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Microscopic electronic inhomogeneity in the high-Tc superconductor Bi2Sr2CaCu2O8+x

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Abstract

The parent compounds of the copper oxide high-transition-temperature (high-Tc) superconductors are unusual insulators (so-called Mott insulators). Superconductivity arises when they are ‘doped’ away from stoichiometry1. For the compound Bi2Sr2CaCu2O8+x, doping is achieved by adding extra oxygen atoms, which introduce positive charge carriers (‘holes’) into the CuO2 planes where the superconductivity is believed to originate. Aside from providing the charge carriers, the role of the oxygen dopants is not well understood, nor is it clear how the charge carriers are distributed on the planes. Many models of high-Tc superconductivity accordingly assume that the introduced carriers are distributed uniformly, leading to an electronically homogeneous system as in ordinary metals. Here we report the presence of an electronic inhomogeneity in Bi2Sr2CaCu2O8+x, on the basis of observations using scanning tunnelling microscopy and spectroscopy. The inhomogeneity is manifested as spatial variations in both the local density of states spectrum and the superconducting energy gap. These variations are correlated spatially and vary on the surprisingly short length scale of 14 Å. Our analysis suggests that this inhomogeneity is a consequence of proximity to a Mott insulator resulting in poor screening of the charge potentials associated with the oxygen ions left in the BiO plane after doping, and is indicative of the local nature of the superconducting state.

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Acknowledgements

We acknowledge P. A. Lee and E. W. Plummer for their comments. We also thank P. W. Anderson, A. Balatsky, D. A. Bonn, A. Castro-Neto, E. Carlson, M. Franz, L. H. Greene, X. Hu, T. Imai, B. Keimer, S. A. Kivelson, K. Kitasawa, R. B. Laughlin, D.-H. Lee, A. H. MacDonald, A. Millis, N. P. Ong, Z.-X. Shen, H.-J. Tao, X.-G. Wen, Z.-Y. Weng, N.-C. Yeh, G.-M. Zhang and Z.-X. Zhong for helpful discussions. This work was supported by the NSF, the DOE, the Sloan Research Fellowship, the Research Corporation, the Miller Institute for Basic Research and a Grant-in-Aid for Scientific Research on Priority Area and a COE Grant from the Ministry of Education, Japan.

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Correspondence to S. H. Pan.

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Further reading

Figure 1: Topographic image and associated integrated LDOS map of an optimally oxygen-doped, nominally pure single crystal of Bi2Sr2CaCu2O8+x.
Figure 2: A comparison of an integrated LDOS map and its corresponding superconducting gap map, including their associated statistical results.
Figure 3: Spatial variation of the tunnelling differential conductance spectrum.
Figure 4: A scatter plot of the superconducting gap versus integrated LDOS.

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