Tamifusa Matsuura

Relation between resistivity and effective mass in heavy-fermion and A15 compounds

Abstract The coefficient A of the T 2 term in the resistivity and the linear specific heat coefficient γ appear to have the relation A / γ 2 = constant in heavy-fermion compounds. This is explained on the basis of large frequency dependence of the conduction-electron self-energy and the analytic properties of the self-energy. Nb 3 Sn and V 3 Si, typical A15 compounds, show the same universality, which strongly suggests that their large γs are due to many-body effects as in heavy-fermion systems.

Critical study of strongly correlated systems: A unified description of the incoherent and coherent regimes

Abstract By using a modified version of the infinite- U Anderson lattice model, the properties of mixed valence systems over the whole temperature range including both the coherent and incoherent states are studied employing the boson technique with the help of the Feynman diagrammatic expansion developed earlier by P. Coleman. In the present model, we assume the large spin-orbit degeneracy N of the f-electrons but with keeping the total degrees of freedom of the conduction electrons to be a constant independent of N (instead of keeping NV 2 sf ( V sf is the sf hybridization energy) to be a constant as usually done). The system turns out to be in the Fermi liquid state with heavy fermion bands at low temperature T ≲ T 0 ( T 0 is the coherence temperature), while it is described by a collection of single Kondo impurities at T ⪢ T 0 . These two different regimes change their character continuously without showing any broken symmetry in contrast to the results obtained by the mean field theories. The Luttinger sum rule is preserved in the present model with N ⪢1.

A Model for Cooper Pairing in Heavy Fermion Superconductor

Cooper pair fonnation in the heavy fennion system is discussed on the basis of a tight-binding model for the Kondo lattice system. The attractive interaction between heavy fennions stems from the coupling with phonons, which arises through the modulations of the transfer to the nearest neighbor site and the single-particle level of the heavy fennion owing to the lattice vibrations. The interaction relevant to the Cooper pair fcinnation is written in the fonn of the superposition of separable fonns with SO, d- and p-like symmetries. A comparison among the transition temperatures for various types of Cooper pairing shows that the singlet pairing is always favorable compared to the triplet one, and that, of singlet pairings, the d-like one is most favorable in the weak coupling case and so is the s-like one in the strong coupling case. The manner in which the d- and s-components mix together below Tc is discussed in the Ginzburg-Landau region, from which it is shown that the possible type of pairing is purely s-like one or d-like one with s-like admixtures, depending whether the highest Tc occurs for s-like or d-like pairing. This implies that the low lying excitations can be nearly gapless for appropriate values of parameters characterizing the model.

Electronic State of the Anderson Lattice over the Whole Temperature Range

Earlier we proposed a modified version of the Anderson lattice model where the intersite coherence effects can be properly included even in the large limit of the spin-orbital degeneracy N and investigated the low temperature Fermi liquid properties rigorously in the limit N →∞. In the present study, using the same model as above, we investigate properties over the whole temperature range through explicit numerical calculations for N =6. We find that a coherence temperature T 0 of order of one tenth of the Kondo temperature T K exists; at lower temperature \(T\lesssim{T}_{0}\) the system is in the Fermi liquid state, while at higher temperature T ≫ T 0 the electrons are scattered incoherently by the Kondo impurities. At intermediate temperature, with increasing T , the hybridization gap appeared in the Fermi liquid state decreases rapidly at \(T\gtrsim{T}_{0}\), then smoothly turns into the pseudo gap at T ≃ T K , and finally the dense Kondo regime is realized at \(T\gtrsim{T}_{\text{K}}\).

Heavy fermion aspects of strong electron-phonon coupling compound

Heavy fermion aspects of strong electron-phonon coupling compounds are discussed on the basis of the idea that two-level systems (TLS) play a role of the localized spins in the dense Kondo system. Using the Vladar and Zawadowskis results that, at low temperatures, a single TLS interacting with electrons can be transformed to spin-1/2 Kondo system, it is shown that a heavy fermion band is formed through the coherence among virtual bound states around each TLS. The effective interaction between thus obtained heavy fermions consists of on-site repulsion and intersite attraction; so that the Cooper pair becomes inevitably anisotropic. It is shown that the local-phonon model can be transformed to TLS when the adiabatic potential for an ion becomes double-well. For this, it is essential to take into account the term quadratic in ionic displacement in the local-phonon model.

Charge fluctuation and superconductivity in the d-p model

We investigate effects of the repulsive interaction U p d between holes on the nearest-neighbor Cu- and O-sites in the d - p model with infinite repulsion on the Cu site using the auxiliary boson technique and the 1/ N expansion method. It is found that the condition for a metal-insulator (M-I) transition to occur at the half-filled case is modified. We then investigate the charge susceptibility; it is found that the thermodynamic charge susceptibility is enhanced by U p d except for the region around the critical point of the M-I transition. It is found to diverge at a certain value of U p d , which implies an instability toward a phase separation. On the basis of these calculations, we obtain phase diagrams, which consist of the metallic, unstable (phase-separating) and insulating phases. Furthermore, we calculate the quasiparticle interaction and find that the charge fluctuation induces attractive interaction comparable in magnitude to the repulsive interaction originating from the on-site repulsion U ...

Electronic States in High-Tc Copper-Oxide Superconductors

The electronic states in the CuO 2 -layer have been studied by using the p - d model with the help of the auxiliary boson method for treating the local electronic correlation properly. What have been found are: at sufficiently low temperature the electronic states form coherent bands, whose width and intensity are strongly depending upon number of doped carriers (more significantly for holes than electrons) and the p - d charge transfer energy. In particular, when the number of doped-holes n h is small, a new band form around the Fermi level inside the energy gap and its width and intensity are proportional to n h . Even then the Fermi surface is still determined by the total number of unpaired electrons which are mainly of the localized d -electrons.

Metal-insulator transition and charge fluctuation in the d-p model

Electronic states in the d-p model, particularly those around the half-filled case, are investigated with the help of the auxiliary boson method and the 1/ N -expansion. The critical value of the charge transfer energy for a metal-insulator transition to occur at the half-filled case is evaluated. The analytic expression for the charge susceptibility is given. Then characteristics of the charge fluctuation around the transition point are clarified; it is found that the charge susceptibility vanishes on the critical point. Furthermore, dependence of the charge susceptibility on the hole number and on the charge transfer energy is studied. Relevance to high temperature superconducting materials is then discussed.

Copper-Oxide Superconductors as Mixed-Valence Heavy Fermions:Spin-Fluctuation Mechanism of Superconductivity

It is proposed that the physics of high- T c superconductors with a CuO 2 network can be understood in a way parallel to that of heavy-fermion superconductors where the spin fluctuations play an important role. On the basis of a periodic Wolff model with aspects specific to copper oxides, it is shown that the charge carriers are the p -like holes when the antibonding band is filled less than half. It is also shown that these charge carriers form relatively heavy Fermi liquid with an effective bandwidth T 0 ∼10 3 K, which is consistent with experiments.

Heavy fermion aspects of A15 compounds and other strong-coupling superconductors

We show that various anomalous properties of A15 compounds and other high T c and high H c2 superconductors are well understood by regarding them as kinds of heavy femion systems and that interaction between electrons and local phonons is reduced to the s - d type interaction or Anderson model. Comparing with discussion of the usual dense Kondo systems, we discussed physical properties of the compounds.