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

Abstract

Using a combination of muon-spin relaxation (μSR), inelastic neutron scattering (INS), and nuclear magnetic resonance (NMR), we investigated the novel iron-based superconductor with a triclinic crystal structure (CaFe1-xPtxAs)10Pt3As8 (Tc=13 K), containing platinum-arsenide intermediary layers. The temperature dependence of the superfluid density obtained from the μSR relaxation-rate measurements indicates the presence of two superconducting gaps, Δ1»Δ2. According to our INS measurements, commensurate spin fluctuations are centered at the (π,0) wave vector, like in most other iron arsenides. Their intensity remains unchanged across Tc, indicating the absence of a spin resonance typical for many Fe-based superconductors. Instead, we observed a peak in the spin-excitation spectrum around ?ω0=7 meV at the same wave vector, which persists above Tc and is characterized by the ratio ω0/kBTc≈6.2, which is significantly higher than typical values for the magnetic resonant modes in iron pnictides (∼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/T1T, observed by NMR immediately below T∗ without any notable subsequent anomaly at Tc, indicates that T∗ could mark the onset of a pseudogap in (CaFe1-xPtxAs)10Pt3As8, which is likely associated with the emergence of preformed Cooper pairs. © 2015 American Physical Society.