Katsurou Hanzawa

The Electric Field Gradients from the On-Site Holes at Cu and O Nuclei in YBa2Cu3O7

A consistent explanation of the electric field gradients at Cu and O nuclei in YBa 2 Cu 3 O 7 is obtained provided that on-site 3 d and 2 p holes give the dominant contributions. The number of holes per Cu(2) site in the planer d x 2 - y 2 - p σ antibonding orbital is estimated to be nearly one (that is, the orbital is nearly half-filled), more than half of which are attributed to the oxygen 2 p holes. For oxygen sites, it is necessary to consider both the p σ and p π holes, while more than 80% of 2 p holes reside on the p σ orbitals in all oxygen sites.

Magnetoresistance Due to Ce Impurities in Metals

In order to calculate magnetoresistances due to the Ce impurities coupled with conduction electrons by the s - f exchange interaction, conditions for determining the number of local conduction electrons around Ce are given, which are valid for arbitrary spin-orbit and crystal-field splittings of Ce ion. General arguments for resistivity and magnetoresistance are given. In order to discuss the resistivities and magnetoresistances of the dense Kondo Ce compounds quantitatively, we adopt the Yosida theory and carry out the wave number integration numerically. It is shown that the resistivity along the crystal c -axis becomes small for crystalline fields with tetragonal or hexagonal symmetry and that a peak of magnetoresistance is expected due to the Kondo effect re-enhancement.

Spin-Peierls Transition in Ce Compounds

A possible spin-Peierls (dimerization) transition in Ce Compounds is investigated theoretically, in connection with the novel phase transition recently reported to occur in CeRu 2 Al 10 . It is pointed out that one-dimensional zigzag chains are profiled by connecting nearest-neighbor (n.-n.) Ce ions in this compound. If only the RKKY interactions between n.-n. Ce 3+ ions are considered in the mean-field approximation, a spin-Peierls transition occurs at T =27 K for the zero-temperature gap parameter Δ 0 =90 K, regardless of the magnetic anisotropy. The characteristics of CeRu 2 Al 10 can be accounted for semiquantitatively, including the finite value of susceptibility for T →0 K due to a Van-Vleck contribution of the excited states of the crystalline electric field at ≈10 2 K above the ground-state doublet.

Magnetization in Heavy-Fermion Compounds

We calculate the magnetization as a function of magnetic field in the mean-field approximation for the periodic Coqblin-Schrieffer model. Prior to a metamagnetic second-order transition from heavy-fermion to local-moment state, a rapid increase in the magnetization, which resembles the metamagnetic-like behavior in CeRu 2 Si 2 , appears if the anisotropy of the hybridization matrix element is taken into account.

Hyperfine Interactions in CeB6.

It is shown that the main origin of the hyperfine field at B nuclei in CeB 6 is the transferred hyperfine interaction of the nuclear moment with the electronic spin carried by the electrons in the 2 s and 2 p states of the B ion hybridized with the 4 f states of neighboring Ce ions. The dipole-dipole interaction with the magnetic moments of Ce ions also contributes substantially. In the antiferro-quadrupolar ordering phase II, it is essential to assume the ordering of Γ 5 -type moments. The reason why the RKKY interaction between the Γ 5 -type quadrupole moments is strongest is also discussed, on the basis of the estimate of the strength of p-f mixing which plays a vital role in many respects in CeB 6 .

Analysis of the Electric Field Gradients and the Knight Shifts at All Cu and O Nuclei in YBa2Cu3O7

Electronic configurations and local susceptibilities at all copper and oxygen sites in YBa 2 Cu 3 O 7 are determined from the analysis of the electric field gradients (EFGs) and the Knight shifts (shifts). Besides main contributions from 3 d holes for Cu sites and 2 p σ holes for O sites, it is necessary to consider 4 p electrons of Cu and 2 p π holes of O for accurate analysis of the EFGs, and 4 s electrons of Cu and 2 s holes of O for the shifts. Difference in the observed EFGs between Cu(1) and Cu(2) is due to the 4 p -electron contributions, and the 3 d -electron states are determined to be quite similar. Each site of Cu and O shares almost same amount of holes. The local spin susceptibilities, however, are fairly different in order of magnitude as Cu(2), Cu(1), O(2, 3), O(4), implying a strong correlation in Cu(2).

Fermi Liquid Theory on Heavy Electron Systems

Theory of heavy electron is reviewed on the basis of the Fermi liquid theory. The importance of the orbital degeneracy in realizing the heavy electrons is stressed by using the scaling theory on the Kondo temperature. With use of the periodic Anderson Hamiltonian, the expressions for the physical quantities, such as electronic specific heat, magnetic susceptibility, conductivity, relaxation time of nuclear spin and anomalous Hall coefficient are derived. These results explain the essential properties of heavy electrons. For further development the extension to the orbital degenerate case is essential and discussed mainly on the specific heat and the susceptibility

A Heavy-Fermion State with Singular Density of States

The metamagnetic behavior of CeRu 2 Si 2 and the non Fermi-liquid behavior of CeNi 2 Ge 2 are described by the periodic Coqblin-Schrieffer model with anisotropic c - f exchange interaction in the mean-field theory. Only the Kramers doublet of M J =±5/2 for the 4 f states of Ce ions is considered, for the case of which the density of quasi-particle states has singularity of | ω| -1/2 around the effective f -level energy. In the asymmetric case applicable to CeRu 2 Si 2 , the chemical potential reaches the singularity at a finite magnetic field, at which the magnetization shows a metamagnetic-like increase. On the other hand, in the symmetric case applicable to CeNi 2 Ge 2 , the chemical potential is on the singularity at zero magnetic field, and non-Fermi-liquid-like behaviors emerge in the susceptibility and the specific heat.

Comments on the Hyperfine Fields at B Nuclei in CeB6.

It is pointed out that the hyperfine field at B nuclei in CeB 6 is attributed to the dipole-dipole interaction of the nuclear moment with the magnetic moments of Ce ions and the transferred hyperfine interaction due to the 2 s and 2 p states of B ion hybridized with the 4 f states of Ce ions. Both contributions have the same order of magnitude, and account for the hyperfine fields in the antiferro-quadrupolar ordering phase II and the paramagnetic phase I.

Fermi Liquid Theory on the Basis of the Periodic Anderson Model with Spin-Orbit Coupling and Crystalline Field

In order to discuSs the normal-state properties of heavy-fermion systems, the Fermi liquid theory is developed on the basis of the periodic Anderson model with arbitrary spin-orbit coupling and crystalline field. For the unperturbed (U =0) case, the electronic band structure is determined; then the expressions of T-linear coefficif:mt of specific heat and magnetic susceptibility are derived. For U =1= 0, the general expressions for T -lineGlr coefficient of specific heat and magnetization are derived. However, the expression for magnetic susceptibility cannot generally be brought into such a usual form that includes only the quantities on the Fermi surface because of the existence of off-diagonal elements of I-electron self-energy. Therefore, the susceptibility is calculated in the special case that only the lowest Kramers doublet is taken into account.