Superconducting properties and pseudogap from preformed Cooper pairs in the triclinic (CaFe_(1-x)Pt_xAs)_(10)Pt_3As_8

摘要

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 (CaFe_(1-x)Pt_xAs)_(10)Pt_3As_8 (T_c = 13 K), containing platinum-arsenide intermediary layers. The temperature dependence of the superfluid density obtained from the μ SR relaxation-rate measurements indicates the presence of two superconducting gaps, Δ_1 » Δ_2. 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 T_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 hω_0 = 7 meV at the same wave vector, which persists above T_c and is characterized by the ratio hω_0/k_B T_c ≈ 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/T_1T, observed by NMR immediately below T~* without any notable subsequent anomaly at T_c, indicates that T~* could mark the onset of a pseudogap in (CaFe_(1-x)Pt_xAs)_(10)Pt_3As_8, which is likely associated with the emergence of preformed Cooper pairs.