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...

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 .

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We have performed the 125 Te-nuclear magnetic resonance (NMR) measurement in the field along the b axis on the newly discovered superconductor UTe 2 , which is a candidate of a spin-triplet superconductor. The nuclear spin-lattice relaxation rate divided by temperature 1/T 1 T abruptly decreases below a superconducting (SC) transition temperature T c without showing a coherence peak, indicative of UTe 2 being an unconventional superconductor. It was found that the temperature dependence of 1/T 1 T in the SC state cannot be understood by a single SC gap behavior but can be explained by a two SC gap model. The Knight shift, proportional to the spin susceptibility, decreases below T c , but the magnitude of the decrease is much smaller than the decrease expected in the spin-singlet pairing. Rather, the small Knight-shift decrease as well as the absence of the Pauli-depairing effect can be interpreted by the spin triplet scenario.

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