Electric transport of a single-crystal iron chalcogenide FeSe superconductor: Evidence of symmetry-breakdown nematicity and additional ultrafast Dirac cone-like carriers

An SDW antiferromagnetic (SDW-AF) low-temperature phase transition is generally observed and the AF spin fluctuations are considered to play an important role for the superconductivity pairing mechanism in FeAs superconductors. However, a similar magnetic phase transition is not observed in FeSe superconductors, which has caused considerable discussion. We report on the intrinsic electronic states of FeSe as elucidated by electric transport measurements under magnetic fields using a high quality single crystal. A mobility spectrum analysis, an ab initio method that does not make assumptions on the transport parameters in a multicarrier system, provides very important and clear evidence that another hidden order, most likely the symmetry broken from the tetragonal C4 symmetry to the C2 symmetry nematicity associated with the selective $d$-orbital splitting, exists in the case of superconducting FeSe other than the AF magnetic order spin fluctuations. The intrinsic low-temperature phase in FeSe is in the almost compensated semimetallic states but is additionally accompanied by Dirac cone-like ultrafast electrons $\sim {10}^4{\mathrm{cm}}^2{\left(\mathrm{VS}\right)}^{-1}$ as minority carriers.