水戸 毅

The nuclear spin relaxation rate between T=0.09 and 100K in a new heavy fermion (HF) compound was presented. The new HF compound was found to be more itinerant than known Cerium based HFs from the nuclear quadrupolar resonance (NQR) measurement. The bulk superconductivity was found setting in at 0.40K below which the coherence peak was absent thereby suggesting unconventional superconductivity with line-node gap.

We report Cu nuclear-quadrupole-resonance results under pressure (P) on a homogeneous CeCu2Si2 (T-c = 0.65 K) that revealed critical magnetic fluctuations at the border to an antiferromagnetic (AF) phase. This exotic superconducting (SC) phase evolves into the typical heavy-fermion SC phase at minute pressures exceeding P(c)similar to0.2 GPa. The nuclear spin-lattice relaxation data in a P range 0.85 - 2.58 GPa are shown to be accountable by the spin-fluctuation theory based on the nearly AF Ferni-liquid model. In 0 less than or equal toP<P-c<similar to>0.2 GPa, by contrast, we conclude that the exotic SC phase manifests itself under the unconventional normal state where AF waves propagate over a long range without any trace of AF order and thereby the heavy-fermion state breaks up.

We report temperature dependence of the Gaussian component of spin-echo decay rate (1/T2G) in under, optimally and overdoped single crystals Bi2Sr2CaCu2O8+δ (Bi2212) with the superconducting (SC) transition temperatures Tc = 79, 86, and 77.3 K, respectively. For all the samples, 1/T2G was found to decrease below a temperature T*c lower than T* but higher than Tc. Here T* is the temperature below which the `pseudogap' emerges. T*c undergoes a similar variation with Tc as function of doping level. This result is corroborated by those results of nuclear spin-lattice relaxation rate 1/T1 and the Knight shift reported previously. These anomalies below T*c are suggested to be precursive to the SC transition. Based on the NMR results, three crossover temperatures, TmK, T* and T*c are marked in the phase diagram on Bi2212.

We report (formula presented) nuclear-quadrupole-resonance (NQR) measurements of the pressure (formula presented)-induced superconductor (formula presented) in the antiferromagnetic (AF) and superconducting (SC) states. In the AF region, the internal field (formula presented) at the In site is substantially reduced from (formula presented) at (formula presented) to 0.39 kOe at (formula presented) while the Néel temperature slightly changes with increasing P. This suggests that either the size in the ordered moment (formula presented) or the angle (formula presented) between the direction of (formula presented) and the tetragonal c axis is extrapolated to zero at (formula presented) at which a bulk SC transition is no longer emergent. In the SC state at (formula presented) the nuclear spin-lattice relaxation rate (formula presented) has revealed a (formula presented) dependence without the coherence peak just below (formula presented) giving evidence for the unconventional superconductivity. The dimensionality of the magnetic fluctuations in the normal state is also discussed. © 2001 The American Physical Society.

We report Cu nuclear-quadrupole-resonance results under pressure (P) on a homogeneous CeCu2Si2 (Tc=0.65 K) that revealed critical magnetic fluctuations at the border to an antiferromagnetic (AF) phase. This exotic superconducting (SC) phase evolves into the typical heavy-fermion SC phase at minute pressures exceeding Pc∼0.2 GPa. The nuclear spin-lattice relaxation data in a P range 0.85 - 2.58 GPa are shown to be accountable by the spin-fluctuation theory based on the nearly AF Fermi-liquid model. In 0≤P< Pc∼0.2 GPa, by contrast, we conclude that the exotic SC phase manifests itself under the unconventional normal state where AF waves propagate over a long range without any trace of AF order and thereby the heavy-fermion state breaks up.

We report Cu-NQR results under pressure on heavy-fermion (HF) superconductor CeCu2Si2 close to a magnetic phase, Ce 0.99Cu2.02Si2 and CeCu2(Si 0.98Ge0.02)2. It is found that an application of a minute pressure exceeding 0.3 GPa markedly suppresses low-energy magnetic fluctuations observed at ambient pressure (P = 0) in Ce0.99Cu 2.02Si2. Correspondingly, anomalous superconducting (SC) characteristics evolve into conventional HF-SC ones with increasing pressure. In the latter, the Ge substitution works as a negative chemical pressure. CeCu2(Si0.98Ge0.02)2 reveals a magnetic phase transition at TN= 0.75 K followed by the SC phase transition at Tc ∼ 0.4 K at P = 0. It is suggested experimentally that the magnetic fluctuations seen in Ce0.99Cu 2.02Si2 at P = 0 possesses a quantum critical character of both SC and magnetically ordered states. © 2001 Elsevier Science B.V. All rights reserved.

