Tokunaga Yo

Anisotropic Superconducting Gap in Transuranium Superconductor PuRhGa5: Ga NQR Study on a Single Crystal

69,71 Ga NMR/NQR studies have been performed on a single crystal of the transuranium superconductor PuRhGa 5 with T c ≃9 K. We have observed a 69 Ga NQR line at ∼29.15 MHz, and assigned it to the 4 i Ga site using the NMR results. The 69 Ga NQR spin–lattice relaxation rate 1/ T 1 shows no coherence peak just below T c , but obeys T 3 behavior below T c . This result strongly suggests that PuRhGa 5 is an unconventional superconductor having an anisotropic superconducting gap. The gap amplitude 2Δ 0 ( T →0)≃5 k B T c and the residual density of states N res ( T =0)/ N 0 ( T = T c )≃0.25 have been determined assuming a simple polar function of the form Δ(θ,φ)=Δ 0 cos θ, where θ and φ are angular parameters on the Fermi surface.

27Al NMR Evidence for the Strong-Coupling d-Wave Superconductivity in NpPd5Al2

We present 27 Al NMR studies for a single crystal of the newly developed superconductor NpPd 5 Al 2 ( T c = 4.9 K). In the superconducting state, the 27 Al NMR spectrum shifts to lower frequencies owing to a decrease of the spin part of the Knight shift, and exhibits broadening characteristic of the local field distribution associated with a vortex lattice. We have also found that the 27 Al nuclear spin–lattice relaxation rate (1/ T 1 ) decreases just below T c with no coherence peak and shows T 3 behavior at low temperatures. The observed temperature dependences of the Knight shift and 1/ T 1 in general accord with those assuming a line-node superconducting gap with the gap value of 2Δ/ k B T c ≃6.4. Therefore, the present NMR data provides evidence for strong-coupling d -wave superconductivity in NpPd 5 Al 2 .

NMR Shift Measurements of 69Ga in Unconventional Superconductor PuRhGa5

The NMR shift ( K ) at the Ga(2) site (the 4 i site) has been measured in PuRhGa 5 , a compound which shows anisotropic superconductivity below T c ≈9 K. In the normal state, K ( T ) is very nearly independent of T below ∼30 K. This result is consistent with the relaxation rate measurements. This Korringa-like behavior indicates the formation of Fermi-liquid state below ∼30 K. The transferred hyperfine coupling constants have been estimated from the relation between K and the bulk susceptibility, showing a strong anisotropy. Measurements of K ( T ) in the superconducting state have also been performed. A decrease of K in the superconducting state has been observed, which strongly suggests a spin-singlet pairing.

NMR Study of Antiferromagnet UPtGa5

We report spin–lattice relaxation time ( T 1 ) measurements at Ga and Pt sites in a single crystal sample of an itinerant antiferromagnet UPtGa 5 ( T N =27 K). In the paramagnetic state at high temperatures, the T -dependence of static susceptibility and 1/ T 1 T at Ga and Pt sites shows a similar behavior, indicating the dynamical susceptibility \textImχ( q ,ω) has no strong q -dependence at high temperatures. This behavior suggests a localized character of this compound at high temperatures. Around T N , 1/ T 1 T at Ga site increases rapidly due to critical slowing down behavior, in contrast a saturation behavior is observed at Pt site as the antiferromagnetic fluctuation cancels at Pt site. The characteristic spin fluctuation energy Γ is estimated using T 1 and previously obtained hyperfine parameters.

Anisotropic Spin Fluctuations in the Heavy Fermion Systems: Case Studies of CePd5Al2 and NpPd5Al2

In the so-called 115 family members with chemical formulae of Ce T In 5 ( T = Co, Rh, Ir) and A n T Ga 5 ( A n = U, Np, Pu), our analysis of the Knight shift and spin–lattice relaxation rates suggests that larger XY-type anisotropy in the antiferromagnetic spin fluctuations in the normal states would favor d -wave superconductivity with higher T c . In order to examine this working hypothesis, the anisotropy ratios of antiferromagnetic fluctuations are derived from our NMR results for the superconductor NpPd 5 Al 2 and antiferromagnet CePd 5 Al 2 , which are tetragonal compounds with a 115-related structure. CePd 5 Al 2 shows Ising-type anisotropy, which is the same anisotropy as in the ordered state. On the other hand, NpPd 5 Al 2 shows XY-type anisotropy just above T c .

