Isotropic superconducting gaps with enhanced pairing on electron Fermi surfaces in FeTe${}_{0.55}$Se${}_{0.45}$

We used angle-resolved photoemission spectroscopy to reveal directly the momentum distribution of the superconducting gap in FeTe${}_{1-x}$Se${}_x$, which has the simplest structure of all Fe-based superconductors. We found isotropic superconducting gaps on all Fermi surfaces whose sizes can be fitted by a single gap function derived from a strong coupling approach, promoting local antiferromagnetic exchange interactions as a serious candidate for the pairing origin.

Figure 1

(a)–(d) ARPES intensity plots along $\Gamma$-M. (a) Near $\Gamma$, in the normal state ($T=25$ K). Blue and black dotted lines indicate the $k$${}_F$ positions of the ${\alpha }^{\prime }$ and $\beta$ bands, respectively. (b) Same as (a) but in the SC state ($T=6$ K). (c) Near M, normal state ($T=25$ K). (d) Near M, SC state ($T=6$ K). The corresponding EDCs are given below each panel in (e)–(h). Thick bars are guides for the eye.

Figure 2

(a) Temperature dependence of the SC gap of the $\beta$ band. (b) Comparison of the SC gaps of the ${\alpha }^{\prime }$, $\beta$, and $\gamma$ bands. (c) and (d) $k$ dependence of symmetrized EDCs along the $\beta$ and $\gamma$ FSs, respectively. (e) Schematic FS indicating positions where the SC gap size has been measured. Dotted FSs are folded from the BZ of a pure Fe plane due to the alternate position of the chalcogen atoms above and below the Fe plane. (h) Polar plot of the SC gaps with dotted circles indicating the average values on each FS pocket.

Figure 3

(a) and (b) ARPES intensity plots along the $\Gamma$-M high-symmetry line measured with $S$ and $P$ polarizations, respectively. (c) Schematic band structure and dominant orbital character of each band at $\Gamma$. (d) SC gap correspondence between ARPES and STM measurements. Blue, red, and green lines are $k_F$ symmetrized EDCs of the ${\alpha }^{\prime }$, $\beta$, and $\gamma$ band, respectively. Dashed arrows indicate the correspondence between ARPES and STM (black curve, from Ref. 22) features. (e) Comparison of ARPES data with optical conductivity (black curve, from Ref. 23). Colored dashed lines position the 2$\Delta$ values of ${\alpha }^{\prime }$, $\beta$, and $\gamma$ band, respectively.

Figure 4

(a) 3D representation of the SC gap with the FS topology. (b) Fit of the SC gap data with a $|{\Delta }_2\cos k_x\cos k_y|$ gap function. (c) Fit of the SC gap data with a $|{\Delta }_2\cos k_x\cos k_y-{\Delta }_3(\cos 2k_x+\cos 2k_y)/2|$ gap function. The horizontal and vertical error bars represent the uncertainties on $k_F$ positions and their corresponding SC gap values. (d) In-plane $k$ distribution of the $|{\Delta }_2\cos k_x\cos k_y-{\Delta }_3(\cos 2k_x+\cos 2k_y)/2|$ SC gap function along with the FSs.