednesday, March 7, 2018 4:06PM  4:18PM 
P10.00007: Anomalous Transport in TypeII Weyl Semimetals Robert McKay, II, Tim McCormick, Nandini Trivedi Weyl semimetals (WSM) possess monopoles of Berry curvature in momentum space. We calculate the effects of these Berry monopoles on the anomalous transport in the absence of an external magnetic field for lattice models of typeII WSM. Our calculations reveal that the anomalous transverse thermoelectric transport coefficient is enhanced with increasing nodal tilt, so long as the Fermi pockets comprising the nodes are separated. For very large tilt, the system undergoes a Lifshitz transition where the electron and hole pockets merge and the anomalous thermoelectric coefficient is suppressed. We find that the anomalous Hall coefficient is uniformly suppressed with increasing tilt. Furthermore, we find nonmonotonic temperature dependence of the Hall and transverse thermoelectric transport coefficients is due to the temperature dependence of the chemical potential. Lastly, we make connections with measurable quantities and our work indicates that typeII WSM show promise for thermoelectric applications. 
Wednesday, March 7, 2018 4:18PM  4:30PM 
P10.00008: TypeII SymmetryProtected Topological Dirac Semimetals TayRong Chang, Suyang Xu, Daniel Sanchez, WeiFeng Tsai, ShinMing Huang, Guoqing Chang, ChuangHan Hsu, Guang Bian, Ilya Belopolski, Zhiming Yu, Shengyuan Yang, Titus Neupert, HorngTay Jeng, Hsin Lin, Zahid Hasan The recent proposal of the typeII Weyl semimetal state has attracted significant interest. In this work, we propose the concept of the threedimensional typeII Dirac fermion and theoretically identify this new symmetryprotected topological state in the large family of VAl_{3} family (VAl_{3}, Nb Al_{3}, TaAl_{3}, NbGa_{3}, and TaGa_{3}). We show that the VAl_{3} family features a pair of strongly Lorentzviolating typeII Dirac nodes and that each Dirac node can be split into four typeII Weyl nodes with chiral charge ±1 via symmetry breaking. Furthermore, we predict that the Landau level spectrum arising from the typeII Dirac fermions in VAl_{3} is distinct from that of known Dirac or Weyl semimetals. We also demonstrate a topological phase transition from a typeII Dirac semimetal to a quadratic Weyl semimetal or a topological crystalline insulator via crystalline distortions [1]. 
Wednesday, March 7, 2018 4:30PM  4:42PM 
P10.00009: Quantum Oscillations in the TypeII Dirac Semimetals: MAl_{3} (M = V, Nb and Ta) KuanWen Chen, Xiujun Lian, YuChe Chiu, Wangwei Lan, Niraj Aryal, You Lai, David Graf, Efstratios Manousakis, Luis Balicas, Ryan Baumbach Recent work shows that MAl_{3} (M = V, Nb and Ta) are typeII topological Dirac semimetals with a pair of strong Lorentzviolating Dirac nodes. We synthesized single crystals by using an Al flux method and measured the de Hassvan Alphen effect by using torque magnetomery. From the angulardependence of the quantum oscillations of MAl_{3}, the geometry of the Fermi surface is identified and found to be consistent with firstprinciple calculations. The Dirac points are located the Fermi level at about 100 meV, 230 meV and 250 meV in VAl_{3}, NbAl_{3} and TaAl_{3}, respectively. We also reveal that one of the hole orbits formed by the tilted Dirac cone displays a nontrivial Berry phase π. These results support the existence of tilted Dirac cones above the Fermi level within the VAl_{3} family. 
