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Extending Universal Nodal Excitations Optimizes Superconductivity in Bi2Sr2CaCu2O8+δ

Science  26 Jun 2009:
Vol. 324, Issue 5935, pp. 1689-1693
DOI: 10.1126/science.1174338

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Abstract

Understanding the mechanism by which d wave superconductivity in the cuprates emerges and is optimized by doping the Mott insulator is one of the major outstanding problems in condensed-matter physics. Our high-resolution scanning tunneling microscopy measurements of the high–transition temperature (Tc) superconductor Bi2Sr2CaCu2O8+δ show that samples with different Tc values in the low doping regime follow a remarkably universal d wave low-energy excitation spectrum, indicating a doping-independent nodal gap. We demonstrate that Tc instead correlates with the fraction of the Fermi surface over which the samples exhibit the universal spectrum. Optimal Tc is achieved when all parts of the Fermi surface follow this universal behavior. Increasing the temperature above Tc turns the universal spectrum into an arc of gapless excitations, whereas overdoping breaks down the universal nodal behavior.

  • * These authors contributed equally to this work.

  • Present address: Department of Physics, Columbia University, New York, NY 10027, USA.

  • Present address: Department of Physics, Stanford University, Stanford, CA 94305, USA.

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