We report 115In nuclear-quadrupole-resonance (NQR) measurements of the pressure(P)-induced superconductor CeRhIn5 in the antiferromagnetic (AF) and superconducting (SC) states. In the AF region, the internal field Hint at the In site is substantially reduced from Hint = 1.75 kOe at P = 0 to 0.39 kOe at P = 1.23 GPa, while the Néel temperature slightly changes with increasing P. This suggests that either the size in the ordered moment MQ(P) or the angle θ(P) between the direction of MQ(P) and the tetragonal c axis is extrapolated to zero at P* = 1.6±0.1 GPa at which a bulk SC transition is no longer- emergent. In the SC state at P =2.1 GPa, the nuclear spin-lattice relaxation rate 115(1/T1) has revealed a T3 dependence without the coherence peak just below Tc, giving evidence for the unconventional superconductivity. The dimensionality of the magnetic fluctuations in the normal state is also discussed.

In order to investigate the evolution of magnetic properties when the ground state changes from magnetism to superconductivity, we have performed In-NQR measurements under hydrostatic pressure in heavy-fermion antiferromagnetic compound, CeIn3. Peaks in the In-NQR spectra were shifted in the magnetically ordered state due to the appearance of the asymmetric parameter η, which enables us to determine the ordering temperature TN, precisely. TN in our measurement is in good agreement with that from the resistivity measurement. Nuclear spin-lattice relaxation rate 1/T1 increases gradually upon cooling in the paramagnetic state at pressure (P) smaller than 15 kbar, whereas 1/T1 shows slightly decreasing behavior with decreasing T above TN at P>15 kbar. It is suggested that the magnetic property at TN may change from the localized to itinerant regime around 15 kbar, above which the Kondo temperature TK is higher than TN.

In order to investigate the magnetic properties at the vicinity of magnetic to superconducting (SC) phase boundary, we have performed Cu-NQR measurements under hydrostatic pressure (P) on two off-stoichiometric Ce1+xCu2+ySi2. Ce0.99Cu2.02Si2 exhibits the anomalous ground state so-called A-phase, dominated by critical magnetic correlations and affected by a presence of minor SC fraction at ambient P. With increasing P which enhances the Kondo temperature, the fraction of the A-phase is markedly suppressed, but the SC fraction is increased, although Tc stays nearly constant in the range of 1 bar < P < 14.4 kbar. This evidences that the P-induced A- to SC phase change is of first order. In Ce0.975Cu2Si2 which undergoes a static magnetic ordering (SMO) at ambient P below TN ∼ 0.6 K, no superconductivity has been induced up to 19.4 kbar. © 2000 Elsevier Science B.V. All rights reserved.

In order to investigate magnetic properties in the vicinity of the magnetic to superconducting (SC) phase boundary, we have performed Cu-NQR measurements under hydrostatic pressure (P) on two off-stoichiometric Ce1+xCu2+ySi2polycrystals: Ce0.99Cu2.02Si2 (denoted as Ce 0.99) exhibits the anomalous ground state so-called A phase dominated by critical magnetic correlations and Ce0.975Cu2Si2 (Ce 0.975) shows a static magnetic ordering at ambient P. With increasing P which enhances the Kondo temperature, the fraction of the A phase becomes markedly suppressed, and the SC fraction increases correspondingly, although Tc stays nearly constant in the measured pressure range, P ≤ 14.4 kbar. This shows that the P-induced transition from the A to the SC phases is of a first-order type, without passing through a quantum (T → 0) phase transition. However, no superconductivity has been attained in Ce 0.975 up to 19.4 kbar. It is considered that the P-induced superconductivity is sensitive to the sample quality.

We report temperature dependence of the Gaussian component of spin-echo decay rate (1/T2G) in single crystals Bi2Sr2CaCu2O8+y(Bi2212). We found that 1/T2G starts to decrease at a certain temperature, Tc*, which is higher than Tc and lower than the pseudogap temperature, T*, suggesting the formation of local Cooper pairing without global phase coherency. Based on our NMR results, we develop the phase diagram in the Bi2212 system which includes three crossover temperatures, TmK, T* and Tc*. © 2000 Elsevier Science B.V. All rights reserved.

Recent NMR measurements on high-Tc and related material Sr2RuO4 by the Osaka group are reviewed. The following results are presented: (1) Study of YBa2Cu3O7 + Ni has revealed that the attractive force is spin in origin. (2) Analysis of T1, T2 and Knight shift (K) in various high-Tc materials supports the antiferromagnetic spin-fluctuation mediated superconducting model. (3) Temperatures at which a pseudogap starts to open obtained by 1/T1T, dK/dT, photoemission and resistivity agree with each other in the Bi2Sr2CaCu2O8+δ system. (4) 17O Knight shift measurements in the related material Sr2RuO4 suggests that a triplet pairing is realized in this compound as in the heavy-fermion superconductor UPt3. © 1999 Published by Elsevier Science B.V. All rights reserved.

