深澤 英人

We have clarified that the internal magnetic field in α-Mn is suppressed by pressure and that a first-order phase transition occurs at around 1.4-1.5 GPa by zero-field nuclear magnetic resonance. In the pressure-induced ordered state above about 1.4 GPa, the scenario of the canted-antiferromagnetic order or the ferrimagnetic order seems to be preferable to account for the weak ferromagnetism.

© 2020 American Physical Society. Nuclear quadrupole resonance (NQR) measurements were performed on the heavy-fermion superconductor Ce3PtIn11 with Tc=0.32K. The temperature dependence of both spin-lattice relaxation rate 1/T1 and NQR spectra evidences the occurrence of two successive magnetic transitions with TN1≃2.2K and TN2≃2.0K. In successive magnetic transitions, even though the magnetic moment at the Ce(2) site plays a major role, the magnetic moment at the Ce(1) site also contributes to some extent. While a commensurate antiferromagnetic ordered state appears for TN2<T<TN1, a partially incommensurate antiferromagnetic ordered state is suggested for T<TN2.

© 2017 We performed 63,65Cu and 139La NMR measurements of T’-La1.8Eu0.2CuO4+δ (T’-LECO) with the Nd2CuO4-type structure (so-called T’-structure). As a result, we detected the 63,65Cu NMR signal under finite magnetic fields and found superconductivity without antiferromagnetic (AF) order only in the reduced T’-LECO, where excess apical oxygen atoms are properly removed. This indicates that the intrinsic ground state of the ideal T’-LECO is a paramagnetic and superconducting (SC) state. Below Tc, the Knight shift was found to rapidly decrease, which indicates the emergence of bulk superconductivity due to spin-singlet Cooper pairs in the reduced T’-LECO. In the SC state of the reduced T’-LECO, moreover, a characteristic temperature dependence of the spin-lattice relaxation rate 1/T1 was observed, which implies the existence of nodal lines in the SC gap. These findings suggest that the superconductivity in the reduced T’-LECO probably has d-wave symmetry. In the normal state of the reduced T’-LECO, on the other hand, AF fluctuations were found to exist from the temperature dependence of 1/T1T, though no clear pseudogap behavior was observed. This suggests that the AF correlation plays a key role in the superconductivity of undoped high-Tc cuprate superconductors with the T’-structure.

© 2016 American Physical Society. We have carried out the measurements of high-pressure B11-nuclear magnetic resonance on the intermediate-valence compound SmB6 to investigate the effects of pressure on Sm 4f states and the quasiparticle band. From the measurements of spin-lattice relaxation time, just below the critical pressure Pc of nonmagnetic-magnetic phase transition, we find that quasiparticle bandwidth clearly decreases with pressure, while the insulating gap is almost constant or slightly increases. The latter is consistent with the result of a band-structure calculation. These pressure induced modifications in the band structure indicate the enhancement of the density of states of the quasiparticles when approaching Pc. The pressure dependence of the Sm 4f states and the origin of the insulating gap are well explained in terms of exchange interactions between conduction and 4f electrons.

Single crystals of Ba1-xKxFe2As2 with 0.15 = x = 1 have been successfully synthesized by a KAs self-flux method. The potassium (K) concentration x of the grown crystals was systematically changed by changing the mixing ratio of Ba to Fe in the starting materials. The crystals have flat surfaces corresponding to the crystallographic (001) planes, whose planar dimensions increase with decreasing thickness when x increases. The superconducting transition temperature systematically changes with x, ranging from 38K at x = 0.4 to 3.4K at x = 1.0, in good agreement with the results for polycrystalline samples.

© 2016 The Physical Society of Japan. For the understanding of Ce-doped high-Tc cuprates with the so-called TA-structure, we have performed 63,65Cu and 139La NMR measurements in T′-Pr1.3-xLa0.7CexCuO4+δ single crystals with x = 0.10 and 0.15. The Néel temperature (TN) was evaluated from the temperature (T) dependence of the 139La NMR line width. For x = 0.10, TN decreased from 190 K in the as-grown crystal to 60 K in the oxygen-reduced crystal annealed at 800°C. For x = 0.15, TN of 170 K in the as-grown crystal decreased to zero through the same heat treatment. The oxygen reduction strongly suppressed the antiferromagnetic (AF) order. 63,65Cu NMR spectra had a narrow center line and broad satellites with a small electric field gradient in the paramagnetic state. In the AF state, in contrast, 63,65Cu NMR spectra had a large electric field gradient. The T dependence of the 63Cu nuclear spin-lattice relaxation rate (1/T1) showed the existence of large spin fluctuations in the paramagnetic state. The spin fluctuations seem to play a key role in the appearance of superconductivity in the TA-cuprates.

