Superconductivity, cuprate

As mentioned above, La(2 x)BaxCu04 is superconducting below 35 K. The mother compound La2Cu04 is a cuprate and an insulator at T = 0 like CuO and all cuprates where copper has the well-defined oxidation state -1-2. CuO, the black, semiconducting oxide of copper, is formally not a cuprate (since a cuprate has two different metal ions). Cuprates, in contradistinction to CuO, can be made superconducting by oxidative or reductive doping. Cuprates are different from CuO by the presence of the Cu02 plane in the structure. 

There is no consensus on the mechanisms of high T superconductivity in doped cuprates. There is also no generally accepted assignment of the spectrum of pure as well as doped cuprates. Convincing experimental evidence shows, however, that the doping sites, consisting of Cu + ions, are local. 

The Cu + sites with the neighboring oxygen ions are local and positive, since they miss one electron compared to the Cu + site. The energy needed for an intermetal transition where the electron ends on a site near a Cu + site must be much less than if it ends on a Cu + site for away from. We may conclude that the Hubbard gap is reduced in size locally in the CUO2 plane close to the Cu(III). 

The CUO2 plane is the xy-plane. The metal-ligand covalent bonds are in the xy-plane because one electron is missing in the antibonding 02p - 3d(x - y ) MO. The CuO distance is therefore smaller in the CUO2 plane than in the perpendicular direction, where the 3dz orbital is fully occupied. This is the Jahn-Teller effect in action, ordering all the copper ions of the plane. The reorganization of the structure due to the number of electrons present at the site has little to do with the Jahn-Teller effect, however. When we apply the Jahn-Teller theorem, the number of electrons is constant. 

The occurrence of superconductivity in the cuprates still had no accepted explanation when this book was written in 2011, 100 years after superconductivity was 

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Key words Cuprates, superconductivity, photoemission, films, strain, Tc, laser ablation... 

Abstract Since the discovery of high 7C cuprates, superconductivity has been sought... 

Electron—hole asymmetry is encountered in cuprate superconductors.3 In the cuprates, superconductivity occurs in the electron-doped regime, although not as prominently as in the hole-doped regime. The electron-... 

THE ISING, SMALL-BIPOLARON THEORY OF CUPRATE SUPERCONDUCTIVITY... 

The Ising, small-bipolaron (ISB) theory of cuprate superconductivity (Section 8) employs NUHH where, just as in BCS theory, it is a consequence of vibronic interaction which overrides the positive U. 

The theory of the cuprate superconductivity must explain all above experimental facts, and the mechanism of the superconductivity itself. The strong Coulomb repulsion is certainly a very important ingredient as is manifested by the fact that parents compounds are Mott insulators. However, we will show that, in addition to it, strong hole-lattice interactions are also very important. In this review, we will present explanations for some of the above anomalies by including the strong hole-lattice interactions. 

In the previous section, we have presented the evidence that the small polaron is an important ingredient of the cuprate superconductivity. In this section, we show that the spin-vortex is another important ingredient. 

By now, we have identified three important ingredients in the cuprate superconductivity strong on-site Coulomb repulsion, small polaron formation, and spin-vortex formation. With all these ingredients, however, the conventional transport theory based on Bloch electrons will predict that the system is an insulator. In order to... 

Table 4.7 High-temperature cuprate superconducting oxides...

We have seen in the previous section that the single-layer materials exhibit substantial R-Cu exchange coupling, and they also have modest (for cuprates) superconducting transition temperatures. For the materials that contain midtiple Cu-O layers the Tc values... 

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