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We investigate the orthorhombic phase $({T}_{d})$ of the layered transition-metal dichalcogenide ${\mathrm{MoTe}}_{2}$ as a Weyl semimetal candidate. ${\mathrm{MoTe}}_{2}$ exhibits four pairs of Weyl points lying slightly above $(\ensuremath{\sim}6\phantom{\rule{0.16em}{0ex}}\mathrm{meV})$ the Fermi energy in the bulk band structure. Different from its cousin ${\mathrm{WTe}}_{2}$, which was recently predicted to be a type-II Weyl semimetal, the spacing between each pair of Weyl points is found to be as large as 4% of the reciprocal lattice in ${\mathrm{MoTe}}_{2}$ (six times larger than that of ${\mathrm{WTe}}_{2}$). When projected onto the surface, the Weyl points are connected by Fermi arcs, which can be easily accessed by angle-resolved photoemission spectroscopy due to the large Weyl point separation. In addition, we show that the correlation effect or strain can drive ${\mathrm{MoTe}}_{2}$ from a type-II to a type-I Weyl semimetal.

Three-dimensional Dirac semimetals (DSMs) are materials that have massless Dirac electrons and exhibit exotic physical properties. It has been suggested that structurally distorting a DSM can create a topological insulator but this has not yet been experimentally verified. Furthermore, Majorana fermions have been theoretically proposed to exist in materials that exhibit both superconductivity and topological surface states. Here we show that the cubic Laves phase ${\mathrm{Au}}_{2}\mathrm{Pb}$ has a bulk Dirac cone that is predicted to gap on cooling through a structural phase transition at 100 K. The low temperature phase can be assigned a ${\mathrm{Z}}_{2}=\ensuremath{-}1$ topological index, and this phase becomes superconducting below 1.2 K. These characteristics make ${\mathrm{Au}}_{2}\mathrm{Pb}$ a unique platform for studying the transition between bulk Dirac electrons and topological surface states as well as studying the interaction of superconductivity with topological surface states, combining many different properties of emergent materials---superconductivity, bulk Dirac electrons, and a topologically nontrivial ${\mathrm{Z}}_{2}$ invariant.

We report an observation of a pressure-induced breakdown of the three-dimensional Dirac semimetal (3DDSM) state in Cd3As2, evidenced by a series of in situ high-pressure synchrotron x-ray diffraction and single-crystal transport measurements. We find that Cd3As2 undergoes a structural phase transition from a metallic tetragonal phase in space group I4(1)/acd to a semiconducting monoclinic phase in space group P2(1)/c at critical pressure 2.57 GPa; above this pressure, an activation energy gap appears, accompanied by distinct switches in Hall resistivity slope and electron mobility. These changes of crystal symmetry and corresponding transport properties manifest the breakdown of the 3DDSM state in pressurized Cd3As2.

We report the magnetic and electronic properties of ${\mathrm{CaMn}}_{2}{\mathrm{Bi}}_{2}$, which has a structure based on a triangular bilayer of Mn, rather than the ${\mathrm{ThCr}}_{2}{\mathrm{Si}}_{2}$ structure commonly encountered for 122 compounds in intermetallic systems. ${\mathrm{CaMn}}_{2}{\mathrm{Bi}}_{2}$ has an antiferromagnetic ground state, with a ${\mathrm{T}}_{N}$ of 150 K, and for a 250 K temperature range above ${\mathrm{T}}_{N}$ does not exhibit Curie-Weiss behavior, indicating the presence of strong magnetic correlations at high temperatures. Resistivity measurements show that ${\mathrm{CaMn}}_{2}{\mathrm{Bi}}_{2}$ exhibits semiconducting properties at low temperatures, with an energy gap of only 62 meV, indicating it to be a very narrow band gap semiconductor. The electronic structure of ${\mathrm{CaMn}}_{2}{\mathrm{Bi}}_{2}$, examined via ab-initio electronic structure calculations, indicates that Mn 3d orbital hybridization is essential for the formation of the band gap, suggesting that ${\mathrm{CaMn}}_{2}{\mathrm{Bi}}_{2}$ may be a hybridization-gap semiconductor.

The layered, noncentrosymmetric heavy element ${\text{PbTaSe}}_{2}$ is found to be superconducting. We report its electronic properties accompanied by electronic-structure calculations. Specific heat, electrical resistivity, and magnetic-susceptibility measurements indicate that PbTaSe${}_{2}$ is a moderately coupled, type-II BCS superconductor (${T}_{c}=3.72$ K, Ginzburg--Landau parameter $\ensuremath{\kappa}=17$) with an electron-phonon coupling constant of ${\ensuremath{\lambda}}_{ep}=0.74$. Electronic-structure calculations reveal a single bulk three-dimensional Dirac cone at the $K$ point of the Brillouin zone derived exclusively from its hexagonal Pb layer; it is similar to the feature found in graphene except there is a 0.8 eV gap opened by spin-orbit coupling. The combination of large spin-orbit coupling and lack of inversion symmetry also results in large Rashba splitting on the order of tenths of an eV.