Hisao Jichu

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 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.

Nearly Gapless Singlet-Pairing in Pure System of Heavy Fermion Superconductor

Heavy fermion superconductors, recently found in compounds with I-electrons (such as CeCu2Si2 and UBed, have attracted much attention.i)-6) One of the recent· issues is concerning the type of Cooper pairing; singlet (even parity) or triplet (odd parity) type? Although a nature of heavy fermion itself has not been completely clarified from a theoretical point of view, it is likely from experimental evidences that heavy fermions behave as a fermi liquid with very small Fermi energy of order TK. There was an attempt to explain, along this line, heavy fermion superconductors within the singlet BCS scheme.1l Since the motion of heavy fermion is slower than those of light-electrons and ions, the Coulomb interaction between heavy fermions is screened instantaneously by rapid motions of the latter; so that the interaction potential in k-space between heavy fermions is given by Vk=4Ke 2 /k €(k, 0»0, where €(k, 0) is the static dielectric constant. The reason why the triplet case is considered to be a likely possibility seems to be that Vk is positive for the whole regions of k-space. Indeed, several theorists have recently claimed that the triplet pairing is the most likely possibility.S),9) However, the present authors recently pointed out that s-wave pairing is still possible owing to the spatial correlations among particles, at least in principle, even if Vk is always positive. IO) The experimental results concerning the above issues are also very controversiaL For CeCu2Si., Assmus et aL 5) claimed that the Landau parameter Foa is positive which is in favour of singlet pairing_ On the other hand, Ott et aL6) recently showed that the low temperature specific heat of UBel3 exhibits T 3-dependence, which is in favour of the p-wave superconductivity of ABM-type. Moreover, Kitaoka et aL ll) recently found an unusual nuclear relaxation for CeCu2Si2 which cannot be understood by neither s-wave scheme of simple BCS-type nor p-wave scheme of ABMtype. The purpose of this paper is to point out that a singlet-pairing with nearly gapless excitations is possible for a model in which the interaction between fermions on the same site is strongly repulsive while that on the neighbour sites is appropriately attractive, and that consequences of this model are consistent with existing experiments of heavy fermion superconductors. In particular, a puzzling behavior of nuclear relaxation of CeCu2Si. found by Kitaoka et aL ll) is qualitatively explained by this model. On the basis of a quasi-particle theory of Kondo lattice,10).12) we simulate a heavy fermion system as a half-filled band of fermions with a Hamiltonian

An Origin of an Anomalously Narrow Energy-Band in a Dense Kondo System

3) on the basis of the modified-periodi6Anderson Hamiltonian in which bare I-electrons:are assumed to form a narrow band. In this short note, we study electronic states in a dense Kondo system and show an origin of a very narrow energy-band formed around the Fermi level in this system. We consider the following periodiC Anderson Hamiltonian given

Superconductivity in the dense Kondo system

Abstract An anisotropic s-symmetry superconductivity, possibly with a line gapless region, is shown to be most favorable under the strong on-site repulsive and the phonon-mediated inter-site attractive interaction. Spin-fluctuation-mediated indirect interaction does not seem to benefit triplet pairing.

An origin of heavy fermions in dense Kondo systems

Abstract Self-energy corrections due to the coherence between virtual bound states in a dense Kondo system are studied and shown to give a narrow band with a width of the order of the Kondo temperature.

Vacancy Motion in Quantum Crystals of Fermions under Strong Magnetic Field

An effective Hamiltonian is derived which describes the coupling between a vacancy and the spin fluctuation in Quantum crystals of fermions. The self·energy of the vacancy is calculated up to the first order in the magnitude of the magnon density in the case where spins are polarized by a strong magnetic field. The detailed analysis shows an enhancement of the effective mass and a narrowing of the band due to the coupling with the spin fluctuation.