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.

P and 75 As NMR measurements were performed in superconducting BaFe 2 ͑As 0.67 P 0.33 ͒ 2 with T c =30 K. The nuclear-spin-lattice relaxation rate T 1 −1 and the Knight shift in the normal state indicate the development of antiferromagnetic fluctuations, and T 1 −1 in the superconducting ͑SC͒ state decreases without a coherence peak just below T c , as observed in ͑Ba 1−x K x ͒Fe 2 As 2 . In contrast to other iron arsenide superconductors, the T 1 −1 ϰ T behavior is observed below 4 K, indicating the presence of a residual density of states at zero energy.Our results suggest that strikingly different SC gaps appear in BaFe 2 ͑As 1−x P x ͒ 2 despite a comparable T c value, an analogous phase diagram, and similar Fermi surfaces to ͑Ba 1−x K x ͒Fe 2 As 2 .

We report across-bandgap p-type and n-type control over the Seebeck coefficients of semiconducting single-wall carbon nanotube networks through an electric double layer transistor setup using an ionic liquid as the electrolyte. All-around gating characteristics by electric double layer formation upon the surface of the nanotubes enabled the tuning of the Seebeck coefficient of the nanotube networks by the shift in gate voltage, which opened the path to Fermi-level-controlled three-dimensional thermoelectric devices composed of one-dimensional nanomaterials.

From detailed angle-resolved NMR and Meissner measurements on a ferromagnetic (FM) superconductor UCoGe (TCurie ∼ 2.5 K and TSC ∼ 0.6 K), we show that superconductivity in UCoGe is tightly coupled with longitudinal FM spin fluctuations along the c axis. We found that magnetic fields along the c axis (H c) strongly suppress the FM fluctuations and that the superconductivity is observed in the limited magnetic-field region where the longitudinal FM spin fluctuations are active. These results combined with model calculations strongly suggest that the longitudinal FM spin fluctuations tuned by H c induce the unique spin-triplet superconductivity in UCoGe. This is the first clear example that FM fluctuations are intimately related with superconductivity.PACS numbers: 71.27.+a 74.25.nj, 75.30.Gw The discovery of superconductivity in ferromagnetic (FM) UGe 2 opened up a new paradigm of superconductivity [1,2], since most unconventional superconductivity has been discovered in the vicinity of an antiferromagnetic (AFM) phase [3]. From the theoretical point of view, in an itinerant FM superconductor with the presence of a large energy splitting between the majority and minority spin Fermi surfaces, exotic spintriplet superconductivity is anticipated, in which pairing is between parallel spins within each spin Fermi surface. In addition, it has been argued that critical FM fluctuations near a quantum phase transition could mediate spin-triplet superconductivity [4]. However, there have been no experimental results indicating a relationship between FM fluctuations and superconductivity.Among the FM superconductors discovered so far, UCoGe is one of the most readily explored experimentally, because of its high superconducting (SC) transition temperature (T SC ) and low Curie temperature (T Curie ) at ambient pressure [5]. Microscopic measurements have shown that superconductivity occurs within the FM region, resulting in microscopic coexistence of ferromagnetism and superconductivity [6,7]. Studies of the SC upper critical field (H c2 ) and its angle dependence along each crystalline axis have reported remarkable enigmatic behavior [8,9]: superconductivity survives far beyond the Pauli-limiting field along the a and b axes, whereas H c2 for fields along the c direction (H c c2 ) is as small as 0.5 T. Colossal H c2 for fields along the a and b axes seems to suggest spin triplet pairing. In addition, a steep angle dependence of H c2 was reported when the field was tilted slightly from the a axis toward the c axis [9]. The observed characteristic H c2 behavior is one of mysterious features of SC UCoGe and its origin can be related to the mechanism of the superconductivity.Unlike the three dimensional crystal structure, magnetic properties are strongly anisotropic [8]. The magnetization has Ising-like anisotropy with the c axis as a magnetic easy axis, and direction-dependent nuclear-spin lattice relaxation rate (1/T 1 ) measurements on a single crystalline sample have revealed the magnetic fluctuations in UCoGe to be Ising-...

Unambiguous evidence for the microscopic coexistence of ferromagnetism and superconductivity in UCoGe (T Curie $ 2:5 K and T SC $ 0:6 K) is reported from 59 Co nuclear quadrupole resonance (NQR). The 59 Co-NQR signal below 1 K indicates ferromagnetism throughout the sample volume, while the nuclear spin-lattice relaxation rate 1=T 1 in the ferromagnetic (FM) phase decreases below T SC due to the opening of the superconducting (SC) gap. The SC state is found to be inhomogeneous, suggestive of a self-induced vortex state, potentially realizable in a FM superconductor. In addition, the 59 Co-NQR spectrum around T Curie shows that the FM transition in UCoGe possesses a first-order character, which is consistent with the theoretical prediction that the low-temperature FM transition in itinerant magnets is generically of first-order.KEYWORDS: ferromagnetic superconductor, U-based heavy-fermion, UCoGe, nuclear quadrupole resonance DOI: 10.1143/JPSJ.79.023707After the discovery of superconductivity in UGe 2 under pressure, 1) the coexistence of superconductivity and ferromagnetism becomes one of the major topics in condensedmatter physics. This is because ferromagnetism and spinsinglet superconductivity are thought to be mutually exclusive. 2,3) In the presence of a large splitting between the majority and minority spin Fermi surfaces, as in a ferromagnetic (FM) state, more-exotic spin-triplet superconductivity is allowed, in which parallel spins pair within each spin Fermi surface. While FM superconductors such as UIr 4) and URhGe 5) have recently been demonstrated to occur experimentally, proof that the same charge carriers participate simultaneously in both phenomena has remained elusive.In 2007, new ambient-pressure ferromagnetic (FM) superconductor UCoGe was discovered by Huy et al.6) UCoGe is a weak ferromagnet with T Curie ¼ 3 K and the ordered moments s ¼ 0:03 B , and shows superconductivity at the transition temperature T SC ¼ 0:8 K, 6) highest among FM superconductors. In order to investigate the correlation between ferromagnetism and superconductivity, nuclear quadrupole resonance (NQR) measurements are ideally suited, since they provide microscopic information about the electronic and magnetic properties without applying external fields. In a magnetically ordered state, the NQR signal splits or shifts due to internal fields at the nuclear site, and the nuclear spin-lattice relaxation rate 1=T 1 provides siteselective information about the density of states at the Fermi level and thus about the superconducting (SC) gap structure. UCoGe is a FM superconductor suitable for NQR measurements, since it contains an NQR-active element of 59 Co. In the previous letter, we reported 59 Co-NQR studies in a polycrystalline UCoGe with T Curie ¼ 2:5 K and the SC onset temperature T onset SC ¼ 0:7 K. 7) We found inhomogeneous ferromagnetism below T Curie in the polycrystalline sample, from the observation of the FM and paramagnetic (PM) NQR spectra at lowest temperature. In addition, the SC anomaly was observed in 1=T 1 mea...