<p>La1.8Eu0.2CuO4+δは電子ドープ型と同じT' 構造を有し、酸素を完全に取り除くことにより電子ドープなしに超伝導が発現するノンドープ銅酸化物超伝導体として注目されている。今回は、700℃で還元処理を施した試料（Tc=22K）について主にCu, La-NMRを行ない、超伝導状態と常伝導状態での磁気揺らぎを吟味したので報告する。</p>

© 2015 American Physical Society. We report a phenomenon intimately related to the spin-triplet superconductivity. It is well known that the spin susceptibility decreases below the superconducting transition temperature in almost all superconductors because of spin-singlet pair formation, while it may remain unchanged in a handful of spin-triplet exceptions. Here we report the observation in Sr2RuO4 with nuclear magnetic resonance that the spin susceptibility originating from the Ru-4d electron slightly increases by ∼2% of the total and becomes inhomogeneous in the superconducting state. These are reasonably explained if the electron pairs form the equal-spin pairing in the mixed state. A similar phenomenon was predicted for superfluid He340 years ago, but had never been demonstrated in any superconductor.

© 2014 American Physical Society. We performed a laser angle-resolved photoemission spectroscopy (ARPES) study on a wide doping range of the Ba1-xKxFe2As2 (BaK) iron-based superconductor. We observed a robust low-binding energy (BE) kink structure (kink1) in the dispersion which is doping dependent whereby its energy peaks at the optimally doped level (x∼0.4) and decreases towards the underdoped and overdoped sides. We attribute this kink to electron-mode coupling in good agreement with the inelastic neutron scattering (INS) and scanning tunneling microscopy (STM) results on the same compound where a similar bosonic mode associated with spin excitations was observed. The relation between the mode energy (Ω) and the SC transition temperature (Tc) deduced from our laser ARPES data follow the universal relation deduced from INS and STM. In addition, we could resolve another kink at higher BE (kink2) showing less doping and temperature dependence compared to kink1 and which thus may be of different origin.

© 2014 The Physical Society of Japan. We carried out combined transport and optical measurements for BaFe2As2 and five isostructural transition-metal (TM) pnictides. The low-energy optical conductivity spectra of these compounds are, to a good approximation, decomposed into a narrow Drude (coherent) component and an incoherent component. The iron arsenides BaFe2As2 and KFe2As2 are distinct from other pnictides in their highly incoherent charge dynamics or bad metallic behavior with the coherent Drude component occupying a tiny fraction of the low-energy spectral weight. The fraction of the coherent spectral weight or the degree of coherence, which is a measure of the strength of electronic correlations, is shown to be well correlated with the TM-pnictogen bond angle and the electron filling of TM 3d orbitals. The iron arsenides are thus strongly correlated systems, and the doping into BaFe2As2 controls the strength of electronic correlations. This naturally explains a remarkable asymmetry in the charge dynamics of electron- and hole-doped systems, and the unconventional superconductivity appears to emerge when the correlations are strong.

In high-transition-temperature superconducting cuprates and iron arsenides, chemical doping plays an important role in inducing superconductivity. Whereas in the cuprate case, the dominant role of doping is to inject charge carriers, the role for the iron arsenides is complex owing to carrier multiplicity and the diversity of doping. Here, we present a comparative study of the in-plane resistivity and the optical spectrum of doped BaFe 2 As 2, which allows for separation of coherent (itinerant) and incoherent (highly dissipative) charge dynamics. The coherence of the system is controlled by doping, and the doping evolution of the charge dynamics exhibits a distinct difference between electron and hole doping. It is found in common with any type of doping that superconductivity with high transition temperature emerges when the normal-state charge dynamics maintains incoherence and when the resistivity associated with the coherent channel exhibits dominant temperature-linear dependence.

