Reconstruction of Band Structure Induced by Electronic Nematicity in an FeSe Superconductor

We have performed high-resolution angle-resolved photoemission spectroscopy on an FeSe superconductor ($T_c\sim 8\mathrm{K}$), which exhibits a tetragonal-to-orthorhombic structural transition at $T_s\sim 90\mathrm{K}$. At low temperature, we found splitting of the energy bands as large as 50 meV at the $M$ point in the Brillouin zone, likely caused by the formation of electronically driven nematic states. This band splitting persists up to $T\sim 110\mathrm{K}$, slightly above $T_s$, suggesting that the structural transition is triggered by the electronic nematicity. We have also revealed that at low temperature the band splitting gives rise to a van Hove singularity within 5 meV of the Fermi energy. The present result strongly suggests that this unusual electronic state is responsible for the unconventional superconductivity in FeSe.

Figure 1

(a) ARPES-intensity mapping for FeSe at $E_F$ in a 2D wave-vector plane around the $\mathrm{\Gamma }\text{−}\mathit{M}$ cut obtained with $\mathrm{He}\text{−}\mathrm{I}\alpha$ photons ($h\nu =21.218\mathrm{eV}$). The map is obtained by integrating the spectral intensity within $\pm 5\mathrm{meV}$ of $E_F$. (b),(c) The ARPES intensity and corresponding EDCs, respectively, measured along the $\mathrm{\Gamma }\text{−}\mathit{M}$ cut (the green line) in (a). (d),(e) Near-$E_F$ ARPES intensity around the $\mathrm{\Gamma }$ and $\mathit{M}$ points (along the red and blue lines), respectively, in (a) divided by the Fermi-Dirac distribution function for $T=120\mathrm{K}$ convoluted with the instrumental resolution. Dashed curves are a guide for eyes.

Figure 2

(a),(b) ARPES intensity and corresponding EDCs, respectively, measured along the $\mathrm{\Gamma }\text{−}\mathit{M}$ direction. Dashed curves in (a) trace the $M$-centered holelike bands. (c) Schematic band diagram around the $M$ point below and above $T_s$. Red and blue curves indicate the $d_{yz}$ and $d_{zx}$ orbitals. Solid and dashed curves depict the band dispersion along the (0, 0)-($\pi$, 0) and (0, 0)-(0, $\pi$) directions (long and short Fe-Fe directions) of the untwinned crystal, respectively. (d),(e) The second-derivative plot of the near-$E_F$ ARPES intensity around the $\mathrm{\Gamma }$ point for $T=30$ and 120 K. (f) Temperature dependence of band dispersion around the $\mathrm{\Gamma }$ point, extracted by tracing the peak maxima of the EDCs divided by the Fermi-Dirac distribution function.

Figure 3

(a) Second-derivative near-$E_F$ ARPES intensity plotted as a function of binding energy (B.E.) and wave vector. The data were taken around the around the $M$ point along the $\mathbf{k}$ cut shown by the blue line in Fig. 1 measured using the the He $\mathrm{I}\alpha$ line, at various temperatures. (b),(c), Temperature dependence of the EDC at the $M$ point and its second derivative, respectively. Blue and red dots in (c) indicate the local minima corresponding to the peak position in (b). (d) Temperature dependence of the peak energies in the EDC at the $M$ point [same as dots in (c)]. (e) Same as (a) but measured with circularly polarized 40 eV photons. In addition to the high-energy holelike band seen in (a), other $M$-centered holelike and electronlike bands are observed at $T=30\mathrm{K}$, as indicated by the dashed curves. The dashed curve at $T=80\mathrm{K}$ is a guide for eyes, tracing the near-$E_F$ electronlike band (this band is indistinguishable at $T=30\mathrm{K}$ due to the band degeneracy with the low-energy holelike band). (f) Comparison of the temperature dependence of the electrical resistivity recorded from the same sample used for the ARPES (red curve) [21], the Fe-Fe distance estimated from the x-ray diffraction (green squares) [15], and the magnitude of the band splitting at the $M$ point (purple circles). The blue dashed line in the resistivity plot highlights the anomaly above $T_s$.