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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.

We measured the optical reflectivity R(ω) for an underdoped (Ca0.935La0.065)10(Pt3As8)(Fe2As2)5 single crystal and obtained the optical conductivity $${\sigma }_{1}(\omega )$$ σ 1 ( ω ) using the K-K transformation. The normal state $${\sigma }_{1}(\omega )$$ σ 1 ( ω ) at 30 K is well fitted by a Drude-Lorentz model with two Drude components (ωp1 = 1446 cm−1 and ωp2 = 6322 cm−1) and seven Lorentz components. Relative reflectometry was used to accurately determine the temperature dependence of the superconducting gap at various temperatures below Tc. The results clearly show the opening of a superconducting gap with a weaker second gap structure; the magnitudes for the gaps are estimated from the generalized Mattis-Bardeen model to be Δ1 = 30 and Δ2 = 50 cm−1, respectively, at T = 8 K, which both decrease with increasing temperature. The temperature dependence of the gaps was not consistent with one-band BCS theory but was well described by a two-band (hence, two gap) BCS model with interband interactions. The temperature dependence of the superfluid density is flat at low temperatures, indicating an s-wave full-gap superconducting state.

We investigate changes in the vortex pinning mechanism caused by proton irradiation through the measurement of the in-plane electrical resistivity for H//c in a pristine and two proton-irradiated (total doses of 1 × 1015 and 1 × 1016 cm−2) SmBa2Cu3O7-δ (SmBCO) superconducting tapes. Even though proton irradiation has no effect on the critical temperature (Tc), the resulting artificial point defect causes an increase in normal state electrical resistivity. The electrical resistivity data around Tc shows no evidence of a phase transition to the vortex glass state but only broadens with increasing magnetic field due to the vortex depinning in the vortex liquid state. The vortex depinning is well interpreted by a thermally activated flux flow model in which the activation energy shows a nonlinear temperature change $${\boldsymbol{U}}{\boldsymbol{(}}{\boldsymbol{T}},{\boldsymbol{H}}{\boldsymbol{)}}{\boldsymbol{=}}{{\boldsymbol{U}}}_{{\boldsymbol{0}}}{\boldsymbol{(}}{\boldsymbol{H}}{\boldsymbol{)}}{{\boldsymbol{(}}{\bf{1}}-{\boldsymbol{T}}{\boldsymbol{/}}{{\boldsymbol{T}}}_{{\boldsymbol{c}}}{\boldsymbol{)}}}^{{\boldsymbol{q}}}$$ U ( T , H ) = U 0 ( H ) ( 1 − T / T c ) q (q = 2). The field dependence of activation energy shows a $${{\boldsymbol{U}}}_{{\bf{0}}}{\boldsymbol{ \sim }}{{\boldsymbol{H}}}^{-{\boldsymbol{\alpha }}}$$ U 0 ~ H − α with larger exponents above 4 T. This field dependence is mainly due to correlated disorders in pristine sample and artificially created point defects in irradiated samples. Compared with the vortex pinning due to correlated disorders, the vortex pinning due to the appropriate amount of point defects reduces the magnitude of Uo(H) in the low magnetic field region and slowly reduces Uo(H) in high magnetic fields.

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.

We investigate the doping dependence of flux pinning in superconducting NaFeCo x As (x = 0.01, 0.03, 0.05 and 0.07) single crystals grown by the Bridgman method. The electronic specific heat displays a pronounced anomaly in a sample series at superconducting transition temperature, which hardly shows any residual part at low temperature. We found that Co doping plays an important role in signifying the secondary peak in the magnetic hysteresis of optimally doped (x = 0.03) and heavily doped (x = 0.05, 0.07) crystals. Furthermore, the dependence of the relaxation rate S = d ln M/d ln t on magnetic field and temperature exhibits a decreasing trend within a certain range corresponding to the secondary peak effect in the optimally and heavily doped samples. The magnetic relaxation rate combined with the Maley analysis of the current-dependent creep energy shows a single-vortex pinning in the lightly doped sample dominant at low applied fields, and plastic pinning at high applied fields, without showing a secondary peak. However, in the optimally and heavily doped samples, the magnetic relaxation rate and U(J) isothermal analysis show that the collective pinning that dominates below H peak crosses over to plastic pinning for fields above H peak.

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 .

The thermally activated flux flow effect has been studied in epitaxial FeSe0.6Te0.4 thin film grown by a PLD method through the electrical resistivity measurement under various magnetic fields for B//c and B//ab. The results showed that the thermally activated flux flow effect is well described by the nonlinear temperature-dependent activation energy. The evaluated apparent activation energy U0(B) is one order larger than the reported results and showed the double-linearity in both magnetic field directions. Furthermore, the FeSe0.6Te0.4 thin film shows the anisotropy of 5.6 near Tc and 2D-like superconducting behavior in thermally activated flux flow region. In addition, the vortex glass transition and the temperature dependence of the high critical fields were determined. Keywords: Thermally activated flus flow, Apparent activation energy, FeSe0.6Te0.4 thin film