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For high-Tc superconductors, clarifying the role and origin of the pseudogap is essential for understanding the pairing mechanism. Among the various models describing the pseudogap, the preformed Cooper pair model is a potential candidate. Therefore, we present experimental evidence for the preformed Cooper pair model by studying the pseudogap spectrum observed in the optical conductivity of a Ca10(Pt4As8)(Fe2As2)5 (Tc = 34.6 K) single crystal. We observed a clear pseudogap structure in the optical conductivity and observed its temperature dependence. In the superconducting (SC) state, one SC gap with a gap size of Δ = 26 cm−1, a scattering rate of 1/τ = 360 cm−1 and a low-frequency extra Drude component were observed. Spectral weight analysis revealed that the SC gap and pseudogap are formed from the same Drude band. This means that the pseudogap is a gap structure observed as a result of a continuous temperature evolution of the SC gap observed below Tc. This provides clear experimental evidence for the preformed Cooper pair model.

The temperature dependence of the electric resistivity measured in various magnetic fields was analyzed by the vortex glass theory and the thermally activated flux motion (TAFM) theory. The vortex glass-to-vortex liquid (GTL) transition Tg obtained from the analysis shows a temperature dependence of B g T = B 0 1 - T / T c m . The vortex liquid region is divided into the critical region existing in a finite temperature region just above Tg and the TAFM region present in the finite temperature region above it. In the critical region, the activation energy is expressed as U e f f = k B T T c - T / ( T c - T g ) , whereas in the TAFM region, the activity energy is expressed as temperature-nonlinear U T , B = U 0 B 1 - t q . In the GTL transition, (Ca0.85La0.15)10(Pt3As8)(Fe2As2)5 maintains the 3D vortex structure without exhibiting dimension crossover of the vortex, but Ca10(Pt3As8)((Fe0.92Pt0.08)2As2)5 exhibits the dimension crossover from the 3D vortex glass to the 2D vortex liquid.

For newly synthesized hole-doped Ca8.5Na1.5(Pt3As8)(Fe2As2)5 single crystals, we measured the infrared reflectivity spectrum and the magnetic field dependence of magnetoresistivity and Hall resistivity. The results of these two experiments in normal states are well described by two band models. In the normal state below 150 K, the optical conductivity spectrum shows a transfer of spectral weights from the mid-infrared region to the near-infrared region. Meanwhile, the magnetoresistance and Hall resistance show a significant decrease in carrier density at 150 K. These two phenomena are due to the conversion of itinerant electrons to heavy electrons by the strong correlation effect, Hund's coupling. In the superconducting state, the spectral weight in the low frequency region by the superconducting condensate is completely suppressed, which is well analyzed by the generalized Mattis-Bardeen (M-B) model with a two superconducting gap.

We have measured the reflectivity of the optimally doped ${\mathrm{Ca}}_{8.5}{\mathrm{La}}_{1.5}({\mathrm{Pt}}_{3}{\mathrm{As}}_{8})({\mathrm{Fe}}_{10}{\mathrm{As}}_{10})$ single crystal $({T}_{c}=32.8\phantom{\rule{0.16em}{0ex}}\mathrm{K})$ over the broad frequency range from $40{\text{cm}}^{\ensuremath{-}1}$ to 12 $000{\text{cm}}^{\ensuremath{-}1}$ and for temperatures from 8 K to 300 K. The optical conductivity spectra of the low-frequency region $(l1000\phantom{\rule{4pt}{0ex}}{\text{cm}}^{\ensuremath{-}1})$ in the normal state (80 K $lT\ensuremath{\le}300$ K) is well fitted with two Drude forms, which indicates the presence of multiple bands at the Fermi level. Decreasing temperature below 80 K, this low-frequency Drude spectra develops a pseudogap (PG) hump structure at around $100\phantom{\rule{4pt}{0ex}}{\text{cm}}^{\ensuremath{-}1}$ and continuously evolves into the fully opened superconducting (SC) gap structure below ${T}_{c}$. Theoretical calculations of the optical conductivity with the preformed Cooper pair model provide an excellent description of the temperature evolution of the PG structure above ${T}_{c}$ into the SC gap structure below ${T}_{c}$. The extracted two SC gap sizes are ${\mathrm{\ensuremath{\Delta}}}_{S}=4.9$ $\text{meV}$ and ${\mathrm{\ensuremath{\Delta}}}_{L}=14.2$ $\text{meV}$, suggesting ${\mathrm{Ca}}_{8.5}{\mathrm{La}}_{1.5}({\mathrm{Pt}}_{3}{\mathrm{As}}_{8})({\mathrm{Fe}}_{10}{\mathrm{As}}_{10})$ as a multiple-gap superconductor with a mixed character of the weak-coupling and strong-coupling superconductivity.

The influence of proton irradiation on the fluctuation-induced magnetoconductivity of high quality FeSe1−xTex (x=0.4, 0.55) (FST) thin films has been investigated. The measurements were performed with magnetic fields up to 13 T applied in the two main crystal directions. The results were interpreted in terms of the Ginzburg–Landau approach for three-dimensional materials under a total-energy cutoff. The analysis shows that properly-tuned proton irradiation does not appreciably affect fundamental superconducting parameters like the Tc value, the upper critical fields or the anisotropy. This has important consequences from the point of view of possible applications due to the enhancement of vortex pinning induced by irradiation. YSK was supported by the NRF grant funded by the Ministry of Science, ICT and Future Planning (No. NRF-2015M2B2A9028507 and NRF-2016R1A2B4012672). TP was supported by the NRF grant funded by the Ministry of Science, ICT and Future Planning of Korea (No. 2012R1A3A2048816). JM acknowledges support by project FIS2016-79109-P (AEI/FEDER, UE) and by the Xunta de Galicia (project AGRUP 2015/11). SL was supported by the Global Research Network program through the NRF funded by the Ministry of Science and ICT & Future Planning (NRF-2014S1A2A2028361) SI

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.

Terahertz and far-infrared spectroscopy in the temperature range, 4–300 K were used to study the normal and superconducting states of superconductor BaFe1.8Co0.2As2 with Tc = 22.5 K. At T < Tc, the vanishing of optical conductivity caused by the unity approach and flat behavior in reflectivity were observed below 45 cm−1. This feature indicates the formation of a superconducting energy gap due to the formation of Cooper pairs. The introduction of the two Drudes model well reproduced the normal state optical conductivity, indicating the multiband nature of this superconductor. Two superconducting energy gaps were estimated as = 2.90 meV and = 6.75 meV by the BCS model. Using the sum rule and dielectric function, the superconducting plasma frequency () can be estimated as 5170 ± 270 cm−1, yielding that the magnetic penetration depth (λ) is 3090 ± 160 A. This material was observed to fall on the universal scaling line, Nc ~ 4.4 .