Theory of Copper Oxide Superconductors

The objective of this book is to provide an up-to-date comprehensive descr- tion of the Kamimura–Suwa model, which is the ?rst of the present rep- sentative two-component theories in high temperature superconductivity. In 1986 George Bednorz and Karl Alex Muller ¨ made the remarkable discovery ofsuperconductivitywithanunbelievinglyhighvalueofT = 35K,bysubs- c 2+ 3+ tuting Ba ions for La ions in the antiferromagnetic insulator La CuO . 2 4 Soon after this discoveryT rose to 90K by synthesizing YBa Cu O with c 2 3 7?? ade?citinoxygen. Furtherexplorationfornewcopperoxidesuperconducting materials with higherT led to the discovery of Bi–Sr–Ca–Cu–O, Tl–Ba–Ca– c Cu–O and Hg–Ba–Ca–Cu–O compounds in subsequent years. The new class of copper oxide compounds mentioned above is called “cuprates”. At present T = 135K under ambient pressure andT = 164K under 31 GPa observed c c in HgBa Ca Cu O are the highest value so far obtained. The Kamimura– 2 2 3 8 Suwa model, which was originally developed in 1993, is a theory of these real copper oxide superconducting materials. Since undoped La CuO is a 2 4 Mott–Hubbard antiferromagnetic insulator, its electronic structure can not beexplained bytheordinaryone-electron energybandtheory. Inthiscontext the important role of electron-correlation was pointed out. 2+ On theother hand, ad-hole state in each Cu ion in theligand ?eld with octahedral symmetry is orbitally doubly-degenerate so that it is subject to strong Jahn–Teller interaction in La CuO. Asaresult,aCuO octahedron 2 4 6 in La CuO is elongated along thec-axis due to the Jahn–Teller distortion.