中井 祐介

We present the results of nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements on the layered tin pnictide superconductors NaSn2As2 (the superconducting transition temperature Tc = 1.3 K) and Na1−xSn2P2 (Tc = 2.0 K). In NaSn2As2, a broad 75As NQR spectrum indicates a large charge distribution at the As site, and a broad 119Sn NMR spectrum indicates a distributed local density of states (DOS) at the Sn site. Almost the same level of microscopic disorder in the electronic state is also found at the Sn site in Na1−xSn2P2 as well. These results clearly indicate the presence of a local atomic disorder in these superconductors. We have also measured the nuclear spin–lattice relaxation rate 1=T1 to gain insight into the partial DOS at each site. Our NMR=NQR results are consistent with previous band calculations, indicating that Na deficiency results in an increase in the DOS at the Fermi energy (EF) due to the decrease in EF. The agreement between the experimental and band calculation results makes it more plausible that the increase in the Tc of Na1−xSn2P is due to the increase in the DOS at EF caused by the Na deficiency. This study thus demonstrates that NMR=NQR measurements can be a very effective tool in revealing the local electronic structure of materials by comparing experimental results with theoretical band calculations.

We applied the Bayesian framework to the analysis of nuclear spin-lattice relaxation curves. In the simplest case, nuclear spin-lattice relaxation is described by a single exponential function. However, relaxations to which the exponential function does not fit well are often observed in real NMR experiments. One of the reasons for those relaxations is the sample inhomogeneity. For such cases, it is common practice to analyze the curve by fitting a stretched exponential function exp(-(t=T1)beta) to the data, where T1 is the nuclear spin-lattice relaxation time. Another possibility is the coexistence of different relaxation mechanisms. In this situation, the relaxation curves should be fitted by the summation of several relaxation functions. For scientists to interpret relaxation curves, a data-driven method is required to determine which of the two possibilities is more plausible. In this paper, we propose a Bayesian method to select the appropriate relaxation function and even the number of relaxation components. We demonstrate that our method effectively determines the relaxation model on the basis of numerical experiments and the experimental data of nuclear spin-lattice relaxation of the nonmagnetic semiconductor SmS.

Dirac fermion systems form a unique Landau level at the Fermi level-the so-called zero mode-whose observation itself will provide strong evidence of the presence of Dirac dispersions. Here, we report the study of semimetallic black phosphorus under pressure by ^{31}P-nuclear magnetic resonance measurements in a wide range of magnetic field up to 24.0 T. We have found a field-induced giant enhancement of 1/T_{1}T, where 1/T_{1} is the nuclear spin lattice relaxation rate: 1/T_{1}T at 24.0 T reaches more than 20 times larger than that at 2.0 T. The increase in 1/T_{1}T above 6.5 T is approximately proportional to the squared field, implying a linear relationship between the density of states and the field. We also found that, while 1/T_{1}T at a constant field is independent of temperature in the low-temperature region, it steeply increases with temperature above 100 K. All these phenomena are well explained by considering the effect of Landau quantization on three-dimensional Dirac fermions. The present study demonstrates that 1/T_{1} is an excellent quantity to probe the zero-mode Landau level and to identify the dimensionality of the Dirac fermion system.

The zero-field state in the antiferroquadrupole (AFQ) ordered phase of CeB6 has been investigated by a B-11-nuclear quadrupole resonance (NQR) measurement that is sensitive to changes in local charge distribution. In order to achieve this experiment, we have improved our low-frequency NQR techniques, so that the B-11-NQR spectrum has been successfully observed at an NQR frequency nu(Q) similar to 560 kHz, exceptionally low frequency among direct NQR measurements on strongly correlated electron systems. The temperature dependence of the NQR spectrum reveals that nu(Q) rapidly decreases just below AFQ ordering temperature T-Q = 3.3 K, while the spectral shape does not show any sign of site splitting below T-Q. The latter is incompatible with the theoretical prediction based on the O-xy-type AFQ order, suggesting that the ordered state in the zero-field region differs from that in the nonzero-field region. Some new possible interpretations to explain the obtained results are proposed.

