Yamagami Hiroshi

de Haas–van Alphen Effect in LaFePO with Two-Dimensional Cylindrical Fermi Surfaces

We have succeeded in observing the de Haas–van Alphen (dHvA) effect in LaFePO, the first reported iron-based superconductor in the iron–oxypnictide family. The four detected dHvA frequencies, which correspond to the cross-sectional areas of Fermi surfaces, approximately follow the 1/cos θ dependence, where θ is the field-tilted angle from the [001] direction to the [100] and [110] directions in the tetragonal structure. This indicates that all the Fermi surfaces are nearly cylindrical, with the two-dimensional electronic state. The cyclotron effective mass, which was determined from the temperature dependence of the dHvA amplitude, is relatively large, ranging from 1.82 to 2.34 m 0 . The result of energy band structure calculation revealed that the conduction bands mainly consist of 3 d electrons of Fe atoms.

Pressure-Induced Superconductivity in Antiferromagnet CePd5Al2

A layered compound CePd 5 Al 2 , which is an analog of a heavy fermion superconductor NpPd 5 Al 2 , is found to be an antiferromagnet with an easy-axis along the tetragonal [001] direction but becomes superconductive in the pressure region from 9 to 12 GPa, with the superconducting transition temperature, T sc = 0.57 K and the upper critical field at 0 K, H c2 (0) = 2.5 kOe, at 10.8 GPa. The quasi-two dimensional Fermi surfaces are also theoretically calculated for CePd 5 Al 2 on the basis of the 4 f -itinerant band model.

Electronic Band Structure and Fermi Surface of Heavy-Fermion Neptunium Superconductor NpPd5Al2

Electronic band structure calculations were carried out for a new neptunium superconductor NpPd 5 Al 2 using a relativistic linear-augmented plane wave method within a local density approximation. The Fermi energy in the band structure is found to be located just on the 5 f -bands of the j =5/2 state with the extremely narrow band width and therefore the theoretical electronic specific heat coefficient γ b is extremely large, being γ b = 74.0 mJ/(K 2 ·mol), which is compared to γ= 390 mJ/(K 2 ·mol) estimated experimentally. NpPd 5 Al 2 is an uncompensated metal. The main electron Fermi surface corresponds to a doughnut-like flat Fermi surface centered at the Γ point which is connected with cylindrical Fermi surfaces elongated along the X–W–P line.

High-Quality Single Crystal Growth and Unique Electronic States under Magnetic Field and Pressure in Rare Earth and Actinide Compounds

The f electrons in rare earth and actinide compounds exhibit variety of characteristic properties including heavy fermions and unconventional superconductivity. Fermi surface properties in rare earth and actinide compounds such as CeSn 3 , USi 3 , NpGe 3 and PuIn 3 are clarified by the de Haas–van Alphen experiments on the basis of the results of energy band calculations. An abrupt nonlinear increase of magnetization, namely metamagnetic behavior is found in the heavy fermion compounds including YbT 2 Zn 20 (T: Co, Rh, Ir). An effect of pressure on the electronic state of YbIr 2 Zn 20 and antiferromagnets CeRhIn 5 and CeIrSi 3 are also studied in magnetic fields. The electronic instability including unconventional superconductivity occurs at about 2.4 GPa in CeRhIn 5 and 2.6 GPa in CeIrSi 3 .

Observation of Itinerant Ce 4f Electronic States in CeIrSi3 Studied by Angle-Resolved Ce 3d→4f Resonance Photoemission Spectroscopy

We have applied angle-resolved Ce 3 d →4 f resonance photoemission spectroscopy ( h ν∼882 eV) in the soft X-ray region to the analysis of the non-centrosymmetric pressure-induced superconductor CeIrSi 3 and obtained the 4 f band structure and Fermi surfaces. We have found that the Ce 4 f states are located mainly near the Fermi level and that the photoemission feature corresponding to the dispersive conduction bands across the Fermi level shows considerable resonant enhancement. In addition, the band structure and Fermi surfaces of CeIrSi 3 differ from those of the non- f reference compound LaIrSi 3 , and the differences are well explained by the band structure calculated within local density approximation, in which the Ce 4 f states are assumed to be itinerant. These results strongly suggest that the Ce 4 f electrons in CeIrSi 3 hybridized well with conduction bands and form itinerant electronic states.

