Yoshihumi Tokiwa

Magnetic and Thermal Properties of CeIrIn5 and CeRhIn5

We studied the magnetic properties in a non-magnetic heavy-fermion compound CeIrIn 5 and an antiferromagnetic compound CeRhIn 5 with the tetragonal structure. High-field magnetization of CeIrIn 5 s...

Fermi surface and de Haas-van Alphen oscillation in both the normal and superconducting mixed states of UPd2Al3

The dHvA oscillation was observed clearly in both the normal and superconducting mixed states for UPd 2 Al 3 with the hexagonal structure. Eight kinds of dHvA branches were detected, which are well explained by the results of band calculations based on a fully-relativistic spin-polarized LAPW method. An inner orbit of a band 41-hole Fermi surface, named ζ, has the largest dHvA frequency of 9.45×10 7 Oe and cyclotron mass of 65 m 0 , reflecting a large electronic specific heat coefficient of 145 mJ/K 2 ·mol. Branch α of a band 40-hole Fermi surface was observed in both the normal and superconducting mixed states, where the dHvA frequency does not change in magnitude between the normal and mixed states. The cyclotron effective mass of branch α is, however, found to be reduced and the corresponding Dingle temperature or scattering rate of the conduction electron increases with decreasing the field in the mixed state. An anisotropic energy gap with a line node for UPd 2 Al 3 is discussed from the angular depe...

Fermi surface properties and de Haas–van Alphen oscillation in both the normal and superconducting mixed states of URu2Si2

Abstract We have succeeded in growing a high-quality single crystal of URu2Si2 with a residual resistivity ratio of 255 and performed magnetoresistance and de Haas–van Alphen (dHvA) experiments. The dHvA oscillations were observed clearly in both the normal and superconducting mixed states. From the magnetoresistance experiments, it is concluded that URu2Si2 is a compensated metal with equal carrier numbers of electrons and holes. In the dHvA experiments we observed three kinds of Fermi surfaces. We studied precisely the Fermi surface properties in both the normal and mixed states. The dHvA frequency does not change in magnitude between the normal and mixed states, while the cyclotron effective mass is reduced, and the corresponding Dingle temperature or scattering rate of the conduction electron increases in the mixed state.

Neutron Scattering Study of the Crystal and Magnetic Structures in Itinerant-5f Antiferromagnets UNiGa5 and UPtGa5

The crystal and magnetic structures of 5 f itinerant antiferromagnets UNiGa 5 and UPtGa 5 were studied by means of neutron scattering. The powder diffraction data were well explained by the HoCoGa 5 tetragonal structure with space group P4/mmm. In UNiGa 5 , uranium spins in the simple tetragonal lattice have a Neel-type structure with a relatively large moment of 0.9 µ B /U along the [0,0,1] direction. On the other hand, magnetic moments of uranium atoms in UPtGa 5 are aligned ferromagnetically in the (0,0,1) plane and are directed along the [0,0,1] direction in an antiferromagnetic sequence (↑↓). An ordered moment is 0.24 µ B /U. The nearest-neighbor interaction might be different in these isostructural compounds.

Magnetic and Fermi Surface Properties of UCoGa5 and URhGa5

We succeeded in growing high-quality single crystals of UCoGa5 and URhGa5 by the self-flux method and measured the electrical resistivity, magnetic susceptibility, and de Hass-van Alphen(dHvA) oscillation. Three kinds of dHvA branches were observed, ranging from 8.6x 105 to 8.1 x 106Oe, which correspond to closed Fermi surfaces. UCoGa5 and URhGas are concluded to be semimetals. The cyclotron mass is small, ranging from 0.6 m0 to 2.9 m0.

Fermi Surface Properties of USi3

We succeeded in growing a single crystal of USi 3 with the cubic crystal structure and measured the transverse magnetoresistance and de Haas-van Alphen(dHvA) oscillation. Fifteen dHvA branches were observed, ranging from 7.37×10 6 to 2.33×10 8 Oe. These branches are in good agreement with theoretical ones based on 5 f -itinerant FLAPW band calculations. Cyclotron masses, which are in the range from 0.39 to 4.17 m 0 , are approximately the same as the corresponding band masses. These results indicate that the itinerant 5 f -electrons form wide bands. The magnetoresistance results confirmed the existence of open orbits, which can be well explained by the band 10-electron Fermi surface.

Magnetic and Fermi Surface Properties of the Ferromagnetic Compound UGa2.

We have studied the magnetic and Fermi surface properties in a ferromagnet UGa 2 . The electrical resistivity, elastic constant, thermal expansion coefficient, thermoelectric power, magnetic susceptibility and Hall coefficient indicate highly anisotropic properties, reflecting the hexagonal structure. The 5 f 2 - and 5 f 3 -CEF schemes, based on the localized 5 f -electron nature, are discussed to understand the magnetic susceptibility and magnetization data. We have also done the magnetoresistance and de Haas-van Alphen (dHvA) experiments to investigate the Fermi surface properties. Both data are very complicated, implying that the Fermi surface is a multiply-connected one. They are explained neither by the localized 5 f 2 - and 5 f 3 -core models nor by the spin-polarized 5 f -itinerant model. The cyclotron mass is, however, rather light, ranging from 0.17 to 6.2 m 0 , which is consistent with the electronic specific heat coefficient of 11 mJ/K 2 ·mol.

De Haas-van Alphen Oscillation in Both the Normal and Superconducting Mixed States of UPd2Al3

We succeeded in growing a high-quality single crystal of the heavy fermion superconductor UPd 2 Al 3 and observed the de Haas-van Alphen (dHvA) oscillation in both the normal and superconducting mixed states. The dHvA frequency, which is proportional to a maximum cross-sectional area of the electron Fermi surface labelled α, does not change in magnitude between the normal and mixed states, while the cyclotron mass of 5.7 m 0 in the normal state is reduced to 3.5 m 0 at 0.7 H c2 , and the corresponding Dingle temperature of 0.20 K in the normal state is increased to 0.39 K at 0.7 H c2 . Here, H c2 of 39 kOe is the upper critical field along the [0001] direction in the hexagonal structure.

Fermi surface study on uranium-based intermetallic compounds

Fermi surface properties of various uranium-based intermetallic compounds, including a 5f-localized compound, Pauli paramagnets, heavy fermions, antiferromagnets and ferromagnets, have been investigated by means of the de Haas-van Alphen effect. For this purpose we have grown high-quality single crystals using a purified uranium metal. Both the topology of the Fermi surface and the cyclotron mass in Pauli paramagnets such as USi3 are well explained by the band calculations. Surprisingly, the topology of the Fermi surface in heavy fermions such as UPt3 and UPd2Al3 also agree with the theoretical calculations, except for their unusually large mass enhancement. We have also succeeded in detecting a dHvA oscillation in the superconducting mixed state in the heavy fermion superconductor, UPd2Al3. Spin-polarized band calculatioins are approximately applicable to some magnetically ordered compounds such as UBi2 and UNiGa5.

Ga and Pt NMR Study of UPtGa5 and UNiGa5

Ga and Pt NMR measurements have been carried out for two isomorphs compounds, UPtGa 5 and UNiGa 5 , which exhibit different magnetic structures below T N . Knight shift K measurements in the parama...