Transport Properties of Semimetallic Transition Metal Dichalcogenides

The Weyl semimetal requires the breaking of either the time-reversal symmetry (TRS) or the lattice inversion symmetry. When the TRS and inversion symmetry coexist, a pair of degenerate Weyl points may exist, leading to the related Dirac semimetal phase. In other words, a Dirac semimetallic state can be regarded as two copies of Weyl semimetal states. In this dissertation, we study tellurium based compounds like the Weyl semimetal candidate MoTe2 and the Dirac semimetal candidate PtTe2 within the transition metal dichalcogenides family. Firstly, we report a systematic study on the Hall-effect of the semi-metallic state of bulk MoTe2, which was recently claimed to be a candidate for a novel type of Weyl semi-metallic state. The temperature (T) dependence of the carrier densities and of their mobilities, as estimated from a numerical analysis based on the isotropic two-carrier model, indicates that its exceedingly large and non-saturating magnetoresistance may be attributed to a near perfect compensation between the densities of electrons and holes at low temperatures. A sudden increase in hole density, with a concomitant rapid increase in the electron mobility below T ∼ 40 K, leads to comparable densities of electrons and holes at low temperatures suggesting a possible electronic phase-transition around this temperature. Secondly, the electronic structure of semi-metallic transition-metal dichalcogenides, such as WTe2 and orthorhombic γ−MoTe2, are claimed to contain pairs of Weyl points or linearly touching electron and hole pockets associated with a non-trivial Chern number. For this reason, these compounds were recently claimed to conform to a new class, deemed type-II, of Weyl semi-metallic systems. A series of angle resolved photoemission experiments (ARPES) claim a broad agreement with these predictions detecting, for example, topological Fermi arcs at the surface of these crystals. We synthesized single-crystals of semi-metallic MoTe2 through a Te flux method to validate these predictions through measurements of its bulk Fermi surface (FS) via quantum oscillatory phenomena. We find that the superconducting transition temperature of γ−MoTe2 depends on disorder as quantified by the ratio between the room- and low-temperature resistivities, suggesting the possibility of an unconventional superconducting pairing symmetry. Similarly to WTe2, the magnetoresistivity of γ−MoTe2 does not saturate at high magnetic fields and can easily surpass 106 %. Remarkably, the analysis of the de Haas-van Alphen (dHvA) signal superimposed onto the magnetic torque, indicates that the geometry of its FS is markedly distinct from the calculated one. The dHvA signal also reveals that the FS is affected by the Zeeman-effect precluding the extraction of the Berry-phase. A direct comparison between the previous ARPES studies and density-functional-theory (DFT) calculations reveals a disagreement in the position of the valence bands relative to the Fermi level εF . Here, we show that a shift of the DFT valence bands relative to εF , in order to match the ARPES observations, and of the DFT electron bands to explain some of the observed dHvA frequencies, leads to a good agreement between the calculations and the angular dependence of the FS cross-sectional areas observed experimentally. However, this relative displacement between electron- and hole-bands eliminates their crossings and, therefore, the Weyl type-II points predicted for γ−MoTe2. Finally, we investigate the electronic structure and transport properties in single crystals of the semi-metallic platinum ditelluride (PtTe2), recently claimed to be a novel type-II Dirac semimetal, via a methodology similar to that applied to γ−MoTe2, i.e. the temperature and angular dependence of the SdH and dHvA effects. Our high-quality PtTe2 crystal displays a large non-saturating magnetoresistance under magnetic field up to 61 T. The dHvA oscillation and SdH effect reveal several high and low frequencies suggesting a rather complex Fermi surface. We also find evidence for a non-trivial Berry phase. The crystal quality improved considerably under subsequent annealing at high-temperatures leading to the observation of linear in field magnetoresistivity. Combined with effective masses in the order of ∼ 0.1 free electron mass, these results further suggest that PtTe2 displays bulk Dirac-like bands.