Electron Mass Enhancement due to Anharmonic Local Phonons

In order to understand how electron effective mass is enhanced by anharmonic local oscillation of an atom in a cage composed of other atoms, i.e., {\it rattling}, we analyze anharmonic Holstein model by using a Green's function method. Due to the evaluation of an electron mass enhancement factor $Z$, we find that $Z$ becomes maximum when zero-point energy is comparable with potential height at which the amplitude of oscillation is rapidly enlarged. Cooperation of such quantum and rattling effects is considered to be a key issue to explain the electron mass enhancement in electron-rattling systems.Comment: 3 pages, 3 figures, to appear in J. Phys. Soc. Jpn. Suppl. (Proceedings for International Conference on Heavy Electrons

Formation of heavy quasiparticle state in two-band Hubbard model

A realization of heavy fermion state is investigated on the basis of two-band Hubbard model. By means of the slave-boson mean-field approximation, it is shown that for the intermediate electron density, n_e=1.5, the inter-band Coulomb repulsion U strongly emphasizes initially small difference between bands, and easily stabilizes integral valence in the lower band. As a result, a strong renormalization takes place in the lower band and the mixing strength between two bands. It gives rise to a sharp peak at the Fermi level in the quasiparticle density of states, as that obtained in the periodic Anderson model. In contrast to a simple insight that the Hund's-rule coupling J reduces the characteristic energy, it turns out to be almost irrelevant to the renormalization for J<U. The required conditions are suitable for LiV_2O_4, the first observed heavy fermion compound in transition metal oxide.Comment: 5 pages, 4 figures, to be published in Phys. Rev.

Electric Dipolar Susceptibility of the Anderson-Holstein Model

The temperature dependence of electric dipolar susceptibility \chi_P is discussed on the basis of the Anderson-Holstein model with the use of a numerical renormalization group (NRG) technique. Note that P is related with phonon Green's function D. In order to obtain correct temperature dependence of P at low temperatures, we propose a method to evaluate P through the Dyson equation from charge susceptibility \chi_c calculated by the NRG, in contrast to the direct NRG calculation of D. We find that the irreducible charge susceptibility estimated from \chi_c agree with the perturbation calculation, suggesting that our method works well.Comment: 4 pages, 4 figure

Kondo Effect in an Electron System with Dynamical Jahn-Teller Impurity

We investigate how Kondo phenomenon occurs in the Anderson model dynamically coupled with local Jahn-Teller phonons. It is found that the total angular moment composed of electron pseudo-spin and phonon angular moments is screened by conduction electrons. Namely, phonon degrees of freedom essentially contribute to the formation of singlet ground state. A characteristic temperature of the Kondo effect due to dynamical Jahn-Teller phonons is explained by an effective $s$-$d$ Hamiltonian with anisotropic exchange interaction obtained from the Jahn-Teller-Anderson model in a non-adiabatic region.Comment: 5 pages, 3 figure

Magnetically Robust Non-Fermi Liquid Behavior in Heavy Fermion Systems with f^2-Configuration: Competition between Crystalline-Electric-Field and Kondo-Yosida Singlets

We study a magnetic field effect on the Non-Fermi Liquid (NFL) which arises around the quantum critical point (QCP) due to the competition between the f^2-crystalline-electric-field singlet and the Kondo-Yosida singlet states by using the numerical renormalization ground method. We show the characteristic temperature T_F^*, corresponding to a peak of a specific heat, is not affected by the magnetic field up to H_z^* which is determined by the distance from the QCP or characteristic energy scales of each singlet states. As a result, in the vicinity of QCP, there are parameter regions where the NFL is robust against the magnetic field, at an observable temperature range T > T_F^*, up to H_z^* which is far larger than T_F^* and less than min(T_{K2}, $Delta).Comment: 8 pages, 9 figur

Realization of Heavy Local Fermi Liquid and Non-Fermi Liquid in f2^2 Crystalline-Electric-Field Singlet-Triplet Configuration

