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We carried out a de Haas–van Alphen (dHvA) experiment under pressure on a heavy-fermion superconductor, URu2Si2. The dHvA frequency, which corresponds to a nearly spherical Fermi surface, increases slightly with increasing pressure, while the corresponding cyclotron mass decreases considerably. Neither the dHvA frequency nor the cyclotron mass exhibit any abrupt change at the critical pressure of 1.5 GPa. The present result is thus inconsistent with a recent phase-separation proposal.

The f-electron systems of the cerium and uranium compounds have attracted attention in the context of heavy fermion and anisotropic superconductivity. We clarified an itinerant f-electron nature for UPt 3 by the de Haas–van Alphen experiments, detecting heavy conduction electrons with 100 m 0 . The dHvA oscillation was also observed clearly in both the normal and superconducting mixed states in CeRu 2 ,URu 2 Si 2 and UPd 2 Al 3 . An anisotropic energy gap with a line node for URu 2 Si 2 was discussed from the angular dependence of the dHvA amplitude in the mixed state.

We have performed 238U Mossbauer spectroscopy of uranium-based heavy fermion superconductors, UPd2Al3 and URu2Si2, in order to investigate their physical properties, mainly their magnetic properties. The slow relaxation of magnetic hyperfine interaction in a paramagnetic state and the static hyperfine field has been observed in an antiferromagnetic ordered state for each compound. The line-widths have maximum at their characteristic temperatures where their magnetic susceptibilities have maximum values.

We have performed a high-resolution angle-resolved photoemission spectroscopy (ARPES) on single-crystal ${\mathrm{URu}}_{2}{\mathrm{Si}}_{2}$ to study the band structure and the Fermi surface in the paramagnetic phase. The valence-band structure consisting of the Ru $4d$ and Si $3p$ states shows a qualitatively good agreement between the ARPES experiment and the band-structure calculation. In the vicinity of the Fermi level ${(E}_{F}),$ we observed a less dispersive band, which crosses ${E}_{F}$ midway between the Z and X points in the Brillouin zone. Comparison with the band calculation as well as with the de Haas\char21{}van Alphen (dHvA) result suggests that the experimental band near ${E}_{F}$ is assigned to the largest hole pocket centered at the Z point, which has a strong U $5f\ensuremath{-}\mathrm{Ru}4d$ hybridized character. The observed remarkable narrowing of the near-${E}_{F}$ band compared with the band-structure calculation suggests a strong renormalization effect due to the electron correlation of U $5f$ electrons.

We have carried out the magnetization measurements in high magnetic fields up to 60 T and in the wide temperature range from 1.3 to 77 K for a heavy femion superconductor URu 2 Si 2 . A sharp metamagnetic transition with three steps is found below 14 K for the field along the tetragonal [001] direction. This transition is changed into a sharp metamagnetic transition with two steps in a narrow temperature range from about 14 to 17 K and furthermore into a broad metamagnetic transition with one step above the Neel temperature T N =17.5 K. The broad transition persists up to a characteristic temperature T χ m a x (=55 K) where the magnetic susceptibility has a maximum in the temperature dependence. These results are discussed on the basis of a change of the electronic state of 5 f -electrons from itinerant to localized and the crystalline electric field model.

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 experiments have been carried out for a heavy fermion superconductor URu 2 Si 2 . We found that the (1 0 0) magnetic Bragg peak intensity is reduced by 1–2% in magnitude below the superconducting transition temperature of 1.15 K. This is direct evidence of the interplay between magnetism and superconductivity in URu 2 Si 2 .

We succeeded in growing high-quality single crystals of CeRu 2 and URu 2 Si 2 and observed the de Haas–van Alphen (dHvA) oscillations in both the normal and superconducting mixed states. The dHvA frequency does not change in magnitude between the normal and superconducting mixed states, while the cyclotron mass is reduced, and the corresponding Dingle temperature or the scattering rate of the conduction electron increases in the mixed state.

