Orbital mixing at the onset of high-temperature superconductivity in ${\mathrm{FeSe}}_{1-x}{\mathrm{Te}}_x/{\mathrm{CaF}}_2$

We perform systematic high-resolution angle-resolved photoemission spectroscopy of iron-chalcogenide superconductor ${\mathrm{FeSe}}_{1-x}{\mathrm{Te}}_x$ films on ${\mathrm{CaF}}_2$ which exhibit a unique paramagnetic nematicity at $x=0$ (pristine FeSe) and a gigantic $T_{\mathrm{c}}$ enhancement at the critical Te concentration ($x_{\mathrm{c}}$) of $x\sim 0.2$. Upon increasing the Te concentration from $x=0$, the electronlike Fermi-surface shape at the Brillouin-zone corner shows a clear change associated with a remarkable energy shift of the $d_{xz/yz}$ orbital, indicative of the suppression of nematicity near $x_{\mathrm{c}}$. Evolution of band structure at the Brillouin-zone center is characterized by a drastic upward shift of the $d_{xy}$ band with increasing $x$, leading to an orbital switching from $d_{xz/yz}$ to $d_{xz/yz}+d_{xy}$ accompanied by a mass enhancement. These results demonstrate that the pristine and high-$T_{\mathrm{c}}{\mathrm{FeSe}}_{1-x}{\mathrm{Te}}_x$ have distinctly different electronic structures. The present study lays the foundation for understanding the origin of high-$T_{\mathrm{c}}$ superconductivity and the interplay with electronic nematicity.

Figure 1

(a) Phase diagram of ${\mathrm{FeSe}}_{1-x}{\mathrm{Te}}_x/{\mathrm{CaF}}_2$ as a function of Te concentration $x$ [23]. Also shown are plots of (b) the APRES intensity and (c) the corresponding second-derivative intensity, measured along the $\mathrm{\Gamma }-M$ line at $T=30$ K for FeSe/${\mathrm{CaF}}_2$ ($x=0$) with linearly polarized 21.2-eV photons. The temperature dependence of the second-derivative ARPES intensity at the $M$ point is shown for (d) $x=0$, (e) $x=0.1$, (f) $x=0.2$, and (g) $x=0.3$. The red curves in (c)–(g) are a guide for the eyes to trace the band dispersions.

Figure 2

Te-concentration dependence of the ARPES-intensity map at $E_{\mathrm{F}}$ as a function of the two-dimensional wave vector for ${\mathrm{FeSe}}_{1-x}{\mathrm{Te}}_x/{\mathrm{CaF}}_2$, with (a) $x=0.0$, (b) $x=0.1$, (c) $x=0.2$, (d) $x=0.3$, and (e) $x=0.4$, at $T=30$ K. Black curves are the schematic of FSs. Also shown is the near-$E_{\mathrm{F}}$ second-derivative ARPES intensity obtained at $T=30$ K along yellow and green lines, around the (f)–(j) $\mathrm{\Gamma }$ and (k)–(o) $M$ points, in (a)–(e), respectively. Cyan and red curves are a guide for the eyes to trace the band dispersions before and after hybridization, respectively. The band dispersions before the hybridization were estimated by extrapolating the dispersions determined in the $(E,\mathbf{k})$ region away from the intersection point. (p) and (q) Comparison of momentum distribution curves at $E_{\mathrm{F}}$ extracted from (f)–(j) and (k)–(o), respectively. Dashed lines are a guide for the eyes to trace the peak position. (r) Te-concentration dependence of an anisotropy of the electronlike FS $(k_a-k_b)/(k_a+k_b)$ (blue circles), compared with the nematic energy scale $2\mathrm{\Delta }E$ (red circles) approximated as twice the energy difference of the $d_{yz}$ band between $T=30$ and 110 K.

Figure 3

(a) Comparison of the experimental near-$E_{\mathrm{F}}$ band dispersions extracted by tracing the peak position of ARPES spectra. (b) Schematics of the band structure around the $\mathrm{\Gamma }$ point for $x<x_{\mathrm{c}}$ (left) and $x\ge x_{\mathrm{c}}$ (right).

Figure 4

(a) Schematic FSs and band dispersions at $T=30$ K for $x<x_{\mathrm{c}}$ (left) and $x\ge x_{\mathrm{c}}$ (right), together with the phase diagram of ${\mathrm{FeSe}}_{1-x}{\mathrm{Te}}_x/{\mathrm{CaF}}_2$. (b) Comparison of the $x$ dependence of the nematic energy scale $2\mathrm{\Delta }E$ [red circles; same as the data in Fig. 2(r)], energy position of the $d_{xy}$-derived hole band at $\mathrm{\Gamma }$, $E_{xy}$ (green circles), and an inverse of $v_{\mathrm{F}}$ for the hole pocket (blue circles).