Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session H04: Dirac/Weyl Semimetals -- Type-II Topological SemimetalsFocus
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Sponsoring Units: DMP Chair: Rongying Jin, Louisiana State University Room: BCEC 107C |
Tuesday, March 5, 2019 2:30PM - 2:42PM |
H04.00001: Polar domains and domain walls in MoTe2 with Weyl semimetallic and trivial semimetallic phases Fei-Ting Huang, Seong Joon Lim, Sobhit Singh, Jinwoong Kim, Lunyong zhang, Jae-Wook Kim, David Vanderbilt, Sang-Wook Cheong Quantum materials with non-trivial electronic topologies are one of the most active fields of current condensed matter research. Among them, Weyl semimetal (WSM) behavior can be realized quite generally in a semimetallic crystal with large spin-orbit coupling by breaking either time-reversal or space inversion symmetry. Interestingly, those known WSMs through the space inversion breaking mechanism often crystallize in polar structures, and thus they are polar WSMs. The question that naturally arises is the possible existence of polar domains and domain walls (DWs), which is particularly important because the Weyl points and the surface Fermi-arcs can depend on the domain reorientation and accompany with intriguing properties. Here, we focus on the type-II WSM MoTe2, which has recently drawn an immense attention due to its phase tunability and unique physical properties. Utilizing in-situ cryogenic transmission electron microscopy, we unveil intriguing polar domain structures in MoTe2. We also discover unexpected domain kinetics under electron beams. Using spatially resolved tunneling spectroscopy, we observe distinct electronic responses of those domains and DWs. These findings are a key step toward engineering the Weyl-node-pair-related physical properties in such polar WSMs. |
Tuesday, March 5, 2019 2:42PM - 2:54PM |
H04.00002: Novel structure in the Mo1-xWxTe2 family Yu Tao, John Schneeloch, Chunruo Duan, Masaaki Matsuda, Sachith Dissanayake, Despina Louca Mo1-xWxTe2 belongs to the family of 2-dimensional transition metal dichalcogenides that are of wide interest because of their fascinating topological properties. Mo1-xWxTe2 undergoes a structural phase transition from the monoclinic 1T' phase at high temperatures, to the orthorhombic Td phase at low temperatures through a first-order structural phase transition. Both phases consist of nearly-identical layers which differ primarily by the layers' in-plane positioning. With elastic neutron scattering, we study the transition between these two structures on single crystals of MoTe2 with and without W-doping. Structural changes including changes in interlayer disorder were observed from the elastic scattering along (2, 0, L) on cooling and warming through the hysteresis. We observed a thus far unreported unit cell doubling phase, Td*, that emerges without disorder on warming from Td, and deduced its layer stacking pattern as 'AABB' rather than the 'ABAB' and 'AAAA' orders for the 1T' and Td phases. We describe the transition in terms of a 1-dimensional Ising model whose interaction coefficients change with temperature. These results clarify in microscopic detail the nature of these phase transitions. |
Tuesday, March 5, 2019 2:54PM - 3:06PM |
H04.00003: Doping dependence of the sliding-layer phase transitions in (Mo,W)Te2 John Schneeloch, Yu Tao, Despina Louca, Chunruo Duan The electronic properties of materials composed of weakly-bound layers can change substantially depending on how the layers are stacked. For example, the monoclinic 1T’ phase of MoTe2 is reported to become a Weyl semimetal when cooled below 250 K into its orthorhombic Td phase, which differs from 1T’ mainly by in-plane displacements of the layers. Substituting Mo for W raises the transition temperature. In this talk, we will focus on the effect of W-doping on Mo1-xWxTe2 crystals up to x=0.5, using neutron scattering to investigate structural changes. The transition temperature continues to increase to at least ~460 K, approximately linearly, as a function of W-doping (as estimated from the c-axis lattice constant). The monoclinic beta angle steadily decreases with doping, from 93.9 to 93.2 degrees. The Td* phase (a thus far unreported phase which will be discussed in Yu Tao’s talk) was present on warming up to x~0.3, but was not seen above this point. Some MoTe2 crystals had broad transitions, likely due to Te vacancies; the existence of a Td* phase in these crystals could not be determined. We discuss these tendencies and their implications for finding new phases in materials with weakly-bound layers. |
Tuesday, March 5, 2019 3:06PM - 3:42PM |
H04.00004: Fermi Surfaces and Topological Character of Dirac and Weyl Type-II Semimetals as Revealed by the de Haas-van Alphen Effect Invited Speaker: Luis Balicas The texture of the Berry phase curvature has led to the observation of novel, but controversial, electrical transport properties |
Tuesday, March 5, 2019 3:42PM - 3:54PM |
H04.00005: Chemical Bonding Induced Topological Lifshitz Transition in Type-II Dirac Semimetal VAl3 Yi-Yuan Liu, Yu-Fei Liu, Gui Xin, Cheng Xiang, Huibing Zhou, Weiwei Xie, Chuang-Han Hsu, Hsin Lin, Tay-Rong Chang, Shuang Jia We report a chemical bonding induced Lifshitz transition of Ti1-xVxAl3 in which the compounds evolve from p-type trivial metal to n-type robust Dirac semimetal. This topological transition is concomitant with an anomalous structural distortion which stems from the interplanar V-Al bond formation. The V-Al bonds in Ti1-xVxAl3 are built up as long as the bonding orbitals of V atoms are fully populated by the electrons. In other words, the type-II Dirac semimetal state of VAl3 is protected by the V-Al bonds whose molecular orbitals are the “gravity center” of the topological electron and hole bands. |