We succeeded in growing a high-quality single crystal of URu 2 Si 2 by the Czochralski pulling method and applying the solid state electro-transport method under ultrahigh vacuum. The sample quality strongly depends on the position in the ingot. For some parts of this ingot, the electrical resistivity, specific heat and de Haas–van Alplhen effect were measured. The electrical resistivity of the surface sample clearly indicates a T -linear temperature dependence below 5 K. The mean free path in this sample was determined by the de Haas–van Alphen effect to be 11000 A.

In order to investigate the metamagnetic anomaly (MA) observed in several heavy-fermion compounds, field dependence of specific heat C(B) and magnetocaloric effect have been measured in the prototypical 4f-electron compound CeRu2Si2, and compared to that of a 5f-electron metamagnet UCoAl. In the C/T versus B curves, an unexpected double-peak structure is found in CeRu2Si2, suggesting that MA is closely related to a sharp peak in the density of states of the hybridized quasiparticle bands. On the other hand, for UCoAl, only a small decrease in C/T is observed, although both compounds exhibit similar bulk magnetic properties.

We have measured the magnetoresistance and de Haas-van Alphen (dHvA) oscillation of URu 2 Si 2 . From the results of the magnetoresistance measurement, URu 2 Si 2 is concluded to be a compensated metal with closed Fermi surfaces. The dHvA branch, which corresponds to a nearly spherical Fermi surface, has been detected in both the normal and superconducting mixed states. The Dingle temperature or the electron scattering rate is found to be strongly enhanced and the cyclotron effective mass to be correspondingly reduced in the mixed state.

The high-field magnetization masurements in UPd 2 Al 3 , URu 2 Si 2 and UPt 3 were carried out up to 55 T at various temperatures. We determined their phase diagrams. The metamagnetic transitions for these compounds were observed up to the characteristic temperatures showing the maxima of the magnetic susceptibilities.

The electrical resistivity and the lattice parameters of the tetragonal heavy-fermion compound CeNi2Ge2 with ThCr2Si2 type of structure have been investigated under high pressure. At room temperatures, the compression curve of an unit-cell volume up to 70 kbar is well fitted to Murnaghan equation of state. In the electrical resistivity at low temperatures, a gradual depression of the Fermi-liquid AT2 term with increasing pressure is observed. From these results, the Gruneisen parameter of the characteristic energy of the Fermi-liquid ground state is determined to be 90.

We have studied the temperature dependence of the electrical resistivity, the magnetic susceptibility and the high-field magnetization of CeNi 2 Ge 2 . Quadratic temperature dependence of the resistivity is observed below 0.7 K. The susceptibility exhibits a broad maximum at 28 K. Magnetization shows metamagnetic behavior at H M ∼42 T as in CeRu 2 Si 2 . These results indicate that CeNi 2 Ge 2 is a new heavy-fermion compound showing the metamagnetic behavior in the nonmagnetic ground state.

We carried out the de Haas–van Alphen experiment under pressure for a heavy fermion antiferromagnet URu2Si2. The detected cyclotron mass for a nearly spherical Fermi surface under pressure changes considerably, reflecting a change of the magnetic moment. The present result is, however, inconsistent with the recent phase separated proposal.

UPd2Al3 and URu2Si2 are considered to be non-s-wave superconductors with an anisotropic energy gap from the NMR and specific heat measurements. We have succeeded in observing clearly the dHvA oscillation in both the normal and superconducting mixed states. The cyclotron mass is found to be reduced and the Dingle temperature increases in the mixed state. An anisotropic energy gap with a line node for UPd2Al3 and URu2Si2 was discussed from the angular dependence of the dHvA amplitude in the mixed state.

The magnetic properties of heavy-fermion superconductors, UPd2Al3 and URu2Si2, were studied by 238U Mossbauer spectroscopy to elucidate the local electronic states of uranium atoms directly. In the paramagnetic states of UPd2Al3 and URu2Si2, we have observed an appreciable change in line width caused by the paramagnetic relaxation, which is the evidence for the formation of the heavy fermion.

