Superconducting properties and pseudogap from preformed Cooper pairs in the triclinic(CaFe1−xPtxAs)10Pt3As8

Using a combination of muon-spin relaxation $(\ensuremath{\mu}\mathrm{SR})$, inelastic neutron scattering (INS), and nuclear magnetic resonance (NMR), we investigated the novel iron-based superconductor with a triclinic crystal structure ${({\mathrm{CaFe}}_{1\ensuremath{-}x}{\mathrm{Pt}}_{x}\mathrm{As})}_{10}{\mathrm{Pt}}_{3}{\mathrm{As}}_{8}$ (${T}_{\text{c}}=13$ K), containing platinum-arsenide intermediary layers. The temperature dependence of the superfluid density obtained from the $\ensuremath{\mu}\mathrm{SR}$ relaxation-rate measurements indicates the presence of two superconducting gaps, ${\ensuremath{\Delta}}_{\text{1}}\ensuremath{\gg}{\ensuremath{\Delta}}_{\text{2}}$. According to our INS measurements, commensurate spin fluctuations are centered at the $(\ensuremath{\pi},\phantom{\rule{0.16em}{0ex}}0)$ wave vector, like in most other iron arsenides. Their intensity remains unchanged across ${T}_{\text{c}}$, indicating the absence of a spin resonance typical for many Fe-based superconductors. Instead, we observed a peak in the spin-excitation spectrum around $\ensuremath{\hbar}{\ensuremath{\omega}}_{0}=7$ meV at the same wave vector, which persists above ${T}_{\text{c}}$ and is characterized by the ratio $\ensuremath{\hbar}{\ensuremath{\omega}}_{0}/{k}_{\text{B}}{T}_{\text{c}}\ensuremath{\approx}6.2$, which is significantly higher than typical values for the magnetic resonant modes in iron pnictides $(\ensuremath{\sim}4.3)$. The temperature dependence of magnetic intensity at 7 meV revealed an anomaly around ${T}^{*}=45$ K related to the disappearance of this new mode. A suppression of the spin-lattice relaxation rate, $1/{T}_{1}T$, observed by NMR immediately below ${T}^{*}$ without any notable subsequent anomaly at ${T}_{\mathrm{c}}$, indicates that ${T}^{*}$ could mark the onset of a pseudogap in ${({\mathrm{CaFe}}_{1\ensuremath{-}x}{\mathrm{Pt}}_{x}\mathrm{As})}_{10}{\mathrm{Pt}}_{3}{\mathrm{As}}_{8}$, which is likely associated with the emergence of preformed Cooper pairs.