SmS, a prototypical intermediate valence compound, has been studied by performing high-pressure nuclear magnetic resonance measurements on a S33-enriched sample. The observation of an additional signal below 15-20 K above a nonmagnetic-magnetic transition pressure Pc2≈2 GPa gives evidence of a magnetic transition. The absence of a Curie term in the Knight shift near Pc2 indicates that the localized character of 4f electrons is entirely screened and the mechanism of the magnetic ordering is not described within a simple localized model. Simultaneously, the line shape in the magnetically ordered state is incompatible with a spin density wave order. These suggest that the magnetic order in SmS may require an understanding beyond the conventional framework for heavy fermions. The fact that hyperfine fields from the ordered moments cancel out at the S site leads us to a conclusion that the ordered phase has a type II antiferromagnetic structure.

<p>The physical and chemical properties of Li_1−xSn_2+xP₂ are affected by Li/Sn mixed occupation with local ordering.</p>

Dynamics of molecules in the solid state holds promise for connecting molecular behaviors with properties of bulk materials. Solid-state dynamics of [60]fullerene (C60) is controlled by intimate intermolecular contacts and results in restricted motions of a ratchet phase at low temperatures. Manipulation of the solid-state dynamics of fullerene molecules is thus an interesting yet challenging problem. Here we show that a tubular host for C60 liberates the solid-state dynamics of the guest from the motional restrictions. Although the intermolecular contacts between the host and C60 were present to enable a tight association with a large energy gain of -14 kcal mol-1, the dynamic rotations of C60 were simultaneously enabled by a small energy barrier of +2 kcal mol-1 for the reorientation. The solid-state rotational motions reached a non-Brownian, inertial regime with an extremely rapid rotational frequency of 213 GHz at 335 K.

Water in a nanoconfined geometry has attracted great interest from the viewpoint of not only basic science but also nanofluidic applications. Here, the rotational dynamics of water inside single-walled carbon nanotubes (SWCNTs) with mean diameters larger than ca. 1.4 nm were investigated systematically using H-2 nuclear magnetic resonance spectroscopy with high-purity SWCNTs and molecular dynamics calculations. The results were compared with those for hydrophilic pores. It was found that faster water dynamics could be achieved by increasing the hydrophobicity of the pore walls and decreasing the pore diameters. These results suggest a strategy that paves the way for emerging high-performance filtration/separation devices. Upon cooling below 220 K, it was found that water undergoes a transition from fast to slow dynamics states. These results strongly suggest that the observed transition is linked to a liquid-liquid crossover or transition proposed in a two-liquid states scenario for bulk water.

The electronic states in isolated single-wall carbon nanotubes (SWCNTs) have been considered as an ideal realization of a Tomonaga-Luttinger liquid (TLL). However, it remains unclear whether one-dimensional correlated states are realized under local environmental effects such as the formation of a bundle structure. Intertube effects originating from other adjacent SWCNTs within a bundle may drastically alter the one-dimensional correlated state. In order to test the validity of the TLL model in bundled SWCNTs, low-energy spin excitation is investigated by nuclear magnetic resonance (NMR). The NMR relaxation rate in bundled mixtures of metallic and semiconducting SWCNTs shows a power-law temperature dependence with a theoretically predicted exponent. This demonstrates that a TLL state with the same strength as that for effective Coulomb interactions is realized in a bundled sample, as in isolated SWCNTs. In bundled metallic SWCNTs, we found a power-law temperature dependence of the relaxation rate, but the magnitude of the relaxation rate is one order of magnitude smaller than that predicted by theory. Furthermore, we found an almost doubled magnitude of the Luttinger parameter. These results indicate suppressed spin excitations with reduced Coulomb interactions in bundled metallic SWCNTs, which are attributable to intertube interactions originating from adjacent metallic SWCNTs within a bundle. Our findings give direct evidence that bundling reduces the effective Coulomb interactions via intertube interactions within bundled metallic SWCNTs.

