Homma Yoshiya

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.

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.

Magnetic Properties of a UPtAl Single Crystal

UPtAl is a ferromagnet with a spontaneous magnetic moment M s =1.38 µ B /f.u. directed along the hexagonal c axis and Curie temperature T C =43.5 K. The uniaxial magnetocrystalline anisotropy is very strong, the estimated anisotropy field B a exceeds 290 T. In a thermally-demagnetized sample the initial susceptibility along the c axis is very low for a wide field interval below a critical field B st . Then the magnetization increases abruptly and saturates in a rather narrow field interval. The low-temperature hysteresis loop is almost rectangular with a 100% remanent magnetization. The coercivity ( B c =0.7 T at 1.7 K) weakens rapidly with increasing temperature and vanishes at 20 K. A zero-field cooled sample exhibits a large thermal hysteresis. In the vicinity of B st and B c , a very pronounced magnetic viscosity is observed. As the main mechanism of the magnetic hysteresis we propose the pinning of domain walls which are very narrow due to the huge anisotropy and a relatively weak exchange interactio...

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.

Magnetic Properties and Heavy Electronic States in the Antiferromagnet NpPtGa5

We succeeded in growing a new neptunium compound NpPtGa 5 by the Ga-flux method, and measured the electrical resistivity, specific heat, magnetic susceptibility, magnetization and de Haas–van Alphe...

Characteristic High-Field Magnetization in a Transuranium Antiferromagnet NpRhGa5

We have measured the high-field magnetization for a transuranium antiferromagnet NpRhGa 5 with two successive antiferromagnetic transition temperatures of T N1 =36 and T N2 =32 K and an ordered moment of 0.96 µ B /Np. For the magnetic field along the easy-axis of the [100] direction of the tetragonal structure, the magnetization indicates a metamagnetic transition at H c =26 T with a sharp step and saturates above H s =38 T, with 0.43 µ B /Np. On the other hand, the hard-axis magnetization for H ∥[001] increases linearly and saturates above H s =28 T, with 0.44 µ B /Np. These magnetization curves were discussed on the basis of reorientation and reduction of the magnetic moment.

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.

Effect of Pressure on Magnetism of UIrGe

We report the effect of hydrostatic pressure on the electronic state of the antiferromagnet UIrGe, which is isostructural and isoelectronic with the ferromagnetic superconductors UCoGe and URhGe. A...

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 .