Wednesday, March 7, 2018 4:42PM  4:54PM 
P10.00010: Fermi surface of the typeII Dirac semimetallic candidates PdTe_{2} and PtTe_{2}. Wenkai Zheng, Qiong Zhou, Niraj Aryal, YuChe Chiu, Rico Schoenemann, KuanWen Chen, Daniel Rhodes, Thomas Martin, Gregory McCandless, Julia Chan, Efstratios Manousakis, Luis Balicas We investigated the electronic structures at the Fermi level and the transport properties via the Shubnikovde Haas (SdH) and de Haas van Alphen (dHvA) effects in singlecrystals of PdTe_{2} and PtTe_{2}, recently claimed to be candidates for a novel typeII Dirac semimetallic state. We synthesized highquality singlecrystals, i.e. with mobilities in excess of 10^{4} cm^{2}/Vs, through a flux method. Both compounds display similar band structures and concomitantly complex their fermi surfaces which are not in perfect agreement with band structure calculations. The effective masses extracted from the LifshitzKosevich formalism were found to be in the order of 0.01 times the free electron mass for both compounds. We also found evidence for a nontrivial Berry phase for PdTe_{2} compound. 
Wednesday, March 7, 2018 4:54PM  5:06PM 
P10.00011: Huge magnetoresistance and extremely large conductivity in the typeII Weyl semimetals WP_{2} and_{ }MoP_{2} Nitesh Kumar, Yan Sun, Chandra Shekhar, Nan Xu, Inge Leermakers, Ming Shi, Uli Zeitler, Claudia Felser The peculiar band structure of semimetals exhibiting Dirac and Weyl crossings can lead to spectacular electronic properties such as large mobilities accompanied by extremely high magnetoresistance. In particular, two closely neighbouring Weyl points of the same chirality are protected from annihilation by structural distortions or defects, thereby significantly reducing the scattering probability between them. Here we present the electronic properties of the transition metal diphosphides, WP_{2 }and MoP_{2}, which are typeII Weyl semimetals with robust Weyl points by transport, angle resolved photoemission spectroscopy and first principles calculations. Our single crystals of WP_{2} display an extremely low residual lowtemperature resistivity of 3 nΩcm accompanied by an enormous and highly anisotropic magnetoresistance above 200 million % at 63 T and 2.5 K. We observe a large suppression of charge carrier backscattering in WP_{2} from transport measurements. These properties are likely a consequence of the novel Weyl fermions expressed in this compound. 
Wednesday, March 7, 2018 5:06PM  5:18PM 
P10.00012: Stable Weyl points and trivial surface states in carrier compensated semimetal WP2 Elia Razzoli, Berend Zwartsenberg, Fabio Boschini, Matteo Michiardi, Ryan Day, Christopher Gutierrez, Ilya Elfimov, Vicky Suess, Claudia Felser, Andrea Damascelli The possible relation between the extremely large magnetoresistance (XMR) and the presence of Weyl points has recently drawn a lot of attention in the study of topological semimetals. Here we combine angleresolved photoemission spectroscopy and density functional theory calculations to identify both surface and bulk electronic states in WP2. While the surface is uncompensated, our results show the bulkband structure of WP2 is particle holecompensated and it is compatible with the presence of at least 4 Weyl points, confirming its topological nature. 
Wednesday, March 7, 2018 5:18PM  5:30PM 
P10.00013: Fermi surface of the Weyl typeII metallic candidate WP_{2} Rico Schoenemann, Niraj Aryal, Qiong Zhou, YuChe Chiu, KuanWen Chen, Thomas Martin, Gregory McCandless, Julia Chan, Efstratios Manousakis, Luis Balicas Here, we present a study of the Fermi surface of WP_{2} via the Shubnikovde Haas (SdH) effect. Compared to other semimetals, WP_{2} exhibits a very low residual resistivity, i.e., ρ_{0} = 10 nΩcm, which leads to perhaps the largest nonsaturating magnetoresistivity [ρ(H)] reported for any compound. For the samples displaying the smallest ρ_{0}, ρ(H) is observed to increase by a factor of 2.5 × 10^{7} % under μ_{0}H = 35 T at T = 0.35 K. The angular dependence of the SdH frequencies is found to be in excellent agreement with the firstprinciples calculations when the electron and hole bands are shifted by 30 meV with respect to the Fermi level. This small discrepancy could have implications for the predicted topological character of this compound. 
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