<p>平均直径が異なる複数の単層カーボンナノチューブ(SWCNT)薄膜について、ゼーベック係数Sを測定した。その結果キャリアドープによるSの最大値が直径にあまり依存しないことが分かった。そこでSWCNTの半導体型と金属型が並列に結合したモデルを用いて、金属型の割合を変化させて熱電物性を計算した。その結果金属型による短絡が原因で、Sの最大値は直径によってほとんど変化せず、金属型の割合によって大きく変化することが分かった。</p>

The Seebeck coefficient S and the electrical resistivity rho of single-wall carbon nanotube (SWCNT) films were investigated as a function of the SWCNT diameter and carrier concentration. The S and rho significantly changed in humid environments through p-type carrier doping. Experiments, combined with theoretical simulations based on the non-equilibrium Green's function theory, indicated that the power factor P can be increased threefold by the enrichment of semiconducting SWCNTs, but the nanotube diameter has little effect. The improvement of the film resistivity strongly enhances the film thermoelectric performance, manifested as increasing the value of P above 1200 mu W/(m&K-2). (C) 2016 The Japan Society of Applied Physics

Doped single-wall carbon nanotube (SWCNT) films were prepared and their Seebeck coefficient (S) and electrical resistivity (rho) were investigated as functions of carrier density. For heavy doping, a second maximum of S (S = 35 mu V/K) was discovered, with its corresponding power factor, P = 85 mu W/(m.K-2), 6 times that of the first maximum for lightly doped films. Calculations for zigzag SWCNTs suggest that the thermoelectric performance can be effectively improved by controlling the multiplicity of the one-dimensional band and tuning the carrier density. This provides a new strategy for achieving higher performance at a lower cost than using high-purity semiconducting SWCNTs. (C) 2016 The Japan Society of Applied Physics

Single-wall carbon nanotubes (SWCNTs) are a good model system that provides atomically smooth nanocavities. It has been reported that water-SWCNTs exhibit hydrophobicity depending on the temperature T and the SWCNT diameter D. SWCNTs adsorb water molecules spontaneously in their cylindrical pores around room temperature, whereas they exhibit a hydrophilic-hydrophobic transition or wet-dry transition (WDT) at a critical temperature T-wd approximate to 220-230 K and above a critical diameter D-c approximate to 1.4-1.6 nm. However, details of the WDT phenomenon and its mechanism remain unknown. Here, we report a systematic experimental study involving X-ray diffraction, optical microscopy, and differential scanning calorimetry. It is found that water molecules inside thick SWCNTs (D &gt; D-c) evaporate and condense into ice Ih outside the SWCNTs at T-wd upon cooling, and the ice Ih evaporates and condenses inside the SWCNTs upon heating. On the other hand, residual water trapped inside the SWCNTs below Twd freezes. Molecular dynamics simulations indicate that upon lowering T, the hydrophobicity of thick SWCNTs increases without any structural transition, while the water inside thin SWCNTs (D &lt; D-c) exhibits a structural transition, forming an ordered ice. This ice has a well-developed hydrogen bonding network adapting to the cylindrical pores of the SWCNTs. Thus, the unusual diameter dependence of the WDT is attributed to the adaptability of the structure of water to the pore dimension and shape. Published by AIP Publishing.

We report the observation of the intrinsic magnetic susceptibility of highly purified SWCNT samples prepared by a combination of acid treatment and density gradient ultracentrifugation (DGU). We observed that the diamagnetic susceptibility of SWCNTs increases linearly with increasing nanotube diameter. We found that the magnetic susceptibility divided by the diameter is a universal function of the scaled temperature. Furthermore, the estimated magnetic susceptibilities of pure semiconducting and pure metallic SWCNT samples suggest that they respond differently to changes in carrier density, which is consistent with theory. These findings provide experimental verification of the theoretically predicted diameter, temperature, and metallicity dependence of the magnetic susceptibility.

