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Register a new userCoexistence of Type-uppercaseexpandafter{romannumeral2} and Type-uppercaseexpandafter{romannumeral4} Dirac Fermions in SrAgBi
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Until now scientists have found tree types of Dirac Fermions in crystals and these three types of Dirac Fermions can be described by one model. Here, we find a type of Dirac fermions that escapes this model. We find this type of Dirac fermions can exist in SrAgBi and is dubbed type-uppercaseexpandafter{romannumeral4} Dirac Fermions. The band near the type-uppercaseexpandafter{romannumeral4} Dirac points is nonlinear and the electron pocket and the hole pocket are from the same band. It is worth pointing out that there is a type-uppercaseexpandafter{romannumeral2} Dirac Fermion near this new Dirac Fermion. So we used two models to describe the coexistence of these two Dirac fermions. Topological surface states of these two Dirac points are also calculated. We envision that our findings will stimulate researchers to study novel physics of type-uppercaseexpandafter{romannumeral4} Dirac fermions, as well as the interplay of type-uppercaseexpandafter{romannumeral2} and type-uppercaseexpandafter{romannumeral4} Dirac fermions.
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Topological semimetals have attracted extensive research interests for realizing condensed matter physics counterparts of three-dimensional Dirac and Weyl fermions, which were originally introduced in high energy physics. Recently it has been proposed that type-II Dirac semimetal can host a new type of Dirac fermions which break Lorentz invariance and therefore does not have counterpart in high energy physics. Here we report the electronic structure of high quality PtSe$_2$ crystals to provide direct evidence for the existence of three-dimensional type-II Dirac fermions. A comparison of the crystal, vibrational and electronic structure to a sister compound PtTe$_2$ is also discussed. Our work provides an important platform for exploring the novel quantum phenomena in the PtSe$_2$ class of type-II Dirac semimetals.
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