To gain insight into the unconventional superconductivity of Fe pnictides with no electron pockets, we measure the thermal conductivity κ and penetration depth λ in the heavily hole-doped regime of Ba1-xKxFe2As2. The residual thermal conductivity (κ/T)T→0K and T dependence of λ consistently indicate the fully gapped superconductivity at x=0.76 and the (line) nodal superconductivity at higher hole concentrations. The magnitude of κTTc|T→0K and the slope of λ(T) at low temperatures, both of which are determined by the properties of the low-energy excitations, exhibit a highly unusual nonmonotonic x dependence. These results indicate a dramatic change of the nodal characteristics in a narrow doping range. We suggest that the observed x dependence is naturally explained by a doping crossover of the gap function between the s-wave states with and without sign reversal between Γ-centered hole pockets. © 2014 American Physical Society.

To gain insight into the unconventional superconductivity of Fe pnictides with no electron pockets, we measure the thermal conductivity. and penetration depth. in the heavily hole- doped regime of Ba1- x K x Fe2As2. The residual thermal conductivity (./ T) T. 0 K and T dependence of. consistently indicate the fully gapped superconductivity at x = 0.76 and the ( line) nodal superconductivity at higher hole concentrations. The magnitude of. T Tc| T. 0 K and the slope of.( T) at low temperatures, both of which are determined by the properties of the low- energy excitations, exhibit a highly unusual nonmonotonic x dependence. These results indicate a dramatic change of the nodal characteristics in a narrow doping range. We suggest that the observed x dependence is naturally explained by a doping crossover of the gap function between the s- wave states with and without sign reversal between - centered hole pockets.

We have investigated the superconducting (SC)-gap anisotropy for several Ba-doped KFe2As2 samples using laser-based angle-resolved photoemission spectroscopy. We show that the SC-gap anisotropy and node positions drastically change with a small amount of Ba doping. Our results totally exclude a possibility of d-wave symmetry and strongly suggest that both spin and orbital fluctuations are important for the pairing interaction in the Ba-doped K122. © 2014 American Physical Society.

The temperature, field, and field-orientation dependences of the electronic specific heat Ce of the iron-pnictide superconductor KFe 2As2 have been investigated. Thermodynamic evidence of the presence of line nodes is obtained from the T and H linear dependences of Ce/T in the low-T and low-H region. Under a magnetic field rotated within the tetragonal ab-plane, a fourfold oscillation is observed in C e with a sign change at 0.08Tc. On the basis of the Doppler-shift analysis, the observed Ce minima in H || [100] at low T indicate the presence of line nodes somewhere on the Fermi surface where the Fermi velocity is parallel to the [100] direction; this is consistent with the octet-line-node scenario proposed recently by a photoemission experiment. In addition, the low-T Ce/T exhibits an unusual upturn on cooling at moderate fields only for H || ab, which is understood in terms of the strong Pauli paramagnetic effect on multiband superconductivity. © 2014 The Physical Society of Japan.

115In nuclear quadrupole resonance measurements were performed in the normal state of the heavy fermion superconductor Ce2PdIn8 under hydrostatic pressure up to about 2.3 GPa. The observed behavior of the spin-lattice relaxation rate revealed a systematic suppression of antiferromagnetic critical fluctuations with increasing pressure.

We report high-pressure studies of X-ray diffraction and ¹¹B-nuclear magnetic resonance (NMR) in the intermediate-valence compound SmB<inf>6</inf>. The pressure dependence of the lattice constant was precisely determined and no anomaly was observed up to 9.1 GPa. The temperature dependence of the nuclear quadrupole resonance frequency \nu_{\text{Q } }, obtained from the ¹¹B-NMR measurements, is predominantly contributed by on-site charge distribution. Using the relationship between \nu_{\text{Q } } and Sm valence at ambient pressure reported previously, we estimate the pressure dependence of the Sm valence up to 6 GPa as well. The increase in the Sm valence accelerates with pressure and reaches an increase of about 10% at 6 GPa. The pressure-induced localization of Sm 4f-holes may be responsible for the long-range magnetic order under pressure.

