Endless Quests

The following sections are included:

• Preface
• Contents

The following sections are included:

• Introduction
• A brief on the development of superconductivity theory
• Dilemma of the BCS theory
• Examinations of the BCS theory
• Issues in electronic theory
• Diagrammatic iteration approach (DIA)
• Summary and conclusions
• References
• Appendix A
• Appendix B

Recent development in the physics of high-temperature superconductivity (SC) is reviewed, with special emphasis on the studies of the low-energy excitations of cuprate and iron-based superconductors. For cuprate superconductors, a phenomenology based on coexisting competing orders with superconductivity in the ground state of these doped Mott insulators is shown to provide a consistent account for a wide range of experimental findings. In the case of iron-based superconductors, studies of the low-energy excitations reveal interesting similarities and differences when compared with cuprate superconductors. In contrast to the single-band cuprate superconductivity with an insulating parent state, the ferrous superconductors are multi-band materials with a semi-metallic parent state and exhibit two-gap superconductivity when doped. On the other hand, both systems exhibit strong antiferromagnetic correlation and Fermi-surface distortion, leading to unconventional pairing symmetries with sign-changing order parameters on different parts of the Fermi surface. These findings suggest that the pairing potentials in both the cuprate and the ferrous superconductors are generally repulsive, thus favoring a pairing mechanism that is electronically driven and a pairing strength that is closely related to the electronic correlation. The physical implications of the unified phenomenology based on antiferromagnetic correlations and remaining open issues associated with the cuprate and ferrous superconductivity are discussed.

We report here the fabrication of our metal (M)-doped PrBa₂Cu₃O₇ (PBCO) or PrBa[Cu_1−xM_x]O₇ (PBCMO) bulk materials and epitaxial thin films in which M = Al, Co, Fe, Ga, Ni, and Zn, and x = 0.00, 0.05, 0.10, 0.15, and 0.20. We will also present the results of our transport and Raman studies on these PBCMO samples. From the results on these studies we are able to identify the doping sites for the dopants and their effects on the electrical resistivity. The transport property studies on the YBCO/PBCMO multilayer allow us to deduce the superconducting coupling length, the finite size effect and the two-dimensional (2D) phase transition of YBCO.

The following sections are included:

• Introduction
• Nanostructures in superconductors
• Superconducting nanostructures
• Conclusion
• Acknowledgements
• References

The following sections are included:

• Introduction
• Effect of excess Mg in the nano-SiC doped Mg-B system
• The combined in-situ/ex-situ technique
• Diffusion technique for high density MgB₂ superconductors
• Conclusion
• Acknowledgements
• References

The following sections are included:

• Introduction
• Rail transit–the mainstream of 21st century transportation
• Maglev train
• Advantages of inherent self-stable maglev systems
• Typical application of the HTS maglev technology
• HTS maglev in the 863 Program
• The special advantage of developing HTS maglev train in China
• Development of “Century”
• Research advances after “Century”
• HTS maglev launch [69]
• Moderate/low-speed HTS maglev train
• Super-high speed HTS maglev train
• Postscript
• Acknowledgments
• References