Electronic Structure of EuAl4 Studied by Photoelectron Spectroscopy

The electronic structure of the divalent Eu compound EuAl4, which shows a charge density wave transition at TCDW = 140 K, was studied by hard X-ray angle-integrated photoelectron spectroscopy (HAXPES) and soft X-ray angle-resolved photoelectron spectroscopy (ARPES). The valence band and core-level spectra obtained by HAXPES are consistent with the divalent nature of Eu atoms in EuAl4. From the ARPES results, the Fermi surface as well as band structure in the vicinity of the Fermi energy (EF) of EuAl4 are very similar to those of its isostructural divalent Sr compound SrAl4, which has no 4f electrons. This suggests that the Eu atoms are divalent in EuAl4, and the 4f electrons are localized below 1.8 eV with the Eu 4f7 electronic configuration in the ground state. The ARPES spectra measured along the Γ–(Σ)–Z high-symmetry line did not show significant temperature dependences above and below TCDW within the energy resolution of 80–90 meV. Moreover, the Fermi surface mapping along the kz direction showed that...

Itinerant U 5f Nature in Antiferromagnet U(Ru0:97Rh0:03)2Si2: Soft X-ray Angle-Resolved Photoemission Spectroscopy

We have carried out soft X-ray angle-resolved photoemission spectroscopy experiments on U(Ru 0.97 Rh 0.03 ) 2 Si 2 , which shows antiferromagnetic (AFM) ordering at low temperatures. We have reveal...

Fermi Surface of ThRu2Si2 as a Reference to the Strongly Correlated Isostructural Metals Investigated by Quantum Oscillations

We have successfully grown a high-quality single crystal of ThRu2Si2 with the residual resistivity ratio (RRR) of 120, which is a reference compound to the heavy-fermion systems CeRu2Si2 and URu2Si2 crystallizing in the tetragonal ThCr2Si2-type structure, and observed the quantum oscillations (de Haas–van Alphen effect). The result is compared with the Fermi surfaces of CeRu2Si2 and URu2Si2 as well as the band structure calculations. The main features of Fermi surface topology of ThRu2Si2 are similar to those of the heavy-fermion compound CeRu2Si2, indicating that the 4f electron in CeRu2Si2 contributes to the conduction band. On the other hand, the Fermi surfaces of ThRu2Si2 significantly differ from those obtained for URu2Si2, suggesting the itinerant nature of 5f electrons rather than the conventionally assumed localized 5f2 configuration.

Magnetic Compton Scattering and Mössbauer Studies of an Itinerant Ferromagnet UFe2

Magnetic Compton scattering and Mossbauer spectroscopy were carried out on an itinerant ferromagnet of UFe 2 . The direction dependence of the magnetic effect in the magnetic Compton scattering indicates the absence of the magnetic anisotropy. 238 U Mossbauer spectra imply the absence of the U magnetic moments. The hyperfine coupling constant obtained by 57 Fe Mossbauer spectroscopy implies that the 3d electrons are hybridized with the U 5f or Fe 3d electrons at the nearest neighbor sites. The results obtained in the present work imply that the magnetic properties of UFe 2 can be interpreted without any contribution of the U 5f electrons.

Band Structure and Fermi Surface of Uranium Compounds: Soft X-ray Angle-Resolved Photoemission Study

We have performed angle-resolved photoemission spectroscopy experiments on paramagnetic UFeGa 5 using soft X-ray synchrotron radiation ( h ν=500 eV) and derived its bulk- and U 5 f -sensitive electronic structure. The topology of the experimental Fermi surface is well explained by the LDA calculation treating U 5 f electrons as being itinerant, suggesting that the U 5 f states of this compound can be essentially understood within the itinerant-electron model.