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.

We performed P-31-NMR measurements on LaFe(As1-xPx)O to investigate the relationship between antiferromagnetism and superconductivity. The antiferromagnetic (AFM) ordering temperature T-N and the moment mu(ord) are continuously suppressed with increasing P content x and disappear at x = 0.3 where bulk superconductivity appears. At this superconducting x = 0.3, quantum critical AFM fluctuations are observed, indicative of the intimate relationship between superconductivity and low-energy AFM fluctuations associated with the quantum-critical point in LaFe(As1-xPx)O. The relationship is similar to those observed in other isovalent-substitution systems, e.g., BaFe2(As1-xPx)(2) and SrFe2(As1-xPx)(2), with the '' 122 '' structure. Moreover, the AFM order reappears with further P substitution (x &gt; 0.4). The variation of the ground state with respect to the P substitution is considered to be linked to the change in the band character of Fe-3d orbitals around the Fermi level.

We found a giant Seebeck effect in semiconducting single-wall carbon nanotube (SWCNT) films, which exhibited a performance comparable to that of commercial Bi2Te3 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, all-carbon, flexible thermoelectric devices. (C) 2014 The Japan Society of Applied Physics

Single-walled carbon nanotubes (SWCNTs) are seamless cylindrical tubes consisting of carbon atoms with diameters ranging from less than one nanometer to a few nanometers. The arrangement of carbon atoms in a SWCNT is uniquely specified by using a pair of integers (n, m) referred to as the chiral indices. While the detailed structures, such as a carbon-carbon bond length, should be important, they have not been fully clarified yet. In this work, we examine the possibility of powder X-ray diffraction (XRD) method to characterize structures of SWCNTs. It is found that the XRD is a useful tool to "fingerprint" the chiral indices of bulk SWCNT samples. Besides, we find that information on the detailed structure within a SWCNT can be obtained from the XRD pattern. The application to a highly concentrated SWCNTs clarifies that the (6,5) SWCNT is expanded along the radial direction compared to that of ideal rolling up structure of graphene, with a negligible change along the tube axis. (C) 2014 Elsevier Ltd. All rights reserved.

The NMR results in iron pnictides BaFe2(As1-xPx)(2) and Ba(Fe1-xCox)(2)As-2 are analyzed based on the self-consistent renormalization (SCR) spin-fluctuation theory. The temperature dependence of the NMR relaxation rate T-1(-1) as well as the electrical resistivity is well reproduced by a SCR model where two-dimensional antiferromagnetic (AF) spin fluctuations are dominant. The successful description of the crossover feature from non-Fermi-liquid to Fermi-liquid behavior strongly suggests that low-lying spin fluctuations in BaFe2(As1-xPx)(2) and Ba(Fe1-xCox)(2)As-2 possess an itinerant AF nature, and that chemical substitution in the two compounds tunes the distance of these systems to an AF quantum critical point. The close relationship between spin fluctuations and superconductivity is discussed compared with the other unconventional superconductors, cuprate and heavy fermion superconductors. In addition, it is suggested that magnetism and lattice instability in these pnictides are strongly linked via orbital degrees of freedom.

A newly developed nano-structural material, zeolite-templated carbon (ZTC), adsorbs a large amount of water, up to 120-170 wt.% with respect to the dry ZTC. Here, we investigated this nanoconfined water by several experimental methods, including specific heat and nuclear magnetic resonance measurements, and molecular dynamics simulations. The water was highly mobile down to 200 K and its rotational correlation time followed an apparent non-Arrhenius behavior. With decreasing temperature, it was suggested that the water freezes into a low-density amorphous ice with a possible glass transition temperature of around 150 K. ZTC provides a new platform to study three-dimensional 'supercooled' water. (C) 2013 Elsevier B.V. All rights reserved.

