Weyl semimetals

Topological semimetals are a peculiar gapless state with massless relativistic electrons and are good candidates to possess high mobility, a key parameter to enhance the efficiency of a device. The band touching point is called Weyl point, which acts as a magnetic monopole in momentum space and always comes in pairs.

Among others, NbP, TaP, NbAs, and TaAs have recently been predicted as a candidate for Weyl semimetal. All have tetragonal structure with non-centrosymmtric space group I4₁md (a Weyl semimetal must have either inversion or time reversal symmetry broken). Recently, we found that NbP combines the main features of the WTe₂-type (extremely large MR) and Cd₃As₂-type (ultrahigh mobility) semimetals in the band structure, exhibiting hole pockets from normal quadratic bands and electron pockets from linear Weyl bands. It exhibits ultrahigh carrier mobility comparable to that of Cd₃As₂ and an extremely large MR surpassing that of WTe₂ owing to the electron-hole resonance situation. Interestingly, a Weyl semimetal should exhibit chiral anomaly in transport measurement in the form of negative magnetoresistance in parallel electric and magnetic field (Adler-Bell-Jackiw anomaly). We observe large and nonsaturating negative magnetoresistance until 14 T external fields in Weyl semimetal. We are interested in systematically investigating such effects in other related materials as well.