Magnetization measurements of a heavy fermion compound URu 2 Si 2 have been performed in the superconducting state. Broad peak structure of the irreversible magnetization was observed around 50 kOe for H ‖ a . The Ginzburg–Landau parameter obtained from the equilibrium magnetization slightly decreases for H ‖ c with decreasing temperature, suggesting the presence of the paramagnetic suppression of the superconductivity. However, the suppression is not so large as expected from the H c2 ( T ) curve. Thermodynamic critical field appears to show unusual temperature dependence.

One branch has been detected in the de Haas–van Alphen (dHvA) experiments in both the normal and superconducting mixed states of URu2Si2 and UPd2Al3 with an anisotropic superconducting energy gap. The corresponding Dingle temperatures increase and the cyclotron effective masses decrease in the mixed states. An anisotropic energy gap with a line node has been discussed from the angular dependence of the dHvA amplitude in the mixed state.

Angular dependence of the transverse magnetoresistance of a heavy-fermion superconductor UBe13 (Tc=0.9 K) was measured on a single crystal grown by the Al-flux method. The resistivity decreases with increasing the magnetic field at low temperatures, indicating that a magnetic scattering is suppressed by the magnetic field. We observed a remarkable angular dependence in magnetoresistance at 0.45 K under a constant field of 13 T, showing a minimum at the [1 1 0] direction. This anisotropy of magnetoresistance is observed below 4 K. Since the large resistivity (200 μΩ cm) of this compound at low temperatures is attributed to the magnetic scattering by uranium ions, the present anisotropy in the magnetic scattering at low temperatures would give important information about the ground state of 5f electrons.

We have performed neutron-scattering experiments in order to study the tiny magnetic moment in a heavy-fermion superconductor URu2Si2. The (1 0 0) magnetic Bragg peak intensity is found to be reduced by about 2% in magnitude below the superconducting transition temperature of about 1 K. This result indicates the interplay between magnetism and superconductivity in URu2Si2.

The de Haas–van Alphen (dHvA) Effect of LaNi2Ge2, a reference material for a non-magnetic heavy electron compound CeNi2Ge2, has been investigated. Many branches with the frequencies from 26 MG to 120 MG have been observed. The angular dependence of the frequencies indicates a complicated Fermi surface which consists of a multi-connected surface. The cyclotron effective masses for each branch were also determined to be 1.2m0–2.7m0.

We have measured the de Haas–van Alphen (dHvA) effect of URu2Si2. One kind of the dHvA branch, which corresponds to an almost spherical Fermi surface, has been detected both in the normal and super-conducting mixed states. The dHvA frequency in the mixed state is unchanged from the normal state. The cyclotron effective mass is clearly found to decrease with decreasing the magnetic field below Hc2 and the corresponding Dingle temperature increases with decreasing the field.

Single crystals of various uranium-based intermetallics such as UPt 3 , UPd 2 Al 3 , U 6 Fe, URu 2 Si 2 , etc., were grown by the Czochralski pulling method in a tetra-arc furnace. Subsequent annealing treatment by the solid-state electrotransport (SSE) method under ultra-high vacuum is found to be quite effective for compounds UPt 3 and CeRu 2 . The residual resistivity ratio (RRR) of CeRu 2 has been improved from 30 to about 300 after the SSE treatment. In the case of UPt 3 , the best annealing temperature is about 900°C, where RRR is increased up to 640.

We have measured the electrical resistivity, thermoelectric power and magnetic susceptibility on single crystalline samples of the tetragonal heavy-fermion compound CeNi 2 Ge 2 which possesses a neither magnetic nor superconducting ground state. The anisotropy of the magnetic resistivity and thermoelectric power are discussed. We found that the susceptibility with H along the c axis goes to maximum near 30 K.

A uranium ingot was successfully purified using the solid state electrotransport (SSE) method. The concentration of impurities was dramatically reduced after the SSE treatment. In particular, magnetic transition metals such as Fe and Cu were removed almost completely. Impurities with high vapor pressure such as Mn and Zn were evaporated. The mechanism of the impurity-reduction was qualitatively explained by three effects: sweeping out by the flow of the electrical current, diffusion and evaporation. It is very important to hold the uranium ingot at a high temperature just below the melting point.