High-pressure NMR measurements on a single crystal of hole-doped two-leg ladder compound, Sr2Ca12Cu24O41, have been carried out up to ∼1.7 GPa. The activated T-dependences of T1 and Knight shift at 1.7 GPa assured the persistence of the spin gap in this system. © 1998 Elsevier Science B. V. All rights reserved.

In under- and optimally doped Bi2Sr2CaCu2O8 (Bi2212), we found a characteristic temperature TK*, below which the spin part of Knight shift Ks starts to decrease steeply and extrapolates to zero at T > 0. TK* is in good agreement with the temperatures at which the pseudogap starts to open in the quasiparticle density of states (DOS) at the Fermi level, and A* at which 1/T1T has a broad peak. We propose that the spin-gap behavior observed in NMR measurements has the same origin as the pseudogap in the quasiparticle DOS. © 1999 Elsevier Science B.V. All rights reserved.

We report that the nuclear spin-lattice relaxation rate divided by temperature, (Formula presented) has a broad maximum around (Formula presented) and ∼100 K in underdoped and overdoped single crystals of (Formula presented) (Bi2212) with (Formula presented) and 77.3 K, respectively, showing the normal-state spin-gap behavior. One result is that the steep decrease in the (Formula presented)Cu Knight shift, (Formula presented) has been observed below (Formula presented) and ∼100 K, suggesting the normal-state pseudogap behavior in quasiparticle density of states (DOS), which is consistent with the pseudogap behavior below (Formula presented) revealed on underdoped Bi2212 from studies of angle-resolved photoemission spectroscopy. We propose that the spin-gap behavior in (Formula presented) has the same origin as the pseudogap in the quasiparticle DOS. © 1998 The American Physical Society.

In order to unravel the nature of spin correlations in the vicinity of the transition between antiferromagnetic (AF) and superconducting phases, 63Cu NMR measurements have been carried out for the 8%-Sr doped La2-xSrxCuO4 (LSCO) and hole-reduced Bi2Sr2CuO4 (Bi2201) with Tc = 0 K. It was found that Bi2201 exhibits neither superconductivity nor long-range order from the NMR experiments. 63(1/T1T) in B12201 shows successive decrease down to 100 K, suggesting the presence of the pseudo spin-gap in contrast to its significant increase in 8%-LSCO.

We have investigated an influence of Zn- and Ni-doping at CuO2 plane in various high-Tc compounds. In optimum-doped LSCO, La1.825Sr0.175CuO4, magnetic Ni as well as nonmagnetic Zn induce a large residual density of states (RDOS) in the superconducting state. The induction of RDOS by slight Zn-doping is considered to be a common feature in optimum-doped high-Tc compounds.

In order to unravel tile nature of spin correlation in the vicinity of the antiferromagnetic (AF) phase boundary, the 63Cu nuclear spin lattice relaxation rate, 63(1/T1), has been measured for the 8%-Sr doped superconducting (SC) La2-xSrxCuO4 (LSCO) by Cu NMR and NQR techniques. The temperature dependence of 63(1/T1T) has revealed a continuous increase down to Tc without the spin pseudogap behavior observed in underdoped YBa2Cu3O6+x and YBa2Cu4O8.

Results of our NMR measurements of high Tc superconductors are reviewed. From systematic measurements of NQR frequency, Knight shift, nuclear spin lattice relaxation rate and nuclear transverse relaxation rate over a wide hole concentration range from light to heavy doped compounds together with the impurity and pressure effect, following results are concluded. The numbers of the holes at the respective sites of Cu and O site in the CuO2 plane govern the several physical properties of the system. There are optimum values of these hole numbers to produce highest Tc. The superconducting properties are well explained consistently by the d-wave pairing model. Tc is found to be correlated intimately with the antiferromagnetic spin fluctuation parameters that governs the nuclear relaxation. The mechanism for the superconductivity is magnetic in origin.

The spin-lattice relaxation rate 1/T1 and the Knight shift were measured for La1.85Sr0.15CuO4 and YBa2Cu4O8 under pressure to 1.4 GPa. The results together with the previously reported pressure dependence of the nuclear quadrupole frequency, νQ, give evidence that the local hole distribution changes under pressure. In La1.85Sr0.15CuO4, the hole density in the Cu 3d orbit decreases, accompanying a suppression of the antiferromagnetic spin fluctuation of the low-energy component. In YBa2Cu4O8, by contrast, the spin fluctuation changes little while the total hole number in the CuO2 plane increases. We argue that such changes in the hole distribution are responsible for the Tc increase in the respective compounds. We also found that the so-called "spin-gap" behavior in YBa2Cu4O8 becomes more pronounced under pressure. © 1995.

Both 63Cu NQR frequency νQ and 1/T1T were found to decrease under high pressure up to 14kbar, indicating the decrease of hole density at Cu site. The enhancement of Tc under pressure is explained as due to the transfer of hole density from Cu to oxygen, which increases the effective carrier density. © 1994.