Metallic nonsuperconducting phase and D-wave superconductivity in Zn-substituted La1.85Sr0.15CuO4

Insights from the study of high-temperature interface superconductivity

A brief overview is given of the studies of high-temperature interface superconductivity based on atomic-layer-by-layer molecular beam epitaxy (ALL-MBE). A number of difficult materials science and physics questions have been tackled, frequently at the expense of some technical tour de force, and sometimes even by introducing new techniques. ALL-MBE is especially suitable to address questions related to surface and interface physics. Using this technique, it has been demonstrated that high-temperature superconductivity can occur in a single copper oxide layer-the thinnest superconductor known. It has been shown that interface superconductivity in cuprates is a genuine electronic effect-it arises from charge transfer (electron depletion and accumulation) across the interface driven by the difference in chemical potentials rather than from cation diffusion and mixing. We have also understood the nature of the superconductor-insulator phase transition as a function of doping. However, a few important questions, such as the mechanism of interfacial enhancement of the critical temperature, are still outstanding.

Disorder and transport in cuprates: weak localization and magnetic contributions

We report resistivity measurements in underdoped YBa 2Cu3O6.6 and overdoped Tl 2Ba2CuO6+x single crystals in which the concentration of defects in the CuO (2) planes is controlled by electron irradiation. Low T upturns of the resistivity are observed in both cases for large defect content. In the Tl compound the decrease of conductivity scales as expected from weak localization theory. On the contrary, in YBa 2Cu3O6.6 the much larger low T contribution to the resistivity is proportional to the defect content and might then be associated with a Kondo-like spin flip scattering term. This would be consistent with the results on the magnetic properties induced by spinless defects.

Zero-temperature d-wave superconducting phase transition

The Ginzburg-Landau-Wilson theory that describes the disordered-metal- d-wave-superconductor phase transition at zero temperature is derived at weak coupling. The theory represents an interacting dissipative system of bosonic Cooper pairs in an effective random potential. I show that there exists a wide crossover regime in the theory controlled by a line of Gaussian fixed points, each of which in two dimensions is characterized by a different universal value of the dc critical conductivity. Relation to experiments on overdoped and underdoped cuprates is discussed.