Mikito Koga

Kondo Effect Due to Impurities with (5f)n Configuration: Fermi Liquid Versus Non-Fermi Liquid

The Kondo effect due to U-type impurities is studied in connection with the stability of the Fermi liquid and the non-Fermi liquid. First, an effective exchange Hamiltonian in terms of irreducible tensor operators is derived for conduction electrons, which are coupled with plural 5 f electrons. It is based on the assumption of strong spin-orbit and Hund couplings. Second, a model for f 2 configuration placed in a tetragonal or hexagonal crystal field is studied by simplifying the internal structure as the non-Kramers doublet ground state and the first singlet excited state. The numerical renormalization group analysis shows that the singlet excited state plays an important role to stabilize the Fermi liquid over the non-Fermi liquid. Third, the difference of the Kondo effect is examined between non-Kramers doublet for f 2 and Kramers doublet for f 3 in the presence of the cubic crystal field. It is shown that for f 3 the Hund coupling reduces the channel number to one, while the situation is different for...

Crystal-Field Excitations, Magnetic-Field-Induced Phase Transition and Neutron-Scattering Spectra in PrOs4Sb12

One possible origin of the field-induced ordered phase (FIOP) in PrOs 4 Sb 12 is an interaction of quadrupolar moments of Pr 4 f electrons in the singlet–triplet crystal-field (CF) levels. We study...

Exciton Mediated Superconductivity in PrOs4Sb12

The most important character of the exotic superconductor PrOs 4 Sb 12 is the existence of low-lying excitations (excitons) with a finite energy gap and it appears as the magnetic field-induced order above 4.5 T. We focus on the a u conduction band, which hybridizes with a Pr 4 f 2 state strongly, coupled to the excitons. It results in an attractive interaction between the conduction electrons. The symmetry of the superconducting order parameter is determined by dispersion relation of the exciton. For the bcc system PrOs 4 Sb 12 , a d -wave state ( k x k y + ω k y k z + ω 2 k z k x , ω=e ±i 2π/3 ) is stabilized with broken time reversal symmetry.

Roles of Multipoles and Excitons in Superconductor PrOs4Sb12 with Th Symmetry

A possibility of exciton-mediated mechanism is studied to describe the superconductivity in a Pr skutterudite PrOs 4 Sb 12 . In this compound the low-lying crystal-field states of Pr form a pseudo-quartet consisting of Γ 1 singlet ground and Γ 4 (2) triplet excited states. The dispersive crystal-field excitations (excitons) have been detected experimentally. The conduction electrons couple both magnetically and nonmagnetically with the pseudo-quartet. Believing that this coupling should be crucial for various properties of PrOs 4 Sb 12 , we determine the effective interaction between conduction electrons and the Pr pseudo-quartet states. It turns out that the nature of exchange processes is related closely to a u and t u components of the conduction electrons that correspond to the molecular orbitals of an Sb 12 cage surrounding a Pr ion. By the second-order perturbation in the exchange coupling, Cooper pairing interactions are derived both for singlet and triplet superconductivities. First, they are clas...

A New Type of Fermi-Liquid Fixed Point in Kondo Effect Due to(5f)^2 Configuration

The Kondo effect due to (5 f ) 2 impurities is studied by restricting the f -electron states to a non-Kramers doublet and a singlet in a tetragonal or hexagonal crystal field. The singlet level plays an important role to stabilize the Fermi liquid over the non-Fermi liquid. The nature of the Fermi-liquid fixed point, which results from the numerical renormalization group calculation, is analyzed. It has turned out to be different from the standard one because of a combined effect of two types of local singlets. One is due to the crystal field and the other is due to the Kondo effect. The leading irrelevant interaction near the fixed point is of the Coulomb type, whose couplings depend on the orbital channels of conduction electrons.

Coexistence of Even- and Odd-Frequency Superconductivities Under Broken Time-Reversal Symmetry

A novel superconducting state under the broken time-reversal symmetry is studied in conventional phonon-mediated superconductors. By solving the Eliashberg equation self-consistently with the mass renormalization effect, it is found that the even- and odd-frequency components of the order parameter coexist in the bulk system as a consequence of the broken time-reversal symmetry. This finding would direct more attention to the odd-frequency pairing that affects physical quantities, especially in strong coupling superconductors.

Longitudinal Spin-Wave Mode Near Quantum Critical Point Due to Uniaxial Anisotropy

A longitudinal excitation mode is important in spin-wave theory for three-dimensional interacting integer spin systems with uniaxial anisotropy of easy plane type. There is a critical value of the ...

Small Energy Scale for Mixed-Valent Uranium Materials

We investigate a two-channel Anderson impurity model with a 5f{sup 1} magnetic and a 5f{sup 2} quadrupolar ground doublet and a 5f{sup 2} excited triplet. Using the numerical renormalization group method, we find a crossover to a non-Fermi-liquid state below a temperature T{sup *} varying as the 5f{sup 2} triplet-doublet splitting to the 7/2 power. To within numerical accuracy, the nonlinear magnetic susceptibility and the 5f{sup 1} contribution to the linear susceptibility are given by universal one-parameter scaling functions. These results may explain UBe{sub 13} as mixed valent with a small crossover scale T{sup *} . {copyright} {ital 1999} {ital The American Physical Society}

Stability of non-Fermi liquid in Kondo effect due to (5f)2configuration

The stability of the non-Fermi liquid (NFL) in Kondo effect is studied by considering the excited states of the 5 f -electrons. First, in order to examine their effects in the strong correlation limit, the exchange model is derived for a tetragonal crystal field by restricting the localized states in f 2 to a non-Kramers doublet and a singlet; all the states are taken into account for f 1 . The numerical renormalization group analysis for this model shows that the singlet state in f 2 and one of the excited states in f 1 play an important role to stabilize the Fermi liquid (FL) over the NFL. Second, the same problem is examined for the six-fold orbitally degenerate Anderson model by considering all the local excited states. The competition between FL and NFL sensitively depends on the Coulomb and Hund couplings, the orbital anisotropy and the hybridization term. The six-fold orbital degeneracy is also important to realize NFL; in fact a similar analysis on the four-fold orbitally degenerate Anderson model...

Exciton-Mediated Triplet Superconductivity in Th System PrOs4Sb12

In PrOs 4 Sb 12 , the lowest-lying singlet and triplet states in a Pr 4 f 2 configuration hybridize with conduction electrons having local a u and t u point-group symmetries. It is shown that for an attractive triplet pairing interaction, the orbital degrees of freedom of the t u component are important. In addition, the T h point-group symmetry characteristic of skutterudites plays an important role in stabilizing triplet superconductivity.