In this review, the authors present a summary of experimental reports on newly discovered iron-based superconductors as they were known at the end of 2008. At the same time, this paper is intended to be useful for experimenters to know the current status of these superconductors. The authors introduce experimental results that reveal basic physical properties in the normal and superconducting states. The similarities and differences between iron-pnictide superconductors and other unconventional superconductors are also discussed.

We report experimental results of 75 As and 139 La nuclear magnetic resonance (NMR) in the iron-based layered LaFeAs(O 1Àx F x ) (x ¼ 0:0, 0.04, and 0.11). In the undoped LaFeAsO, 1=T 1 of 139 La exhibits a distinct peak at T N $ 142 K below which the spectra become broadened due to the internal magnetic field attributed to an antiferromagnetic (AFM) ordering. In the 4% F-doped sample, 1=T 1 T exhibits a CurieWeiss temperature dependence down to 30 K, suggesting the development of AFM spin fluctuations with decreasing temperature. In the 11% F-doped sample, in contrast, pseudogap behavior is observed in 1=T 1 T both at the 75 As and 139 La site with a gap value of Á PG $ 172 K. The spin dynamics vary markedly with F doping, which is ascribed to the Fermi-surface structure. As for the superconducting properties for the 4 and 11% F-doped samples, 1=T 1 in both compounds does not exhibit a coherence peak just below T c and follows a T 3 dependence at low temperatures, which suggests unconventional superconductivity with line-nodes. We discuss similarities and differences between LaFeAs(O 1Àx F x ) and cuprates, and also discuss the relationship between spin dynamics and superconductivity on the basis of F doping dependence of T c and 1=T 1 .

Spin dynamics evolution of BaFe2(As1−xPx)2 was probed as a function of P concentration via 31 P NMR. Our NMR study reveals that two-dimensional antiferromagnetic (AF) fluctuations are notably enhanced with little change in static susceptibility on approaching the AF phase from the superconducting dome. Moreover, magnetically ordered temperature θ deduced from the relaxation rate vanishes at optimal doping. These results provide clear-cut evidence for a quantum-critical point (QCP), suggesting that the AF fluctuations associated with the QCP play a central role in the high-Tc superconductivity.PACS numbers: 74.70. Xa, 74.40.Kb, 74.25.nj Conventional phonon-mediated superconductivity occurs in a normal metal that is well accounted for by Landau's Fermi-liquid (FL) theory. However, the standard FL theory appears to break down above T c in many "exotic" superconductors characterized by unconventional pairing rather than a conventional uniform-sign s-wave pairing function, such as in the heavy fermion materials and cuprates. The origin of the anomalous metallic properties, often referred to as "non-Fermi-liquid" (nFL) behavior, has triggered a growing interest in quantumcritical points (QCPs) that provide a route towards nFL behavior Newly discovered iron-pnictide high-T c superconductivity also appears where antiferromagnetism is suppressed via chemical substitution or pressure [4]. The existence of a QCP in iron pnictides has been suggested [5][6][7][8][9]. Spin-fluctuation-mediated superconductivity associated with the suppression of the antiferromagnetism is one likely scenario [10], but the identification of the mechanism is far from settled [11,12]. The difficulty in examining the superconducting (SC) mechanism could arise from complexity in the materials that can lead to ambiguous interpretations; e.g., non-universal SC gap functions and limited sample quality. It is thus essential to find a suitable model system to examine the mechanism of superconductivity.The isovalent-doped BaFe 2 (As 1−x P x ) 2 can be used as such a model system. It has the highest T c (31 K) among iron-pnictide superconductors known to have line nodes in the SC gap [13][14][15]. Clarifying the mechanism that produces its high-T c nodal gap is thus very important. Since isovalent P-doping is not expected to add carriers [16], BaFe 2 (As 1−x P x ) 2 maintains the compensation condition, i.e. the volume of the hole Fermi surfaces (FSs) is equal to that of the electron FSs. Very clean single-crystals of BaFe 2 (As 1−x P x ) 2 allow de Haasvan Alphen (dHvA) experiments that are the most precise technique to determine FSs, revealing the detailed electronic structure for comparison with band calculations [17,18]. The quasiparticle effective mass increases towards the maximum T c , signaling the enhancement of electron-electron correlation. Such an increase in the quasiparticle mass as well as nFL behavior inferred from resistivity measurements [16,19] can be expected when the system is in proximity to a QCP. However, direct evidence for the...

We found a giant Seebeck effect in semiconducting single-wall carbon nanotube (SWCNT) films, which exhibited a performance comparable to that of commercial Bi 2 Te 3 alloys. Carrier doping of semiconducting SWCNT films further improved the thermoelectric performance. These results were reproduced well by first-principles transport simulations based on a simple SWCNT junction model. These findings suggest strategies that pave the way for emerging printed, allcarbon, flexible thermoelectric devices.