Session P10: Type II Weyl Semimetals

Te vacancy driven superconductivity in type II Weyl semimetal MoTe2

Two-dimensional (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 Te-deficient orthorhombic Td-MoTe2−x with an intrinsic electron-doping, 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 x-ray 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 vacancy-induced superconductivity is emerged in orthorhombic MoTe2, which is predicted as Weyl semimetal, via electron-doping. This chalcogen vacancy induced-superconductivity provides a new route for cultivating superconducting state together with WSM state in 2D van der Waals materials.
  

Origin of Structural Phase Transition and Superconductivity in Different Phases of MoTe2; A First-Principles Study

MoTe₂ has recently attracted much attention due to its rich properties including Weyl Semimetallic (WS) behaviour and superconductivity [1,2]. MoTe₂ 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 (T-P) 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 electron-phonon coupling and superconductivity in both Weyl-semimetal phase (T_d) and 1T' phases with and without the spin-orbit coupling. A detailed understanding the origin of phase transition between T_d and 1T' phases will allow us to tune the final structure in T-P plane and, in turn, to control the physical properties of these interesting systems.

[1] Y. Qi et al, Nat. Commun. 7, 11038 (2016).
[2] J. Jiang et al., Nat. Commun. 8,13973 (2017).   

Correlating the crystal structure and electronic ground state of type-II Weyl semimetal MoTe$_2$: structural measurements

The reported pressure dependent superconductivity of the type-II Weyl semimetal MoTe$_2$ has led to speculation about the possibility of a topologically non-trivial 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 non-centrosymmetric 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.   

Correlating the crystal structure and electronic ground state of type-II Weyl semimetal MoTe2: Transport measurements

The reported pressure dependent superconductivity of the type-II Weyl semimetal MoTe₂ has led to speculation about the possibility of a topologically non-trivial superconducting state in this material. A key question about the topology of this superconducting state is the evolution of the crystal structure of MoTe₂ at pressures and temperatures relevant for superconductivity, as the non-centrosymmetric T_d orthorhombic phase is needed for the Weyl state. Semimetallic MoTe₂ 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 MoTe₂, 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.   

Disorder and the 1T'-Td structural phase transition in MoTe2

The transition metal dichalcogenide MoTe₂ has been much studied recently due to a reported Weyl semimetal state in its low-temperature 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 MoTe₂ 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.   

Disorder-induced phase transitions of type-II Weyl semimetals

Weyl semimetals are a newly discovered class of materials that host relativistic massless Weyl fermions as their low-energy bulk excitations. Among this class of materials, there exist two types of semimetals that are of particular interest: type-I Weyl semimetals, which have broken inversion or time-reversal symmetry, and type-II Weyl semimetals, which additionally break Lorentz invariance, hosting tilted Weyl cone. In this work, we use the Born approximation to demonstrate that the type-I Weyl semimetals undergo a quantum phase transition to type-II Weyl semimetals in the presence of the finite Anderson disorder when non-zero 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 disorder-induced phase transition in Weyl semimetals by using exact diagonalization of a three-dimensional tight-binding model to calculate the resultant phase diagram of the
type-I Weyl semimetal.

Reference : M. J. Park, B. Basa, and M. J. Gilbert Phys. Rev. B 95, 094201 (2017)   

Anomalous Transport in Type-II Weyl Semimetals

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 type-II 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 non-monotonic 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 type-II WSM show promise for thermoelectric applications.   

Type-II Symmetry-Protected Topological Dirac Semimetals

The recent proposal of the type-II Weyl semimetal state has attracted significant interest. In this work, we propose the concept of the three-dimensional type-II Dirac fermion and theoretically identify this new symmetry-protected topological state in the large family of VAl₃ family (VAl₃, Nb Al₃, TaAl₃, NbGa₃, and TaGa₃). We show that the VAl₃ family features a pair of strongly Lorentz-violating type-II Dirac nodes and that each Dirac node can be split into four type-II Weyl nodes with chiral charge ±1 via symmetry breaking. Furthermore, we predict that the Landau level spectrum arising from the type-II Dirac fermions in VAl₃ is distinct from that of known Dirac or Weyl semimetals. We also demonstrate a topological phase transition from a type-II Dirac semimetal to a quadratic Weyl semimetal or a topological crystalline insulator via crystalline distortions [1].

[1] T.-R. Chang et al., Phys. Rev. Lett. 119, 026404 (2017)   

Quantum Oscillations in the Type-II Dirac Semimetals: MAl3 (M = V, Nb and Ta)

Recent work shows that MAl₃ (M = V, Nb and Ta) are type-II topological Dirac semimetals with a pair of strong Lorentz-violating Dirac nodes. We synthesized single crystals by using an Al flux method and measured the de Hass-van Alphen effect by using torque magnetomery. From the angular-dependence of the quantum oscillations of MAl₃, the geometry of the Fermi surface is identified and found to be consistent with first-principle calculations. The Dirac points are located the Fermi level at about 100 meV, 230 meV and 250 meV in VAl₃, NbAl₃ and TaAl₃, 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 VAl₃ family.
  

Fermi surface of the type-II Dirac semimetallic candidates PdTe2 and PtTe2.

We investigated the electronic structures at the Fermi level and the transport properties via the Shubnikov-de Haas (SdH) and de Haas van Alphen (dHvA) effects in single-crystals of PdTe₂ and PtTe₂, recently claimed to be candidates for a novel type-II Dirac semimetallic state. We synthesized high-quality single-crystals, i.e. with mobilities in excess of 10⁴ cm²/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 Lifshitz-Kosevich 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 non-trivial Berry phase for PdTe₂ compound.   

Huge magnetoresistance and extremely large conductivity in the type-II Weyl semimetals WP2 and MoP2

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, WP₂ and MoP₂, which are type-II Weyl semimetals with robust Weyl points by transport, angle resolved photoemission spectroscopy and first principles calculations. Our single crystals of WP₂ display an extremely low residual low-temperature 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 WP₂ from transport measurements. These properties are likely a consequence of the novel Weyl fermions expressed in this compound.   

Stable Weyl points and trivial surface states in carrier compensated semimetal WP2

The possible relation between the extremely large magneto-resistance (XMR) and the presence of Weyl points has recently drawn a lot of attention in the study of topological semimetals. Here we combine angle-resolved 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 bulk-band structure of WP2 is particle hole-compensated and it is compatible with the presence of at least 4 Weyl points, confirming its topological nature.   

Fermi surface of the Weyl type-II metallic candidate WP2

Here, we present a study of the Fermi surface of WP₂ via the Shubnikov-de Haas (SdH) effect. Compared to other semimetals, WP₂ exhibits a very low residual resistivity, i.e., ρ₀ = 10 nΩcm, which leads to perhaps the largest nonsaturating magnetoresistivity [ρ(H)] reported for any compound. For the samples displaying the smallest ρ₀, ρ(H) is observed to increase by a factor of 2.5 × 10⁷ % under μ₀H = 35 T at T = 0.35 K. The angular dependence of the SdH frequencies is found to be in excellent agreement with the first-principles 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.