31P-NMR Study of the Neptunium-based Filled-Skutterudite NpFe4P12

31 P-NMR experiments have been performed on a single crystal of NpFe 4 P 12 with applied fields parallel to the direction for the cubic crystal axis. From K –χ plot above 20 K, the isotropic and anisotropic components of the hyperfine coupling constants between Np-5 f electrons and P nuclei have been estimated. A relatively large value of the isotropic component suggests the importance of a transferred hyperfine mechanism through the 3 s orbitals of P atoms. In the ferromagnetic ordered state, an additional NMR line splitting associated with the reorientation of the Np moments has been observed below T * =19 K. Analysis of the NMR line splitting reveals that the Np-5 f moments are ferromagnetically aligned along the applied field above T * , while they are reoriented along the ferromagnetic easy-axis of the direction below T * .

NMR Studies of Actinide Compounds: The NMR of Actinide Nuclei

In a brief recap of recent work on actinide NMR studies at JAERI, we review the direct observation of 235 U NMR which has been carried out for both UO 2 and USb 2 , as well as the estimate of T 1 ( T ) for 235 U which has been obtained from indirect (i.e., T 2 of 103 Rh) NMR studies of the Pauli paramagnet URh 3 . New data for 1/ T 1 vs T for 17 O in NpO 2 are presented and analyzed. A sizeable fraction of the observed relaxation rate is strongly field-dependent, an effect attributed to a cross-relaxation process from the 100%-abundant, I = 5/2 237 Np nuclear spins. Analysis of this relaxation component leads to quantitative estimates of T 1Np (≃40 ns) and of anomalously enhanced values of the 17 O– 237 Np nuclear spin-spin coupling. These effects as well as future prospects for actinide NMR studies are discussed.

NMR Study on AmO2: Comparison with UO2 and NpO2

Recently, 17 O NMR has been performed for the first time on AmO 2 . We have observed a drastic broadening of the NMR spectrum, along with a sudden drop of NMR signal intensity, below T 0 =8.5 K. These data provide the first microscopic evidence for a phase transition at this temperature as a bulk property of this system. In this paper, we compare NMR results for AmO 2 with those from UO 2 and NpO 2 . In the ordered state we have recorded an 17 O NMR spectrum with a triangular line shape, which resembles neither that of UO 2 nor NpO 2 . This spectrum indicates that the internal local field H int is distributed very nearly randomly from zero to a maximum value, that is, H int max ∼7 kOe. The temperature dependences of the Knight shift and the nuclear relaxation rate are also compared.

Multipolar Phase Transition in NpO2: Comparison with UO2 from 17O-NMR

We review our recent 17 O-NMR results on NpO 2 . The compound is suggested to exhibit a magnetic octupolar phase transition at T 0 =26 K. From the temperature dependence of 17 O-NMR spectrum, the occurrence of two inequivalent oxygen sites has been confirmed below T 0 . Furthermore, a dramatic dependence of nuclear spin-lattice relaxation rates on applied field has been observed above and below T 0 . These NMR results contrast sharply with those for the antiferromagnet UO 2 and thus highlight the exotic nature of the phase transition in NpO 2 .

Observation of Ferromagnetic and Antiferromagnetic Correlations in UIr3B2

A general susceptibility (χ q ,ω ) evaluation from the Knight shift and nuclear spin–lattice relaxation measurements via 11 B NMR technique has been carried out on single crystal UIr 3 B 2 . The experimental results revealed two distinct temperature regimes in which the magnetic state dynamics are different from one another. Above T * ≃50 K, the temperature variation of χ q ,ω is mainly subject to q = 0 fluctuations, suggesting that ferromagnetic correlations develop at high temperatures. On the other hand, antiferromagnetic finite- q components abruptly emerge and show remarkable growth below T * . Even at low temperatures, ferromagnetic correlations persist, but χ q ,ω develops significant dispersion with antiferromagnetic components. The origin of the observed magnetic correlations is discussed in terms of the low dimensionality which is derived from the highly anisotropic structure of UIr 3 B 2 .