Sakai Hironori

Unconventional Heavy-Fermion Superconductivity of a New Transuranium Compound NpPd5Al2

We succeeded in growing a new transuranium compound NpPd 5 Al 2 with the tetragonal crystal structure by the Pb-flux method, and measured the electrical resistivity, specific heat, magnetic susceptibility and magnetization. NpPd 5 Al 2 is found to be a paramagnetic heavy-fermion superconductor with strong coupling Δ C /γ T c = 2.33 [ T c =4.9 K and γ= 200 mJ/(K 2 ·mol)]. The upper critical field H c2 at 0 K is large and highly anisotropic: H c2 (0) =37 kOe for H ∥[100] and H c2 (0) =143 kOe for H ∥[001]. H c2 is found to be strongly suppressed by the magnetic field, and the magnetization curve indicates a step-like increase at H c2 , which are due to the paramagnetic effect. The low-temperature electronic specific heat approximately follows the T 3 -dependence, indicating the existence of point nodes in the energy gap. The d -wave superconductivity with point nodes is most likely realized in this compound.

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 .

Effect of Pressure and Magnetic Field on the Superconducting State of a Heavy Fermion Superconductor NpPd5Al2

We have carried out the electrical resistivity measurements under high pressures and high magnetic fields in NpPd 5 Al 2 in order to investigate a quantum critical behavior and the stability of the superconducting phase. A T -linear dependence of the electrical resistivity is changed into the Fermi liquid relation of ρ∼ A T 2 under the magnetic fields, and A value indicates a divergent feature around the upper critical field H c2 , indicating the existence of a field-tuned quantum critical point (QCP) around H c2 . With increasing pressure, the superconducting transition temperature T sc decreases and becomes zero above 5.7 GPa, together with a decrease of the upper critical field, indicating that the electronic state of NpPd 5 Al 2 is located in the vicinity of QCP at ambient pressure and deviates from QCP with increasing pressure. Furthermore, we have constructed the superconducting phase diagram at 3.71 GPa.

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.

Heavy Fermion Superconductivity with the Strong Pauli Paramagnetic Effect on NpPd5Al2

We measured the electrical resistivity, specific heat, magnetic susceptibility and magnetization in the first Np-based superconductor NpPd 5 Al 2 with the tetragonal crystal structure. NpPd 5 Al 2 is a paramagnet and becomes the superconductivity at T c =4.9 K. The large electronic specific heat coefficient of γ= 200 mJ/K 2 mol at T c indicates that NpPd 5 Al 2 is a heavy fermion superconductor. The upper critical field H c2 is large and highly anisotropic: H c2 (0)=37 kOe for H ∥[100] and H c2 (0)=143 kOe for H ∥[001]. Although the initial slope of H c2 at T c is very large, H c2 is strongly suppressed against the magnetic field for both field direction, indicating the existence of strong Pauli paramagnetic effect. Furthermore, the magnetization curve shows the step-like behaviors at H c2 , corresponding to the first order transition. The d -wave superconductivity with the spin-singlet state is most likely realized in NpPd 5 Al 2 .

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 * .

Single Crystal Growth and Crystal Structure of ThRhIn5 and LuCoGa5

We have succeeded in growing a high-quality single crystal of ThRhIn 5 and LuCoGa 5 with HoCoGa 5 -type crystal structure by flux method. The lattice parameters, positional parameters and thermal parameters were determined by the single-crystal x-ray diffraction measurements. For ThRhIn 5 , we have also measured the electrical resistivity down to 1.9 K.We have succeeded in growing a high-quality single crystal of ThRhIn 5 and LuCoGa 5 with HoCoGa 5 -type crystal structure by flux method. The lattice parameters, positional parameters and thermal parameters were determined by the single-crystal x-ray diffraction measurements. For ThRhIn 5 , we have also measured the electrical resistivity down to 1.9 K.

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.