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We report the observation of pseudogap (PG) behavior in optical conductivity in an underdoped Ba0.6+δK0.4-δFe2As2 (T onset c ≈ 36 K) single crystal far above Tc, up to 100K (≈ 3Tc). Wide separation of the energy scales of the magnetic and superconducting (SC) correlations in the Ba0.6+δK0.4-δFe2As2 sample enabled us to establish that the PG structures observed in optical conductivity in the range of 50-150 cm-1 are uniquely driven by the SC correlation, and the magnetic correlation coexists in the far separated range of about 700 cm-1. Theoretical calculations, based on the preformed Cooper pair model, provided an excellent description of the temperature evolution of the SC correlation in optical conductivity data from below to above Tc. [ABSTRACT FROM AUTHOR]

We studied the electrical, thermal, and thermoelectric properties of the polycrystalline compound of SmNiC2. The electrical resistivity and magnetization measurement show the interplay between the charge density wave at TCDW = 150 K and the ferromagnetic ordering of Tc = 18 K. Below the ferromagnetic transition temperature, we observed the magnon gap formation of Δ ≃ 4.3-4.4 meV by p(T) and CP(T) measurements. The charge density wave is attributed to the increase of the Seebeck coefficient resulting in the increase of the power factor S2σ. The thermal conductivity anomalously increases with increasing temperature along the whole measured temperature range, which implies the weak attribution of Umklapp phonon scattering. The thermoelectric figure of merit ZT significantly increases due to the increase of the power factor at TCDW = 150 K. Here we argue that the competing interaction between electron-phonon and electron-magnon couplings exhibits the unconventional behavior of electrical and thermal properties. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We have measured the reflectivity spectra of the LiFeAs (Tc = 17.6 K) single crystal in the temperature range from 4 to 300 K. In the superconducting (SC) state (T < Tc), the clean opening of the optical absorption gap was observed below 25 cm-1, indicating an isotropic full gap formation. In the normal state (T > Tc), the optical conductivity spectra display typical metallic behavior with the Drude-type spectra at low frequencies, but we found that the introduction of the two Drude components best fits the data, indicating the multiband nature of this compound. A theoretical analysis of the lowtemperature data (T = 4K < Tc) also suggests that two SC gaps best fit the data and their values were estimated as 11 = 3.3 meV and 12 = 1.59 meV, respectively. Using the Ferrell-Glover-Tinkham sum rule and dielectric function ϵ1(ω), the plasma frequency of the SC condensate (ωps) is consistently estimated to be 6665 cm-1, implying that about 65% of the free carriers of the normal state condense into the SC condensate. To investigate the various interband transition processes (for ω > 200 cm-1), we have also performed the localdensity approximation band calculation and calculated the optical spectra of the interband transitions. This theoretical result provided qualitative agreement with the experimental data below 4000 cm-1. [ABSTRACT FROM AUTHOR]

Miniature Hall-probe arrays were used to measure the critical current densities for the three main directions of vortex motion in the stoichiometric LiFeAs superconductor. These correspond to vortex lines along the c axis moving parallel to the ab plane, and to vortex lines in the ab plane moving perpendicular to and within the plane. respectively. The measurements were carried out in the low-field regime of strong vortex pinning, in which the critical current anisotropy is solely determined by the coherence length anisotropy parameter εξ. This allows for the extraction of εξ at magnetic fields far below the upper critical field Bc2. We find that increasing the magnetic field decreases the anisotropy of the coherence length. [ABSTRACT FROM AUTHOR]

The thermal conductivity ? of the iron-arsenide superconductor LiFeAs (Tc≃ 18 K) was measured in single crystals at temperatures down to T≃50 mK and in magnetic fields up to H=17 T, very close to the upper critical field Hc2≃18 T. For both directions of the heat current, parallel and perpendicular to the tetragonal c axis, a negligible residual linear term κ/T is found as T→0, showing that there are no zero-energy quasiparticles in the superconducting state. The increase in ? with magnetic field is the same for both current directions and it follows the dependence expected for an isotropic superconducting gap. These findings show that the superconducting gap in LiFeAs is isotropic in 3D, without nodes or deep minima anywhere on the Fermi surface. We discuss how this behavior of the thermal conductivity may be reconciled with the multiband character of superconductivity in LiFeAs inferred from other measurements. Comparison with other iron-pnictide superconductors suggests that a nodeless isotropic gap is a common feature at optimal doping (maximal Tc). [ABSTRACT FROM AUTHOR]

The variations of in- and inter-plane London penetration depths, Δλ(T), were measured using a tunnel diode resonator in single crystals of the intrinsic pnictide superconductor LiFeAs. This compound appears to be in the clean limit with a residual resistivity of 4 (T→0) to 8 (Tc) μΩ cm and a residual resistivity ratio of 65 to 35, respectively. The superfluid density ρs(T)=λ²(0)/λ²(T) is well described by the self-consistent two-gap γ model. Together with the previous data, our results support the universal evolution of the superconducting gap from nodeless to nodal upon departure from optimal doping even in clean systems. We also conclude that pair-breaking scattering plays an important role in the deviation of the low-temperature behavior of λ(T) from the exponential in Fe-based compounds. [ABSTRACT FROM AUTHOR]