We report high-pressure studies of X-ray diffraction and 11B-nuclear magnetic resonance (NMR) in the intermediatevalence compound SmB6. The pressure dependence of the lattice constant was precisely determined and no anomaly was observed up to 9.1 GPa. The temperature dependence of the nuclear quadrupole resonance frequency νQ, obtained from the 11B-NMR measurements, is predominantly contributed by on-site charge distribution. Using the relationship between νQ and Sm valence at ambient pressure reported previously, we estimate the pressure dependence of the Sm valence up to 6 GPa as well. The increase in the Sm valence accelerates with pressure and reaches an increase of about 10% at 6 GPa. The pressure-induced localization of Sm 4f -holes may be responsible for the long-range magnetic order under pressure. © 2013 The Physical Society of Japan.

Using small angle neutron scattering, the anisotropy of the magnetic vortex lattice (VL), in the heavily hole-doped pnictide superconductor KFe 2As2, was studied. Well-ordered VL scattering patterns were measured with fields applied in directions between B∥c and the basal plane, rotating either towards [100] or [110]. Slightly distorted hexagonal patterns were observed when B∥c. However, the scattering pattern distorted more strongly as the field was rotated away from the c axis. At low field, the arrangement of vortices is affected by the anisotropy of penetration depth in the plane perpendicular to the field. By fitting the distortion with the anisotropic London model, we obtain an estimate of ∼3.4 for the anisotropy factor γ between the in-plane and c-axis penetration depths at the lowest temperature studied. The results further reveal VL phase transitions as a function of field direction. We discuss these transitions using the "hairy ball" theorem. ©2013 American Physical Society.

We report the first high-pressure 11B-NMR studies above 3 GPa on the intermediate-valence semiconductor SmB6. A 11B-NMR line obtained at 4.9 GPa, the highest pressure for the measurements, and at 1.9 K shows quite similar a line shape to that at ambient pressure, indicating no structural or magnetic phase transition up to this pressure. The temperature dependence of the spin lattice relaxation rate 1/T 1 at 4.9 GPa still exhibits an activation-type temperature dependence characteristic of semiconductors, which reveals an obvious decrease in the insulating gap by about 30% compared to the gap at ambient pressure. The present experimental facts of a finite insulator gap and no magnetic order at 4.9 GPa are consistent with recent transport measurements performed under better hydrostatic pressures. © 2013 The Korean Physical Society.

We have completely determined the Fermi surface in KFe2As 2 via de Haas-van Alphen (dHvA) measurements. Fundamental frequencies ε, α, ζ, and β are observed in KFe2As 2. The first one is attributed to a hole cylinder near the X point of the Brillouin zone while the others to hole cylinders at the Γ point. We also observe magnetic breakdown frequencies between α and ζ and suggest a plausible explanation for them. The experimental frequencies show deviations from frequencies predicted by band-structure calculations. Large effective masses up to 19 me for Bâ̂\c have been found, me being the free-electron mass. The carrier number and Sommerfeld coefficient of the specific heat are estimated to be 1.01-1.03 holes per formula unit and 82-94 mJmol-1 K-2, respectively, which are consistent with the chemical stoichiometry and a direct measure of 93 mJmol-1 K-2. The Sommerfeld coefficient is about nine times enhanced over a band value, suggesting the importance of low-energy spin and/or orbital fluctuations, and places KFe2As2 among strongly correlated metals. We have also performed dHvA measurements on Ba 0.07K0.93Fe2As2 and have observed the α and β frequencies. © 2013 American Physical Society.

In iron-pnictide superconductivity, the interband interaction between the hole and electron Fermi surfaces (FSs) is believed to play an important role. However, KFe2As2 has three zone-centered hole FSs and no electron FS but still exhibits superconductivity. Our ultrahigh-resolution laser angle-resolved photoemission spectroscopy unveils that KFe2As 2 is a nodal s-wave superconductor with highly unusual FS-selective multi-gap structure: a nodeless gap on the inner FS, an unconventional gap with "octet-line nodes" on the middle FS, and an almost-zero gap on the outer FS. This gap structure may arise from the frustration between competing pairing interactions on the hole FSs causing the eightfold sign reversal. Our results suggest that the A1g superconducting symmetry is universal in iron-pnictides, in spite of the variety of gap functions.

