Chen Chang-feng

Effective mass and specific heat in the periodic Anderson model with negative U centers

Abstract The periodic Anderson model with negative correlation energy (- U , U 0) has been used to investigate the effective mass and the electronic specific heat in the normal state. The mean occupation number of the local electrons has been treated in a self-consistent method. Numerical calculation shows that these properties of the high T c oxide superconductors can be interpreted qualitatively within this model.

On the theory of superconductivity of mixed valence and heavy fermion systems

Abstract The effective mass and the superconducting transition temperature of mixed valence and heavy fermion systems are discussed under formally equivalent Hamiltonians in the quasiparticle picture. Linearization is applied to the correlation term between up- and down-spin ƒ electrons on the same site. Mixing between ƒ and conduction electrons is decoupled through the Bogoliubov transformation. Numerical results show that proper mixing and correlation enhance the effective mass of these systems. In mixed valence system, both ƒ and conduction electrons make contributions to superconductivity, but basically only ƒ electrons are responsible for superconductivity in heavy fermion system.

Coherent state and superconductivity in the free carrier-bipolaron interacting system

Abstract In this paper a model to describe the free carrier-bipolaron interacting system is proposed. Effective hopping of the bipolaron is studied in the slave-boson approach, and a characteristic temperature T ∗ is obtained, below which the system enters a coherent state. The density of states in the normal state and the superconductivity of the system are discussed in a quasiparticle picture. The results show that the mixing between the free carrier and the bipolaron results in an enhancement of the effective mass of the quasiparticle and meanwhile the renormalized coupling interaction, arising from the negative correlation energy in the bipolaron region, enhances the effective superconducting coupling interaction. Under the most favourable conditions, the superconducting transition temperature T c ∼ ω c , where ω c is the Debye frequency related with local electron-phonon coupling. In general we have T ∗ > T c ⪢ T c0 ( T c0 is the superconducting transition temperature of a usual superconductor). Therefore the system will firstly enter a coherent state before becoming a high- T c superconductor.

Inter-layer coupling and its contribution to superconductivity: Application to high-Tc oxides

Abstract We investigate the role of inter-layer coupling and its contribution to superconductivity. It is concluded that this coupling will reduce Coulomb pseudopotential and hence have considerable effect on the enhancement of Tc. Recently discovered high-Tc superconductivity in some exotic oxides is discussed in this model.

A model for local magnetic moment and the valence of f electrons in metals

In this paper we suggest a model in which f electrons are bound in a narrow band near Fermi level, s-p-d electrons free and hybridization between them is considered. Numerical results show that the local magnetic moment of the f electron possesses certain stability under varying pressures. A phase diagram is plotted. The case of intermediate valence is discussed qualitatively.

A quasiparticle model for heavy fermion superconductivity

Based on a two-branch quasiparticle model, it has been proposed that the heavy fermion systems can be distinguished into two kinds, i.e. one-heavy-branch system and two-heavy-branch system. It has been inferred that one of the three known heavy fermion superconductors, UPt3, belongs to the former kind and the other two, CeCu2Si2 and UBe13, belong to the latter. The low-temperature specific heat coefficient, the jump in specific heat and superconducting transition temperature, the order parameters and the thermodynamic critical field of the two kinds of systems have been discussed, and the results are in fair agreement with the experiments.

Evidence for unconventional s-wave superconductivity in the heavy-fermion compound UPt3

Abstract We have discussed the ultrasonic attenuation and specific heat in the superconducting state of the heavy-fermion compound UPt3 in a quasiparticle picture. Coherence and pair-breaking effect have been taken into account for this system. Our results show that UPt3 is most probably an unconventional s-wave superconductor.

Unconventional even parity superconductivity in UBe13 and CeCu2Si2

Abstract Based on an extended s-wave pairing model, the behavior of the nuclear relaxation rate of the heavy fermion superconductors UBe 13 and CeCu 2 Si 2 is discussed. It is pointed out that the appearance of the pseudogap near the Fermi energy, which results from the coherence in the Kondo lattice system at low temperatures, will lead to a temperature-dependent effective BCS interaction between the electrons forming the Cooper pairs. Therefore, the superconducting order parameter will be rather different from that of the conventional BCS theory. As a result, the nuclear relaxation rate in the superconducting state will have both quantitative and qualitative departures from that of the BCS theory. The peak just below the superconducting transition temperature will be reduced in height, shifted to lower temperature or even eliminated entirely, depending on to what degree is the system being affected by the coherence. Some typical numerical results are given, which are in fair agreement with the experiments.

Coherence and pair-breaking effect in the heavy fermion superconductor UBe13

The specific heat and nuclear relaxation rate of the heavy fermion superconductor UBe13 have been discussed in the framework of an extended BCS theory. The coherence and pair-breaking effect in this Kondo-lattice system have been taken into account. The conclusion is that UBe13 is an s-wave superconductor.

Pressure induced structural phase transition in AgSbTe{sub 2}

We report a novel high pressure structural sequence for the functionally graded thermoelectric, narrow band gap semiconductor AgSbTe