Tuesday, March 5, 2019 3:54PM - 4:06PM |
H04.00006: Fermiology and evidence of conventional superconductivity in the type-II Dirac semimetal PdTe2 Amit Vashist, Radha Krishna Gopal, Yogesh Singh We use electrical transport and heat capacity measurements on high-quality single crystals of the recently discovered superconducting type-II Dirac semimetal PdTe2, to probe the nature of its superconducting phase. The magnitude of the electronic heat-capacity anomaly at Tc, the low temperature exponential T dependence of the heat capacity, the linear H dependence of the T = 0 electronic Somerfield coefficient, and a conventional H − T phase diagram establish that the superconductivity in PdTe2 is conventional in nature despite the presence of a topologically nontrivial Fermi surface band, which contributes to the electrical conduction. The Fermi surface of PdTe2 is investigated by the dHvA oscillations. |
Tuesday, March 5, 2019 4:06PM - 4:18PM |
H04.00007: Topological Nontrivial Surface Plasmon on a Type-II Weyl Semimetal Xun Jia, Xuetao Zhu, Jiandong Guo Type-II Weyl semimetals (WSMs), with the Lorentz invariance broken and hosting tilted Weyl cones giving rise to non-point-like Fermi surfaces (FSs), have attracted plentiful attention recently. The orthorhombic phase of (W, Mo)Te2 is predicted to be a prototypical system of Type-II WSM. However, the topological signatures are not clear enough from the single-electron measurements due to its complicated band structure. Meanwhile, measurements of electronic collective excitations (i. e. plasmons) in topological bands may provide more evidence of the topological nature, since they possess some exotic properties distinct from those in standard electron gases. Here, using momentum resolved inelastic electron scattering, we report an observation of a topological nontrivial surface plasmon mode originated from the topological bands of MoTe2, which displays unique super-linear temperature-dependence matching the predictions for that in a WSM. Moreover, the damping of this nontrivial mode matches well with the boundary of electron-hole particle continuum from Weyl band. These findings provide consolidated evidences of the existence of topological nontrivial bands in MoTe2. |
Tuesday, March 5, 2019 4:18PM - 4:30PM |
H04.00008: Inversion Symmetry Breaking in the Monoclinic Phase of MoTe2 Chunruo Duan, John Schneeloch, Yu Tao, Junjie Yang, Xiaoping Wang, Feng Ye, Despina Louca Among the layered material transition metal dichalcogenides, MoTe2 shows a complex phase diagram and many interesting properties such as magnetoresistance, superconductivity, and potentially Weyl semimetal property. If synthesized by furnace cooling, MoTe2 is stabilized in a hexagonal phase (2H). Quenching from above 1000 °C results in a metastable monoclinic phase (1T’), which becomes orthorhombic (Td) in a first order phase transition when cooled below room temperature. The phase transition is accomplished through layer shifting along the monoclinic tilting direction, and the structure within each layer is not changed. The reported crystal structures indicate that the layer shifting from 1T’ (P21/m) to Td (Pnm21) breaks the inversion symmetry and allows the Weyl physics to emerge. Single crystal neutron diffraction performed at SNS, ORNL provided evidence on a lower symmetry (P21) in the 1T’ phase which is non-centrosymmetric. Ab initio calculations and molecular dynamics simulations based on the refined structure will be discussed. |
Tuesday, March 5, 2019 4:30PM - 4:42PM |
H04.00009: Pressure evolution of the low temperature crystal structure of superconducting Weyl semimetal candidate MoTe2 Colin Heikes, I-Lin Liu, Taner Yildirim, Nicholas Butch, William Ratcliff Orthorhombic MoTe2 has been proposed to be a type II Weyl semimetal. This classification is supported by the observation of rare topological phenomena such as Fermi arcs and Weyl nodes through ARPES measurements. Superconductivity is also observed in this material, with a drastic pressure enhancement of the superconducting transition temperature often associated with a first-order structural transition from the non-centrosymmetric orthorhombic phase to a centrosymmetric monoclinic phase with uncertain band topology. I will discuss the temperature-pressure structural phase diagram of this system as determined by neutron scattering at conditions relevant for superconductivity. I will also comment on possible implications of the structural evolution on the ground state electronic structure. |
Tuesday, March 5, 2019 4:42PM - 4:54PM |
H04.00010: Electronic structure of the candidate Weyl phase in MoTe2 I-Lin Liu, Colin Heikes, Chris Eckberg, Nicholas Butch, William Ratcliff, Johnpierre Paglione Orthorhombic MoTe2 has been proposed to be a type II Weyl semimetal. This classification is supported by the observation of rare topological phenomena such as Fermi arcs and Weyl nodes through ARPES measurements. A first-order structural transition from the centrosymmetric monoclinic phase at room temperature to the orthorhombic phase, without inversion symmetry, was found through both magnetoresistance and neutron scattering measurements. I will discuss quantum oscillations measurements and their ramifications for the electronic band structure, and how this relates to the topological state and corresponding novel quantum phenomena |
Tuesday, March 5, 2019 4:54PM - 5:06PM |
H04.00011: ABSTRACT WITHDRAWN
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