Iterative Perturbation Theory for Strongly Correlated Electron Systems with Orbital Degeneracy

A new scheme of the iterative perturbation theory is proposed for the strongly correlated electron systems with orbital degeneracy. The method is based on the modified self-energy of Yeyati, et al. which interpolates between the weak and the strong correlation limits, but a much simpler scheme is proposed which is useful in the case of the strong correlation with orbital degeneracy. It will be also useful in the study of the electronic structures combined with the band calculations.Comment: 6 pages, 3 Postscript figures, to appear in J. Phys. Cond. Matte

Theory of Anomalous Hall Effect in a Heavy fermion System with a Strong Anisotropic Crystal Field

In a heavy fermion system, there exists the anomalous Hall effect caused by localized $f$-orbital freedom, in addition to the normal Hall effect due to the Lorentz force. In 1994, we found that the Hall coefficient caused by the anomalous Hall effect ($R_H^{AHE}$) is predominant and the relation $R_H^{AHE} \propto \rho^2$ ($\rho$ is the electrical resistivity) holds at low temperatures in many compounds. In this work, we study the system where the magnetic susceptibility is highly anisotropic due to the strong crystalline electric field on $f$-orbitals. Interestingly, we find that $R_H^{AHE}$ is nearly isotropic in general. This tendency is frequently observed experimentally, which has casted suspicion that the anomalous Hall effect may be irrelevant in real materials. Our theory corresponds to corrections and generalizations of the pioneering work on ferromagnetic metals by Karplus and Luttinger.Comment: 4 pages, revtex, to be published in J. Phys. Soc. Jpn. (No.8

Electronic State and Magnetic Susceptibility in Orbitally Degenerate (J=5/2) Periodic Anderson Model

Magnetic susceptibility in a heavy fermion systemis composed of the Pauli term (\chi_P) and the Van-Vleck term (\chi_V). The latter comes from the interband excitation, where f-orbital degeneracy is essential. In this work, we study \chi_P and \chi_V in the orbitally degenerate (J=5/2) periodic Anderson model for both the metallic and insulating cases. The effect of the correlation between f-electrons is investigated using the self-consistent second-order perturbation theory. The main results are as follows. (i) Sixfold degenerate model: both \chi_P and \chi_V are enhanced by a factor of 1/z (z is the renormalization constant). (ii) Nondegenerate model: only \chi_P is enhanced by 1/z. Thus, orbital degeneracy is indispensable for enhancement of \chi_V. Moreover, orbital degeneracy reduces the Wilson ratio and stabilizes a nonmagnetic Fermi liquid state.Comment: 4 pages, revtex, to be published in J. Phys. Soc. Jpn. (No.8

The effect of uniaxial pressure on the magnetic anomalies of the heavy-fermion metamagnet CeRu2Si2

The effect of uniaxial pressure (P_u) on the magnetic susceptibility (X), magnetization (M), and magnetoresistance (MR) of the heavy-fermion metamagnet CeRu2Si2 has been investigated. For the magnetic field along the tetragonal c axis, it is found that characteristic physical quantities, i.e., the temperature of the susceptibility maximum (T_max), the pagamagnetic Weiss temperature (Q_p), 1/X at 2 K, and the magnetic field of the metamagnetic anomaly (H_M), scale approximately linearly with P_u, indicating that all the quantities are related to the same energy scale, probably of the Kondo temperature. The increase (decrease) of the quantities for P_u || c axis (P_u || a axis) can be attributed to a decrease (increase) in the nearest Ce-Ru distance. Consistently in MR and X, we observed a sign that the anisotropic nature of the hybridization, which is believed to play an important role in the metamagnetic anomaly, can be controlled by applying the uniaxial pressure. PACS numbers: 75.20.Hr, 71.27.+a, 74.62.FjComment: 7 pages, ReVTeX, 6 EPS figures : Will appear in Phys. Rev.

Specific Heat Study of Non-Fermi Liquid Behavior in CeNi_2Ge_2: Anomalous Peak in Quasi-Particle Density-of-States

To investigate the non-Fermi liquid (NFL) behavior in a nonalloyed system CeNi_2Ge_2, we have measured the temperature and field dependences of the specific heat C on a CeNi_2Ge_2 single crystal. The distinctive temperature dependence of C/T (~a-b*T^(1/2)) is destroyed in almost the same manner for both field directions of B//c-axis and B//a-axis. The overall behavior of C(T,B) and the low-temperature upturn in magnetic susceptibility can be reproduced, assuming an anomalous peak of the quasi-particle-band density-of-states (DOS) at the Fermi energy possessing (epsilon)^(1/2) energy dependence. Absence of residual entropy around T=0 K in B~0 T has been confirmed by the magnetocaloric effect measurements, which are consistent with the present model. The present model can also be applied to the NFL behavior in CeCu_{5.9}Au_{0.1} using a ln(epsilon)-dependent peak in the DOS. Possible origins of the peak in the DOS are discussed.Comment: 4 pages, LaTeX, using jpsj.sty, to be published in J. Phys. Soc. Jpn. 66 No. 10 (1997), 7 figures available at http://494-475.phys.metro-u.ac.jp/ao/ceni2ge2.htm