Using the numerical renormalization group method, we investigate an extended Anderson model, in which correlated electrons with the $\Gamma_1$(singlet)-$\Gamma4$(triplet) f$^2$ crystalline-electric-field (CEF) configuration hybridize with conduction electrons of $\Gamma_7$(doublet) and $\Gamma_8$ (quartet) under cubic $O_h$ symmetry, from a strong spin-orbit interaction limit. For the case of the parameters relevant to PrFe4P12, the system is under competition between the CEF singlet fixed point and the multichannel Kondo non-Fermi liquid fixed point arising from the quadrupolar coupling between the impurity with pseudospin 1 and the conduction electron with pseudospin 3/2 . We consider that this result reveals the origin of the heaviness of the effective mass and non-Fermi liquid behavior of the Pr- based filled skutterudite compounds.Comment: 9 pages, 5 figure

Dynamics of the Singlet-Triplet System Coupled with Conduction Spins -- Application to Pr Skutterudites

Dynamics of the singlet-triplet crystalline electric field (CEF) system at finite temperatures is discussed by use of the non-crossing approximation. Even though the Kondo temperature is smaller than excitation energy to the CEF triplet, the Kondo effect appears at temperatures higher than the CEF splitting, and accordingly only quasi-elastic peak is found in the magnetic spectra. On the other hand, at lower temperatures the CEF splitting suppresses the Kondo effect and inelastic peak develops. The broad quasi-elastic neutron scattering spectra observed in PrFe_4P_{12} at temperatures higher than the quadrupole order correspond to the parameter range where the CEF splittings are unimportant.Comment: 16 pages, 12 figures, 1 tabl

First Order Bipolaronic Transition at Finite Temperature in the Holstein Model

We investigate the Holstein model by using the dynamical mean-field theory combined with the exact diagonalization method. Below a critical temperature Tcr, a coexistence of the polaronic and the bipolaronic solutions is found for the same value of the electron-phonon coupling $ in the range gc1(T)<g<gc2(T). In the coexistence region, the system shows a first order phase transition from the bipolaronic to the polaronic states as T decreases at T=Tp(<Tcr), where the double occupancy and the lattice fluctuation together with the anharmonicity of the effective ion potential change discontinuously without any symmetry breaking. The obtained bipolaronic transition seems to be consistent with the rattling transition in the beta-pyrochlore oxide KOs2O6.Comment: 5 pages, 5 figures, J. Phys. Soc. Jpn. 79 (2010) 09370

Magnetic Field Effect on Crossover Temperature from Non-Fermi Liquid to Fermi Liquid Behavior in f^2-Impurity Systems with Crystalline-Electric-Field Singlet State Competing with Kondo-Yosida Singlet State

We investigate the magnetic field dependence of the physical properties of f^2-configuration systems with a crystalline-electric field (CEF) singlet ground state, which gives rise to a non- Fermi liquid (NFL) fixed point due to the competition between the Kondo-Yosida singlet and CEF singlet states. On the basis of the numerical renormalization group method, we find that the magnetic field breaks this NFL fixed point via two mechanisms: one causing the polarization of f-electrons and the other giving the "channel" anisotropy. These two mechanisms induce a difference in the magnetic field dependence of the characteristic temperature T_F^{*}(H), the crossover temperature from NFL to Fermi-liquid behavior. While the polarization of f-electrons gives T_F^{*}(H) \propto H^x (x\sim2.0), the "channel" anisotropy gives the H-independent T_F^{*}(H). These two mechanisms cross over continuously at approximately the crossover magnetic field H_c, where an anomalous H-dependence of T_F^{*}(H) appears. Such T_F^{*}(H) well reproduces the NFL behaviors observed in Th_{1-x}U_xRu_2Si_2. We also find that the H-dependence of the resistivity and the magnetic susceptibility are in good agreement with the experimental results of this material. These results suggest that the NFL behaviors observed in Th_{1-x}U_xRu_2Si_2 can be understood if this material is located in the CEF singlet side near the critical phase boundary between the two singlet states.Comment: 8 pages, 8figure