Nuclear quadrupole resonance measurements were performed on the heavy-fermion superconductor Ce 2PdIn 8. Above the Kondo coherence temperature T coh30 K, the spin-lattice relaxation rate 1/T 1 is temperature independent, whereas at lower temperatures, down to the onset of superconductivity at T c=0.64 K, it is nearly proportional to T1 /2. Below T c, 1/T 1 shows no coherence peak and decreases as T3 down to 75 mK. All these findings indicate that Ce 2PdIn 8 is close to the antiferromagnetic quantum critical point, and the superconducting state has an unconventional character with line nodes in the superconducting gap. © 2012 American Physical Society.

Transverse-field muon spin rotation measurements were performed on the iron-based superconductor, KFe2As2, in order to investigate the superconducting gap mechanism. We found that the temperature dependence of the magnetic penetration depth λ shows multiple gap-like behavior and that λ depends on the field. These results suggest the existence of quasiparticle excitations outside the vortices. © 2012 The Physical Society of Japan.

The thermal conductivity κ of the iron arsenide superconductor KFe 2As 2 was measured down to 50mK for a heat current parallel and perpendicular to the tetragonal c axis. A residual linear term at T→0, κ 0/T is observed for both current directions, confirming the presence of nodes in the superconducting gap. Our value of κ 0/T in the plane is equal to that reported by Dong et al. for a sample whose residual resistivity ρ 0 was 10 times larger. This independence of κ 0/T on impurity scattering is the signature of universal heat transport, a property of superconducting states with symmetry-imposed line nodes. This argues against an s-wave state with accidental nodes. It favors instead a d-wave state, an assignment consistent with five additional properties: the magnitude of the critical scattering rate Γ c for suppressing T c to zero; the magnitude of κ 0/T, and its dependence on current direction and on magnetic field; the temperature dependence of κ(T). © 2012 American Physical Society.

We have performed 75As nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements on single-crystalline Ba 1-xK xFe 2As 2 for x - 0:27{1. 75As nuclear quadruple resonance frequency (vq) increases linearly with increasing x. The Knight shift K in the normal state shows Pauli paramagnetic behavior with a weak temperature T dependence. K increases gradually with increasing x. By contrast, the nuclear spin-lattice relaxation rate 1=T1 in the normal state has a strong T dependence, which indicates the existence of large antiferomagnetic (AF) spin fluctuations for all x's. The T dependence of 1=T1 shows a gaplike behavior below approximately 100 K for 0:6 < x < 0:9. This behaviors is well explained by the change in the band structure with the expansion of hole Fermi surfaces and the shrinkage and disappearance of electron Fermi surfaces at the Brillouin zone (BZ) with increasing x. The anisotropy of 1=T 1, represented by the ratio of 1=T 1ab to 1=T 1c, is always larger than 1 for all x's, which indicates that stripe-type AF fluctuations are dominant in this system. The K in the superconducting (SC) state decreases, which corresponds to the appearance of spin-singlet superconductivity. The T dependence of 1=T 1 in the SC state indicates a multiple-SC-gap feature. A simple two-gap model analysis shows that the larger superconducting gap gradually decreases with increasing x from 0.27 to 1 and a smaller gap decreases rapidly and nearly vanishes for x > 0:6 where electron pockets in BZ disappear. © 2012 The Physical Society of Japan.

We report cyclotron resonance (CR) measurements for the Fe-based superconductor KFe2As2. Only one series of CR with an effective mass of 3.4me (me is the free electron mass) is observed. The CR signal is attributed to the quasi-two-dimensional α Fermi surface at the Γ point. Our main finding is that the effective mass obtained from the CR is significantly smaller than that obtained from the de-Haas van Alphen (dHvA) measurements. This is the direct evidence of the mass enhancement by electronic correlation. A comparison between the CR and dHvA measurements shows that both intra- and interband electronic correlations contribute to the mass enhancement in KFe2As2. © Published under licence by IOP Publishing Ltd.

