Why TcT_c of (CaFeAs)10_{10}Pt3.58_{3.58}As8_8 is twice as high as (CaFe0.95_{0.95}Pt0.05_{0.05}As)10_{10}Pt3_3As8_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

Evidence for superconductivity with broken time-reversal symmetry in locally noncentrosymmetric SrPtAs

We report the magnetic and superconducting properties of locally noncentrosymmetric SrPtAs obtained by muon-spin-rotation/relaxation (μSR) measurements. Zero-field μSR reveals the occurrence of small spontaneous static magnetic fields with the onset of superconductivity. This finding suggests that the superconducting state of SrPtAs breaks time-reversal symmetry. The superfluid density as determined by transverse field μSR is nearly flat approaching T=0 K proving the absence of extended nodes in the gap function. By symmetry, several superconducting states supporting time-reversal-symmetry breaking in SrPtAs are allowed. Out of these, a dominantly d+id (chiral d-wave) order parameter is most consistent with our experimental data