Thermoelectric energy harvesting—the transformation of waste heat into useful electricity—is of great interest for energy sustainability. The main obstacle is the low thermoelectric efficiency of materials for converting heat to electricity, quantified by the thermoelectric figure of merit, ZT. The best available n-type materials for use in mid-temperature (500–900 K) thermoelectric generators have a relatively low ZT of 1 or less, and so there is much interest in finding avenues for increasing this figure of merit. Here we report a binary crystalline n-type material, In4Se3-δ, which achieves the ZT value of 1.48 at 705 K—very high for a bulk material. Using high-resolution transmission electron microscopy, electron diffraction, and first-principles calculations, we demonstrate that this material supports a charge density wave instability which is responsible for the large anisotropy observed in the electric and thermal transport. The high ZT value is the result of the high Seebeck coefficient and the low thermal conductivity in the plane of the charge density wave. Our results suggest a new direction in the search for high-performance thermoelectric materials, exploiting intrinsic nanostructural bulk properties induced by charge density waves. [ABSTRACT FROM AUTHOR]

A LiFeAs single crystal with Tconset∼19.7 K was grown in a sealed tungsten crucible using the Bridgeman method. The electrical resistivity experiments revealed a ratio of room temperature to residual resistivity of approximately 46 and 18 for the in-plane and out-of plane directions, respectively. The estimated anisotropic resistivity, γρ=ρc/ρab, was approximately 3.3 at Tconset. The upper critical fields had large Hc2∥ab and Hc2∥c values of 83.4 T and 72.5 T, respectively, and an anisotropy ratio is γH=Hc2∥ab/Hc2∥c∼1.15. The high upper critical field value and small anisotropy highlight the potential use of LiFeAs in a variety of applications. The calculated critical current density (Jc) from the M-H loop is approximately 103 A/cm2 [ABSTRACT FROM AUTHOR]

We report the high thermoelectric figure-of-merit (ZT) on the Se-deficient polycrystalline compounds of In4Se3-x (0.02≤x≤0.5) and the anisotropic electronic band structure. The Se-deficiency (x) has the effect of decreasing the semiconducting band gap and increasing the power factor. The band structure calculation for In4Se3-x (x=0.25) exhibits localized hole bands at the Γ-point and Y-S symmetry line, whereas the significant electronic band dispersion is observed along the c-axis. Here, we propose that the high ZT values on those compounds are originated from the anisotropic electronic band structure as well as Peierls distortion. [ABSTRACT FROM AUTHOR]

We have measured the reflectivity of the optimally doped Ca8.5 La1.5(Pt3 As8) (Fe10 As10) single crystal (Tc = 32.8 K) over the broad frequency range from 40cm-1 to 12000cm-1 and for temperatures from 8 K to 300 K. The optical conductivity spectra of the low-frequency region (< 1000 cm-1) in the normal state (80 K < T ≤ 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 cm-1 and continuously evolves into the fully opened superconducting (SC) gap structure below Tc. 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 Tc into the SC gap structure below Tc. The extracted two SC gap sizes are ΔS = 4.9 meV and ΔL = 14.2 meV, suggesting Ca8.5 La1.5 (Pt3 As8) (Fe10 As10) as a multiple-gap superconductor with a mixed character of the weak-coupling and strong-coupling superconductivity. [ABSTRACT FROM AUTHOR]

The influence of the proton irradiation on the fluctuation-induced conductivity in Ca8.5La1.5(Pt3As8)(Fe2As2)5 single crystal was investigated. The in-plane magnetoconductivity was measured up to μ0H = 13 T. It is observed that the Tc was suppressed up to 30.3 K from 32.5 K as a result of proton irradiation whereas the amplitude of the superconducting fluctuations is almost the same in both pristine and irradiated samples. The magnetoconductivity results analyzed by the Ullah and Dorsey scaling approaches showed that in the pristine sample, the 3D–2D crossover is situated near the Tc. Furthermore, once the 3D–2D crossover occurs, it is found that there is a regime simultaneously described by 2D and 3D fluctuation behaviors in our Ca8.5La1.5(Pt3As8)(Fe2As2)5 sample. Meanwhile, the proton-irradiated sample showed the 3D fluctuation behavior. [ABSTRACT FROM AUTHOR]

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 Å. This material was observed to fall on the universal scaling line, Nc ∼ 4.4 . [ABSTRACT FROM AUTHOR]

The reversible magnetization curves for various magnetic fields are investigated for a single crystal of the optimally doped NaFe0.97Co0.03As superconductor with the superconducting transition temperature Tc = 20.9 K (ρ(T) = 0) in the case where H is parallel to c-axis. A well-formed rounding effect is observed in the temperature dependence of the magnetization, and it widens on increasing the strength of the applied magnetic fields. At 20.2 K, we observe a crossing of the M(T) curves for different applied magnetic fields, indicating important fluctuation effects. The irreversibility line Hirr(T) obeys a universal scaling relation Hirr(T) = H0(1 − T/Tc)n characterized by n ∼ 1.537, which confirms the moderately anisotropic nature of the sample. The scaling analysis of the reversible magnetization in the region near Tc(H) supports the three-dimensional nature of the vortex fluctuations in NaFe0.97Co0.03As. [ABSTRACT FROM AUTHOR]

A correction to the article "Nodeless two-gap superconducting state in single crystals of the stoichiometric iron pnictide LiFeAs" that was published online on March 9, 2011 is presented, which discusses a typographical error on page 3.