We performed the 75As nuclear magnetic resonance measurements of Ba1-xKxFe2As2 (BKFA) for x = 0.27, 0.39, 0.58, 0.64, 0.69 and 1 to understand gap structure in this system. Temperature dependence of spin lattice relaxation rate (1/T1) below superconducting (SC) transition temperature Tc changes gradually from x = 0.39 to 1.0. This result indicates that the SC gap structure changes smoothly from full gap state with maximum Tc into nodal-line structure state for x = 1 (KFe2As2). BKFA does not have distinct SC symmetry change. The nodal gap structure appears continuously and develops gradually with increasing x. © Published under licence by IOP Publishing Ltd.

We report on transverse field muon spin rotation measurements of the magnetic penetration depth λ in the iron-based superconductor KFe 2As2 in order to investigate the superconducting gap mechanism. We have found that temperature dependence of λ shows multiple gap like behavior and λ depends on the field. These results suggests the existence of quasiparticle excitations outside the vortices. © Published under licence by IOP Publishing Ltd.

Cyclotron resonance (CR) measurements for the Fe-based superconductor KFe2As2 are performed. One signal for CR is observed, and is attributed to the two-dimensional α Fermi surface at the Γ point. We found a large discrepancy in the effective masses of CR [(3.4±0.05) me (me is the free-electron mass)] and de Haas-van Alphen results, a direct evidence of mass enhancement due to electronic correlation. A comparison of the CR and de Haas-van Alphen results shows that both intra- and interband electronic correlations contribute to the mass enhancement in KFe 2As2. © 2011 American Physical Society.

By neutron scattering, we have observed a well-ordered magnetic vortex lattice (VL) in KFe2As2 single crystals. With the field along the c axis, a nearly isotropic hexagonal VL is formed, with no symmetry transitions up to high fields, indicating a small anisotropy of the superconducting state around this axis. This rules out line nodes parallel to the c axis, and thus d-wave or anisotropic s-wave pairing. However, the strong temperature dependence of the signal at T?Tc shows that extremely small gap values exist; these may arise from nodal lines perpendicular to the c axis. © 2011 American Physical Society.

We report the 75As nuclear magnetic resonance (NMR), nuclear quadrupole resonance (NQR) and specific heat measurements of the heavily hole-doped superconductor KFe2As2 (superconducting transition temperature Tc ≃ 3.5 K) using high quality single crystals. NMR Knight shifts in superconducting state clearly indicate that spin-singlet superconductivity realizes in this material. It is noted that the electronic specific heat coefficient of the material is 93.0 mJ/K 2mol-fu and that the value is quite heavy for d electron system. © 2011 The Physical Society of Japan.

Filled skutterudites GdRu4P12 and TbRu 4P12 exhibit antiferromagnetic ordering below 22 and 20 K, respectively. In TbRu4P12, an additional anomaly exists at around 10K under a zero field; the origin of this successive transition might be related to multipole ordering. In order to determine the detailed magnetic properties of GdRu4P12 and TbRu4P12, we performed zero-field 31P nuclear magnetic resonance (NMR) and 101Ru nuclear quadrupole resonance (NQR) measurements on them. The zero-field 31P-NMR spectra indicate the occurrence of magnetic transition only at 22K (GdRu 4P12) and 20K (TbRu4P12). We observed an anomalous temperature dependence of the spin-lattice relaxation rate 1=T1 just below 20K and a small anomaly at around 10K in the 101Ru-NQR of TbRu4P12. The temperature dependence of 1=T1 also indicates that the nesting condition in the ordered states of GdRu 4P12 and TbRu4P12 is poorer than that of SmRu4P12. © 2011 The Physical Society of Japan.

A neutron scattering study of heavily hole-overdoped superconducting KFe2As2 revealed a well-defined low-energy incommensurate spin fluctuation at [π(1±2δ),0] with δ=0.16. The incommensurate structure differs from the previously observed commensurate peaks in electron-doped AFe2As2 (A=Ba, Ca, or Sr) at low energies. The direction of the peak splitting is perpendicular to that observed in Fe(Te,Se) or in Ba(Fe,Co)2As2 at high energies. A band structure calculation suggests interband scattering between bands around the Γ and X points as an origin of this incommensurate peak. The perpendicular direction of the peak splitting can be understood within the framework of multiorbital band structure. The results suggest that spin fluctuation is more robust in hole-doped than in electron-doped samples, which can be responsible for the appearance of superconductivity in the heavily hole-doped samples. © 2011 American Physical Society.