Junya Otsuki

Theory of Crystalline Electric Field and Kondo Effect in Pr Skutterudites

Possible Kondo effect in Pr skutterudite is studied with attention to characteristic features of low-lying crystalline electric field (CEF) levels and the conduction band. A mechanism for the small CEF splitting between a singlet and a triplet is proposed as combination of the point-charge interaction and hybridization of 4 f with ligand p states. Provided 4 f 3 configurations dominate over 4 f 1 as intermediate states, p – f hybridization favors the triplet, while point-charge interaction favors the singlet. For realistic parameters for hybridization as well as 4 f 1 and 4 f 3 levels, these singlet and triplet can form a nearly degenerate pseudo-quartet. It is found that one of two spin 1/2 objects composing the pseudo-quartet has a ferromagnetic exchange, while the other has an antiferromagnetic exchange with conduction electrons. The magnitude of each effective exchange depends strongly on a parameter characterizing the triplet wave function under the T h symmetry. It is argued that differences of this...

Continuous-Time Quantum Monte Carlo Method for the Coqblin–Schrieffer Model

An impurity solver based on a continuous-time quantum Monte Carlo method is developed for the Coqblin–Schrieffer model. The Monte Carlo simulation does not encounter a sign problem for antiferromagnetic interactions, and accurately reproduces the Kondo effect. Our algorithm can deal with an arbitrary number N of local degrees of freedom, becomes more efficient for larger values of N , and is hence suitable for models with orbital degeneracy. The dynamical susceptibility and the impurity t -matrix are derived with the aid of the Pade approximation for various values of N , and good agreement is found with other methods and available exact results. We point out that the Korringa–Shiba relation needs correction for a finite value of the exchange interaction.

Superconductivity, antiferromagnetism and phase separation in the two-dimensional Hubbard model: A dual-fermion approach

The dual-fermion approach offers a way to perform diagrammatic expansion around the dynamical mean-field theory. Using this formalism, the influence of antiferromagnetic fluctuations on the self-energy is taken into account through ladder-type diagrams in the particle-hole channel. The resulting phase diagram for the (quasi-)two-dimensional Hubbard model exhibits antiferromagnetism and d-wave superconductivity. Furthermore, a uniform charge instability, i.e., phase separation, is obtained in the low doping regime around the Mott insulator. We also examine spin/charge density wave fluctuations including d-wave symmetry. The model exhibits a tendency towards an unconventional charge density-wave, but no divergence of the susceptibility is found.

Evolution of a large Fermi surface in the Kondo lattice.

The single-particle spectrum of the Kondo lattice model is derived with the use of the continuous-time quantum Monte Carlo method, combined with the dynamical mean-field theory. Crossover behavior is traced quantitatively either to a heavy Fermi-liquid state or to a magnetically ordered state from the local-moment state at high temperatures. The momentum distribution in the low-temperature limit acquires a discontinuity at the location that involves the local-spin degrees of freedom. Even without the charge degrees of freedom for local electrons, the excitation spectra exhibit hybridized bands similar to those in the Anderson lattice. Temperature dependence in the zero-energy component of the self-energy is crucial in forming the Fermi-liquid state with the large Fermi surface.

The Kondo Lattice Model in Infinite Dimensions: II. Static Susceptibilities and Phase Diagram

Magnetic and charge susceptibilities in the Kondo lattice are derived by the continuous-time quantum Monte Carlo (CT-QMC) method combined with the dynamical mean-field theory. For a weak exchange coupling J and near half filling of the conduction band, antiferromagnetic transition occurs as signalled by divergence of the staggered magnetic susceptibility with lowering temperature. With increasing J , the Kondo effect suppresses the divergence, and the critical value of J agrees well with Doniachs estimate which considers the RKKY interaction as competing with the Kondo effect. For low density of conduction electrons, a ferromagnetic ordering is observed where Doniachs estimate does not work. Around quarter filling, a charge-density-wave (CDW) transition is found. The CDW is interpreted from the strong-coupling limit in terms of effective repulsion between Kondo singlets.

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 the 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 4 P 12 at temperatures higher than the quadrupole order correspond to the parameter range where the CEF splittings are unimportant.

Electronic Order with Staggered Kondo and Crystalline Electric Field Singlets

Novel electronic order is found theoretically for a system where even number of localized electrons per site are coupled with conduction electrons. For precise quantitative study, a variant of the Kondo lattice model is taken with crystalline electric field (CEF) singlet and triplet states for each site. Using the dynamical mean-field theory combined with the continuous-time quantum Monte Carlo method, a staggered order with alternating Kondo and CEF singlets is identified for a case with one conduction electron per site being distributed in two conduction bands each of which is quarter-filled. This electronic order accompanies a charge density wave (CDW) of conduction electrons that accumulate more on Kondo-singlet sites than on CEF-singlet sites. Possible relevance of the present order to the scalar order in PrFe 4 P 12 is discussed.

Resolution of Entropy \(\ln\sqrt{2}\) by Ordering in Two-Channel Kondo Lattice

Peculiar property of electronic order is clarified for the two-channel Kondo lattice. With two conduction electrons per site, the order parameter is a composite quantity involving both local and it...

Hybridization Effects and Multipole Orders in Pr Skutterudites

Theoretical account is given of 4f-electron dynamics and multipole orders in Pr skutterudites with particular attention to (i) mechanism of the crystalline electric field (CEF) splitting leading to a pseudo-quartet ground state; (ii) Kondo effect due to exchange interactions involving the pseudo-quartet; (iii) multipole orders in the lattice of the pseudo-quartet in magnetic field. Competition between the point-charge interaction and hybridization between 4f and conduction electrons is identified as the key for controlling the CEF splitting. It is found that one of two pseudo-spins forming the pseudo-quartet has a ferromagnetic exchange, while the other has an antiferromagnetic exchange with conduction electrons. The Kondo effect is clearly seen in the resistivity calculated by the NCA, provided the low-lying triplet above the singlet is mainly composed of the Γ 4 -type wave functions. If the weight of the Γ 5 -type is large in the triplet, the Kondo effect does not appear. This difference caused by the nature of the triplet explains the presence of the Kondo effect in PrFe 4 P 12 , and its absence in PrOs 4 Sb 12 . By taking the minimal model with antiferro-quadrupole (AFQ) and ferro-type intersite interactions for dipoles and octupoles between nearest-neighbors, the mean-field theory reproduces the overall feature of the multiple ordered phases in PrFe 4 P 12 . The AFQ order with the Γ 3 -type symmetry is found to be stable only as a mixture of O 0 2 and O 2 2 components.Theoretical account is given of 4f-electron dynamics and multipole orders in Pr skutterudites with particular attention to (i) mechanism of the crystalline electric field (CEF) splitting leading to a pseudo-quartet ground state;(ii) Kondo effect due to exchange interactions involving the pseudo-quartet;(iii) multipole orders in the lattice of the pseudo-quartet in magnetic field.Competition between the point-charge interaction andhybridization between 4f and conduction electrons is identified as the key for controlling the CEF splitting. It is found that one of two pseudo-spins forming the pseudo-quartet has a ferromagnetic exchange, while the other has an antiferromagnetic exchange with conduction electrons. The Kondo effect is clearly seen in the resistivity calculated by the NCA, provided the low-lying triplet above the singlet is mainly composed of the

Sparse modeling approach to analytical continuation of imaginary-time quantum Monte Carlo data

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