Zhang Li-yuan

Comments on superconductivity of BaLaCuO systems

Abstract In this paper, a model for the newly discovered high Tc materials is proposed. In essence, it is a new type of superconductor with two subsystems. One is the conduction electron subsystem and the other is the negative correlation energy center subsystem. Within the framework of this model, some general features of these materials can be understood and some predictions have been made.

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.

Effect of the local substitution of Li for Cu on the electronic structure of YBa2Cu3O7

Abstract The change of the electronic structure of YBa 2 Cu 3 O 7 caused by the local substitution of Li for Cu has been studied by the recursion method. The characteristics including the partial wave density of state, the types and intensities of bonds, and the valence on a site etc. are obviously affected by Li on the replacing site and its neighbours. In the case of the substitution of Li for the Cu(2) site, the two-dimensional character of the electronic structure in CuO 2 planes is locally destroyed, accompanied by the migration of electrons or holes, which causes the decrease of the hole concentration in the CuO 2 planes. The local substitution of Li for Y has also been discussed. This substitution can strengthen the two-dimensional character of the carriers in the CuO 2 planes. From our point of view, these changes may be responsible for the changes of T c in the Li-doped YBa 2 Cu 3 O 7 material.

On the combination of the Cooper pair and the Ogg pair in the high-Tc oxide superconductor

Abstract In this paper it is argued that the superconductivity of the high- T c oxide superconductor can be a combining mechanism of the Cooper pair and the Ogg pair. As a preliminary test the properties of the superconducting state of Bi 2 Sr 2 Ca 2 Cu 3 O 10 have been calculated under this mechanism. Further, within the framework of this combining picture a two superconducting energy gap structure may appear.

On the Free Carrier-Negative U Center Interacting Model for the High-Tc Oxide Superconductor

Using the free carrier-negative U center interacting model, we have investigated the superconducting transition, temperature Tc, the specific heat jump at Tc, the coherence length, the penetration depth, the thermodynamic critical field and the isotope effect for the high-Tc oxide superconductor. The theoretical values of these physical quantities are consistent with the experimental values. Further, the residual electronic specific heat in the superconducting state is explained as the result of the inherent instability in this superconductive mechanism.

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.

A Possible Model of Heavy Fermion Superconductivity

We used the periodic Anderson Hamiltonian to study the behaviour of heavy fermion systems. It has been argued that the properly large mixing between and the conduction electrons, the strong coulomb correlation between electrons and the related renormalization effect are the main causes of the large effective mass of the quasi-particle. Further, we introduced phenomenoloigcally the BCS attractive interaction between the heavy quasi-particles and explained that the value of and may be quite different from that of the BCS theory as a result of the interaction between two branches of the quasi-particles.

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.