Fermions--elementary particles such as electrons--are classified as Dirac, Majorana or Weyl. Majorana and Weyl fermions had not been observed experimentally until the recent discovery of condensed matter systems such as topological superconductors and semimetals, in which they arise as low-energy excitations. Here we propose the existence of a previously overlooked type of Weyl fermion that emerges at the boundary between electron and hole pockets in a new phase of matter. This particle was missed by Weyl because it breaks the stringent Lorentz symmetry in high-energy physics. Lorentz invariance, however, is not present in condensed matter physics, and by generalizing the Dirac equation, we find the new type of Weyl fermion. In particular, whereas Weyl semimetals--materials hosting Weyl fermions--were previously thought to have standard Weyl points with a point-like Fermi surface (which we refer to as type-I), we discover a type-II Weyl point, which is still a protected crossing, but appears at the contact of electron and hole pockets in type-II Weyl semimetals. We predict that WTe2 is an example of a topological semimetal hosting the new particle as a low-energy excitation around such a type-II Weyl point. The existence of type-II Weyl points in WTe2 means that many of its physical properties are very different to those of standard Weyl semimetals with point-like Fermi surfaces.

/pdf/type-ii-weyl-semimetals-3gz387jrsu.pdf

Type-II Weyl semimetals.

The magnetoresistance effect in WTe2, a layered semimetal, is extremely large: the electrical resistance can be changed by more than 13 million per cent at very high magnetic fields and low temperatures. Apply a magnetic field to a magnetoresistive material and its electrical resistance changes — a technologically useful phenomenon that is harnessed, for example, in the data-reading sensors of hard drives. Mazhar Ali and colleagues have now identified a material (tungsten ditelluride or WTe2) in which the magnetoresistance effect is unusually large: the electrical resistance can be changed by more than 13 million per cent. Its remarkable magnetoresitance is evident at very high magnetic fields and at extremely low temperatures, so practical applications are not yet in prospect. But this finding suggests new directions in the study of magnetoresistivity that could ultimately lead to new uses of this effect. Magnetoresistance is the change in a material’s electrical resistance in response to an applied magnetic field. Materials with large magnetoresistance have found use as magnetic sensors1, in magnetic memory2, and in hard drives3 at room temperature, and their rarity has motivated many fundamental studies in materials physics at low temperatures4. Here we report the observation of an extremely large positive magnetoresistance at low temperatures in the non-magnetic layered transition-metal dichalcogenide WTe2: 452,700 per cent at 4.5 kelvins in a magnetic field of 14.7 teslas, and 13 million per cent at 0.53 kelvins in a magnetic field of 60 teslas. In contrast with other materials, there is no saturation of the magnetoresistance value even at very high applied fields. Determination of the origin and consequences of this effect, and the fabrication of thin films, nanostructures and devices based on the extremely large positive magnetoresistance of WTe2, will represent a significant new direction in the study of magnetoresistivity.

pdf/large-non-saturating-magnetoresistance-in-wte2-5fvpsafroa.pdf

Large, non-saturating magnetoresistance in WTe2.

A review on mechanics and mechanical properties of 2D materials—Graphene and beyond

We discovered that many of the commonly studied two-dimensional monolayer transition metal dichalcogenide (TMDC) nanoscale materials are piezoelectric, unlike their bulk parent crystals. On the macroscopic scale, piezoelectricity is widely used to achieve robust electromechanical coupling in a rich variety of sensors and actuators. Remarkably, our density-functional theory calculations of the piezoelectric coefficients of monolayer BN, MoS2, MoSe2, MoTe2, WS2, WSe2, and WTe2 reveal that some of these materials exhibit stronger piezoelectric coupling than traditionally employed bulk wurtzite structures. We find that the piezoelectric coefficients span more than 1 order of magnitude, and exhibit monotonic periodic trends. The discovery of this property in many two-dimensional materials enables active sensing, actuating, and new electronic components for nanoscale devices based on the familiar piezoelectric effect.

Intrinsic Piezoelectricity in Two-Dimensional Materials

We present k.p Hamiltonians parametrized by ab initio density functional theory calculations to describe the dispersion of the valence and conduction bands at their extrema (the K , Q , Γ , and M points of the hexagonal Brillouin zone) in atomic crystals of semiconducting monolayer transition metal dichalcogenides (TMDCs). We discuss the parametrization of the essential parts of the k.p[ Hamiltonians for MoS2 , MoSe2 , MoTe2 , WS2 , WSe2 , and WTe2 , including the spin-splitting and spin-polarization of the bands, and we briefly review the vibrational properties of these materials. We then use k.p theory to analyse optical transitions in two-dimensional TMDCs over a broad spectral range that covers the Van Hove singularities in the band structure (the M points). We also discuss the visualization of scanning tunnelling microscopy maps.

https://iopscience.iop.org/article/10.1088/2053-1583/2/2/022001/pdf

k · p theory for two-dimensional transition metal dichalcogenide semiconductors

The structures of WTe2 and a high-temperature monoclinic polymorph of MoTe2 have been solved by Patterson methods Single crystals were grown by vapor transport methods Cell dimensions, as measured on precession photographs, are for WTe2, a=6282/~, b= 3496/~, c= 14"073 A Similarly for MoTe2, a=633 A, b=3469/~, c=1386/~, B=93°55 Intensities were measured for both crystals from zeroand first-level Weissenberg photographs WTe2 and MoTe2 are given the space groups Pnm21 and P21/m, respectively Minimum function maps prepared by a superposition method gave approximate trial structures for both compounds which were refined by least-squares methods to R values of 125 % and 13-9 % for WTe2 and MoTe2 respectively Both compounds are layer structures with double sheets of tellurium atoms bound together by interleaving metal atoms An off-center positioning of metal atoms in the tellurium octahedra buckles the tellurium sheets and allows metal atoms in adjacent octahedra to approach each other Each metal atom, therefore, has eight neighbors, six tellurium atoms and two metal atoms, and a significant amount of metal-metal bonding is introduced

The crystal structures of WTe2 and high-temperature MoTe2

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The crystal structures of WTe2 and high-temperature MoTe2