The discovery of superconductivity above 50 K in Fe-pnictides/chalcogenides has broken the "monopoly" of high-temperature superconductivity in cuprates, and generated tremendous interests in the scientific community. During the past four years, considerable progresses have been made in materials synthesis and explorations, physical properties measurement and theoretical understandings. Compared with cuprate superconductors, iron-based superconductors are much more flexible, and have many different parent compounds. Superconductivity could be achieved with either hole or electron-doping, isovalent substitution, application of pressure, or chemical intercalations. From fundamental physics point of view, iron-based superconductors have properties that are more amenable to band structural calculations. That is allowing for quantitative comparison with experiments. In addition, iron-based superconductors may have intrinsic properties favourable for applications, such as insensitive to impurities, large and isotropic upper critical fields, and strong grain boundaries. The book aims to review the progresses made in this fascinating field. All invited contributors are world leading experts in the particular subjects of the field. There is no such review book available up to now. It is a good guide for understanding materials, physical properties, and superconductivity mechanism aspects, and is important for students and beginners to have an overall view of the recent progress in this active field. © 2013 Pan Stanford Publishing Pte. Ltd. All rights reserved.

Static and dynamic magnetic properties of lightly P-substituted BaFe2(As1-xPx)(2) were systematically investigated by P-31 NMR. The averaged internal magnetic field at the P site in the zero-temperature limit evaluated from the broadening of NMR spectra in the antiferromagnetic (AFM) phase is gradually suppressed toward x similar to 0.35 with increasing x, which provides definitive evidence for the existence of an AFM quantum critical point (QCP) at x similar to 0.35. The location of the AFM QCP is consistent with the previous estimation from temperature dependence of spin dynamics in the normal state, and the superconducting transition temperature T-c takes the maximum around the QCP. Our experiments, revealing a signature of a QCP extending up to room temperature, establish BaFe2(As1-xPx)(2) as one of the most accessible systems for unraveling the nature of quantum criticality and the relationship between AFM quantum criticality and unconventional superconductivity.

As-75 and La-139 NMR results for LaFeAs(O1-xFx) (x = 0, 0.025, and 0.04) are reported. Upon F doping, the tetragonal-to-orthorhombic structural phase transition temperature T-S, antiferromagnetic transition temperature T-N, and internal magnetic field mu H-0(int) are gradually reduced for x &lt; 0.04. However, at x = 0.04, T-N is abruptly suppressed to be 30 K along with a tiny mu H-0(int), which is distinct from the continuous disappearance of the ordered phases in the Ba122 systems of Ba(Fe,Co)(2)As-2 and BaFe2(As,P)(2). The anisotropy of the spin-lattice relaxation rate T-1(-1) , (T-1)H-parallel to ab(-1)/(T-1)H parallel to c(-1), in the paramagnetic phase of x = 0 and 0.025 is constant (similar to 1.5) but increases abruptly below T-S due to the enhancement of (T-1)H-parallel to ab(-1) by the slowing-down of magnetic fluctuations. This indicates that the tetragonal-to-orthorhombic structural distortion enhances the anisotropy in the spin space via magnetoelastic coupling and/or spin-orbit interaction.

The coexistence of magnetism and superconductivity in the isovalent-P-substituted BaFe2(As1-xPx)(2) has been investigated microscopically by P-31-NMR measurements. We found that superconducting (SC) transition occurs in a magnetic region with static ordered moments and that the moments are reduced below the SC transition temperature T-c in the samples near the phase boundary of magnetism and superconductivity. Our results indicate that magnetism and superconductivity coexist spatially but compete with each other on the same Fermi surfaces. The coexistence state is qualitatively different from that observed in other unconventional superconductors and gives a strict constraint on the theoretical model for superconductivity in BaFe2(As1-xPx)(2).

From detailed angle-resolved NMR and Meissner measurements on a ferromagnetic (FM) superconductor UCoGe (T-Curie similar to 2.5 K and T-SC similar to 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 parallel to 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 parallel to c induce the unique spin-triplet superconductivity in UCoGe. This is the first clear example that FM fluctuations are intimately related with superconductivity.