Tungsten(IV) telluride ( W Te₂) is an

inorganic In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as '' inorganic chemist ...

semimetal A semimetal is a material with a very small overlap between the bottom of the conduction band and the top of the valence band. According to electronic band theory, solids can be classified as insulators, semiconductors, semimetals, or metals. ...

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chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...

. In October 2014, tungsten ditelluride was discovered to exhibit an extremely large

magnetoresistance Magnetoresistance is the tendency of a material (often ferromagnetic) to change the value of its electrical resistance in an externally-applied magnetic field. There are a variety of effects that can be called magnetoresistance. Some occur in bulk ...

: 13 million percent resistance increase in a magnetic field of 60 Tesla at 0.5 Kelvin. The resistance is proportional to the square of the magnetic field and shows no saturation. This may be due to the material being the first example of a compensated semimetal, in which the number of mobile holes is the same as the number of electrons. Tungsten ditelluride has layered structure, similar to many other transition metal dichalcogenides, but its layers are so distorted that the honeycomb lattice many of them have in common is in WTe₂ hard to recognize. The tungsten atoms instead form zigzag chains, which are thought to behave as one-dimensional conductors. Unlike electrons in other two dimensional semiconductors, the electrons in WTe₂ can easily move between the layers. When subjected to pressure, the magnetoresistance effect in WTe₂ is reduced. Above the pressure of 10.5 GPa magnetoresistance disappears and the material becomes a superconductor. At 13.0 GPa the transition to superconductivity happens below 6.5 K. WTe₂ was predicted to be a

Weyl semimetal Weyl fermions are massless chiral fermions embodying the mathematical concept of a Weyl spinor. Weyl spinors in turn play an important role in quantum field theory and the Standard Model, where they are a building block for fermions in quantum ...

and, in particular, to be the first example of a Type II Weyl semimetal, where the Weyl nodes exist at the intersection of the electron and hole pockets. It has also been reported that terahertz-frequency light pulses can switch the crystal structure of W Te₂ between

orthorhombic In crystallography, the orthorhombic crystal system is one of the 7 crystal systems. Orthorhombic lattices result from stretching a cubic lattice along two of its orthogonal pairs by two different factors, resulting in a rectangular prism with a r ...

and

monoclinic In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in the orthorhombic s ...

by altering the material's atomic lattice. Tungsten ditelluride can be exfoliated into thin sheets down to single layers. Monolayer WTe₂ was initially predicted to remain a Weyl semimetal in the 1T' crystal phase. It was later shown with transport measurements that, below 50K, a single layer of WTe₂ instead acts like an insulator but with an offset current independent of doping by a local electrostatic gate. When using a contact geometry that shorted out conduction along the device edges, this offset current vanished, demonstrating that this nearly quantized conduction was localized to the edge—behavior consistent with monolayer WTe₂ being a two-dimensional

topological insulator A topological insulator is a material whose interior behaves as an electrical insulator while its surface behaves as an electrical conductor, meaning that electrons can only move along the surface of the material. A topological insulator is an ...

. Identical measurements with two- and three-layer thick samples showed the expected semimetallic response. Subsequent studies using other techniques have been consistent with the transport results, including those using

angle-resolved photoemission spectroscopy Angle-resolved photoemission spectroscopy (ARPES) is an experimental technique used in condensed matter physics to probe the allowed energies and momenta of the electrons in a material, usually a crystalline solid. It is based on the photoelec ...

and microwave-impedance microscopy. Monolayer WTe₂ has also been observed to superconduct at moderate doping, with a critical temperature tunable by doping level. Two- and three-layer thick WTe₂ have also been observed to be

polar metal A polar metal, metallic ferroelectric, or ferroelectric metal is a metal that contains an electric dipole moment. Its components have an ordered electric dipole. Such metals should be unexpected, because the charge should conduct by way of the free ...

s, simultaneously hosting metallic behavior and switchable electric polarization. The polarization was theorized to originate from vertical charge transfer between the layers, which is switched by interlayer sliding.

References

{{Use dmy dates, date=March 2018 Tellurides Tungsten compounds Transition metal dichalcogenides Monolayers