Why $T_c$ of (CaFeAs)${}_{10}$Pt${}_{3.58}$As${}_8$ is twice as high as (CaFe${}_{0.95}$Pt${}_{0.05}$As)${}_{10}$Pt${}_3$As${}_8$

Recently discovered (CaFe${}_{1-x}$Pt${}_x$As)${}_{10}$Pt${}_3$As${}_8$ and (CaFeAs)${}_{10}$Pt${}_{4-y}$As${}_8$ superconductors are very similar materials having the same elemental composition and structurally similar superconducting FeAs slabs. Yet the maximal critical temperature achieved by changing Pt concentration is approximately twice higher in the latter. Using angle-resolved photoemission spectroscopy (ARPES) we compare the electronic structure of their optimally doped compounds and find drastic differences. Our results highlight the sensitivity of critical temperature to the details of fermiology and point to the decisive role of band-edge singularities in the mechanism of high-$T_c$ superconductivity.

Figure 1

ARPES data taken on 1048-35. (a) shows the FS map measured using $p$-polarized light with an excitation energy of 80 eV. (b) and (c) show energy distribution map (EDM) cuts taken at $\Gamma$ along the direction shown by arrow on the FS map, measured using $p$- and $s$-polarized lights, respectively. (d) shows EDM cut taken at $M$ along the direction shown by a blue arrow on the FS (inset) map, measured using $s$-polarized light. (e) is the second derivative of EDM shown in (d). (f) and (g) are second derivatives of EDMs shown in (b) and (c), respectively. In the figure double circles and double sided arrow represent $p$- and $s$-polarized lights, respectively.

Figure 2

ARPES data taken on 1048-35. (a)–(d) show photon energy dependent EDMs measured at the zone center. (e) shows momentum distribution curves taken over an integration range of 10 meV with respect to the Fermi level, derived from EDMs shown in (a)–(d). Vertical blue lines in the figure (e) represent peak positions of the bands ${\alpha }_2$ and ${\alpha }_3$.

Figure 3

ARPES data taken on 1038-15. (a) and (b) show EDMs taken at the zone center measured using an excitation energy of 70 eV with $p$- and $s$-polarized lights, respectively. (c) and (d) are second derivatives of EDMs shown in (a) and (b), respectively. (e) is EDM taken at the zone corner and (f) is second derivative of (e). (g) and (h) are EDMs measured with the same photon energy and polarization of (a) and (b), respectively but from a different polar angle which corresponds to a different $k_z$. In the figure double circles and double sided arrow represent $p$- and $s$-polarized lights, respectively.

Figure 4

(a) and (b) are EDMs measured using $p$-polarized light with an excitation energy of 20 eV from 1038-15 and 1048-35, respectively. (c) and (d) are second derivatives of (a) and (b), respectively. (e) shows schematic representation of the experimentally determined electronic structure near $\Gamma$ for both compounds.