Type II Dirac Fermion

The standard model of particle physics describes all the known elementary particles, like electrons and quarks. Many of these particles have analogs in condensed matter, where they arise as collective states, or quasiparticles. One example is an electronic state in graphene that behaves like a massless Dirac fermion—a spin-1/2 particle that is not its own antiparticle. But condensed-matter physics may offer a longer list of “elementary particles” than found in the standard model. This is due to the fact that—unlike fundamental particles—quasiparticles in solids are not constrained by so-called Lorentz invariance. A Lorentz-violating quasiparticle is one whose momentum-energy relation depends on the direction it travels. Three separate teams  have collected the first experimental evidence of quasiparticles called type-II Dirac fermions, which break Lorentz invariance. These electronic states, which have no counterpart in the standard model, could be associated with a new type of superconductivity, which has potential applications in thermoelectric devices and quantum computing.

For type-I Dirac [and Weyl semimetals  which obey Lorentz invariance, massless Dirac fermions with linear dispersions are expected at the Dirac or Weyl points. Type-II Dirac and Weyl fermions  emerge at the topologically protected touching points of electron and hole pockets, and they show highly tilted Dirac cones along certain momentum direction (see schematics bottom figure), thereby breaking the Lorentz invariance . The anisotropic electronic structure can also lead to anisotropic MR, and negative MR is expected only along directions where the cones are not tilted enough to break the Lorentz invariance . Type-II Dirac semimetal can be tuned to a Weyl semimetal or topological crystalline insulator when the crystal symmetry or time reversal symmetry is broken , and therefore they are ideal candidates for investigating topological phase transitions and potential device applications.

Materials: PtTe2, PtSe2,,PdTe2,


https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.119.016401

https://arxiv.org/abs/1607.03643


https://journals.aps.org/prb/abstract/10.1103/PhysRevB.96.041201

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.95.155112

website:
http://nccr-marvel.ch/fr/news/highlights/2017-07-typeII-dirac-fermions-now-experimetally-verified