Decoupled interband pairing in a bilayer iron-based superconductor evidenced by ultrahigh-resolution ARPES

Abstract

We present direct experimental evidence of a weakly coupled multiband superconducting state in the bilayer iron-based superconductor ACa$_2$Fe$_4$As$_4$F$_2$ (A = K, Cs) via ultrahigh-resolution angle-resolved photoemission spectroscopy (ARPES). Remarkably, the K-containing compound exhibits two distinct transition temperatures, corresponding to two separate sets of bilayer-split bands, as evidenced by temperature-dependent superconducting gap and spectral weight near the Fermi energy, while its Cs counterpart displays conventional single transition behavior. These experimental observations are well described by the weakly coupled two-band model of Eilenberger theory, which identifies suppressed interband pairing interactions between the bilayer-split bands as the key mechanism. By exploring quantum phenomena in the weak-coupling limit within a multiband system, our findings pave the way for engineering exotic superconductivity via band-selective pairing control.

Figures (4)

• Figure 1: Fermi surface topology and superconducting gap characteristics of K12442 (upper panel) and Cs12442 (lower panel) measured at 4 K. (a) Photoemission contour for K12442 at a binding energy of 10 meV. (b) Momentum-dependent symmetrized EDCs corresponding to the hole bands at the $\Gamma$ point for K12442, with original EDCs shown in Supplemental Figs. S1 (b), (e) SupplMat. (c) Extracted energy gap size as a function of the marked angle shown in (b). The light hollow circles represent symmetrized data at $\theta$ = 45$^{\circ}$ to highlight the symmetry. (d)--(f) Corresponding measurements on Cs12442. Dashed lines of various colors indicate the corresponding folded bands.
• Figure 2: Temperature-dependent measurements along $\Gamma$-X in K12442 (a--d) and Cs12442 (e--h). (a), (e) High resolution photoemission spectra acquired near the Fermi energy at 4 K. (d), (h) Symmetrized EDCs for the hole bands near the $\Gamma$ point from 4 K to 40 K. Green guide lines highlight the temperature evolution of the superconducting gap. The determination of $k_F$ for the $\beta$ band is discussed in Supplemental Discussion III, and the original EDCs are shown in Supplemental Fig. S2 SupplMat. (b), (f) Temperature dependence of the extracted energy gaps from panels (d) and (h), respectively. Solid and dashed curves represent fits to the BCS gap function. (c), (g) Temperature-dependent intensity integrated within $\pm$ 0.5 meV of the Fermi energy for individual bands.
• Figure 3: Measurements on K12442 near the M point. (a) Photoemission spectrum at 4 K for the momentum cut illustrated in the inset. The dispersions of the $\varepsilon$ and $\delta$ bands are schematized by the solid line. (b) Temperature-dependent symmetrized EDCs for the $\varepsilon$ band (bolded at $T_\text{c}^*$). (c) Temperature dependence of the superconducting gap in the $\varepsilon$ band fitted with a BCS function (dashed line). (d) Spectral weight intensity of the $\varepsilon$ band within $\pm ~0.5$ meV of the Fermi level as a function of temperature.
• Figure 4: Simulated energy gaps as a function of temperature based on the Eilenberger two-band model for varying interband coupling strengths. The interband coupling coefficients are defined as follows: weak coupling ($\lambda_{12} = \lambda_{21} = 0.001$), intermediate coupling ($\lambda_{12} = \lambda_{21} = 0.01$), and strong coupling ($\lambda_{12} = \lambda_{21} = 0.1$). The remaining parameters are fixed at $n_1 = n_2 = 0.5$, $\lambda_{11} = 1$, and $\lambda_{22} = 0.9$. A detailed discussion of the simulation methodology and results is provided in the Supplemental Discussion V SupplMat. The inset shows the temperature-dependent gaps for the bilayer-split bands $\beta_1$ and $\beta_2$ in K12442 under weak interband coupling.