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.

Figure 1

(a) Schematic picture of the transport measurements of FeSe under magnetic fields. (b) X-ray diffraction profile of the FeSe single crystal along the $c$-axis direction at room temperature. (c) Temperature dependence of the resistivity of FeSe single crystal. (d), (e) Temperature evolution of electronic structures of FeSe and magnetic field dependence of magnetoresistance [$R_{xx}\left(B\right)/R\left(0\right)$] and Hall resistivity $\left({\rho }_{yx}\right)$ at various temperatures between 12 and 200 K. Inset of (b) shows a magnified plot of $R_{xx}\left(B\right)/R\left(0\right)$ between 80 and 200 K.

Figure 2

(a), (b) The normalized conductivities $X\left(B\right)$ and $Y\left(B\right)$ calculated from the experimental data. Solid lines are partial conductivities of electronlike (blue), holelike (orange) carriers, $[X^{\left(n\right)}\left(B\right),Y^{\left(n\right)}\left(B\right)]$ and $[X^{\left(p\right)}\left(B\right),Y^{\left(p\right)}\left(B\right)]$ employing the KK transformation. Also the summation of $[X^{\left(n\right)}\left(B\right),Y^{\left(n\right)}\left(B\right)]$ and $[X^{\left(p\right)}\left(B\right),Y^{\left(p\right)}\left(B\right)] \left(\left[X\right(B),Y(B\left)\right]\right)$ was plotted in the solid lines (red).

Figure 3

Mobility spectra of electronlike and holelike carriers for FeSe in the low-temperature orthogonal phase displayed on a semilogarithmic scale.

Figure 4

(a), (b) Temperature dependencies of carrier numbers $\left(n\right)$ and electron (hole) mobility ${\mu }_e \left({\mu }_h\right)$ derived using a two carrier-type semiclassical model in the low $B$ limit, assuming an electron and hole compensated electronic structure. $n_{\mathrm{MS}}$ is the averaged carrier number of electronlike and holelike carriers ($n_{\mathrm{MS}}{}^k,k=n,p$) estimated from the mobility spectra. The error bars of $n,{\mu }_e$, and ${\mu }_h$ are estimated from errors of least square fit of transport parameters and are smaller than size of symbols.