Absence of Hybridization Gap in Heavy Electron Systems and Analysis of YbAl3 in terms of Nearly Free Electron Conduction Band

In the analysis of the heavy electron systems, theoretical models with c-f hybridization gap are often used. We point out that such a gap does not exist and the simple picture with the hybridization gap is misleading in the metallic systems, and present a correct picture by explicitly constructing an effective band model of YbAl_3. Hamiltonian consists of a nearly free electron model for conduction bands which hybridize with localized f-electrons, and includes only a few parameters. Density of states, Sommerfeld coefficient, f-electron number and optical conductivity are calculated and compared with the band calculations and the experiments.Comment: 9 pages, 9 figures, submitted to J. Phys. Soc. Jp

Thermodynamic and Transport Properties of CeMg2Cu9 under Pressure

We report the transport and thermodynamic properties under hydrostatic pressure in the antiferromagnetic Kondo compound CeMg2Cu9 with a two-dimensional arrangement of Ce atoms. Magnetic specific heat Cmag(T) shows a Schottky-type anomaly around 30 K originating from the crystal electric field (CEF) splitting of the 4f state with the first excited level at \Delta_{1}/kB = 58 K and the second excited level at \Delta_{2}/kB = 136 K from the ground state. Electric resistivity shows a two-peaks structure due to the Kondo effect on each CEF level around T_{1}^{max} = 3 K and T_{2}^{max} = 40 K. These peaks merge around 1.9 GPa with compression. With increasing pressure, Neel temperature TN initially increases and then change to decrease. TN finally disappears at the quantum critical point Pc = 2.4 GPa.Comment: 10 pages, 6 figure

Magnetization Process in the One-Dimensional Doped Kondo Lattice Model

The magnetization process in the one-dimensional Kondo lattice model for the doped (n_{c}<1) case is studied by the density matrix renormalization group (DMRG) method. A rapid increase of the magnetization is caused by the collapse of the intersite incommensurate correlation of f spins. On the contrary, the intrasite f-c singlet correlation survives in the larger magnetic field. The crossover from large to small Fermi surfaces for majority and minority spins is observed, whereas the Fermi surfaces are always contributed by f spins. A magnetization plateau appears with the magnitude of 1-n_{c}. Both ends of the plateau are related to the coherence temperature and the Kondo temperature which are characteristic energies essential in heavy electron systems.Comment: 4 pages, 3 eps figure

Crystalline-Electric-Field Effect on the Resistivity of Ce-based Heavy Fermion Systems

The behavior of the resistivity of Ce-based heavy fermion systems is studied using a 1/$N$-expansion method a la Nagoya, where $N$ is the spin-orbital degeneracy of f-electrons. The 1/$N$-expansion is performed in terms of the auxiliary particles, and a strict requirement of the local constraints is fulfilled for each order of 1/N. The physical quantities can be calculated over the entire temperature range by solving the coupled Dyson equations for the Green functions self-consistently at each temperature. This 1/N-expansion method is known to provide asymptotically exact results for the behavior of physical quantities in both low- and high-energy regions when it is applied to a single orbital periodic Anderson model (PAM). On the basis of a generalized PAM including crystalline-electric-field splitting with a single conduction band, the pressure dependence of the resistivity is calculated by parameterizing the effect of pressure as the variation of the hybridization parameter between the conduction electrons and f-electrons. The main result of the present study is that the double-peak structure of the $T$-dependence of the resistivity is shown to merge into a single-peak structure with increasing pressure.Comment: 37 pages, 22 figure

The Isotope Effect in d-Wave Superconductors

Based on recently proposed anti-ferromagnetic spin fluctuation exchange models for $d_{x^2-y^2}$-superconductors, we show that coupling to harmonic phonons {\it{cannot}} account for the observed isotope effect in the cuprate high-$T_c$ materials, whereas coupling to strongly anharmonic multiple-well lattice tunneling modes {\it{can}}. Our results thus point towards a strongly enhanced {\it{effective}} electron-phonon coupling and a possible break-down of Migdal-Eliashberg theory in the cuprates.Comment: 12 pages + 2 figures, Postscript files, all uuencoded Phys. Rev. Lett. (1995, to be published