On the electron pairing mechanism of copper-oxide high temperature superconductivity
Shane O’Mahony, Wangping Ren, Weijiong Chen, Yi Xue Chong, Xiaolong Liu, Hiroshi Eisaki, S. Uchida, Mohammad Hamidian, J. C. Davis
Abstract
The elementary CuO 2 plane sustaining cuprate high-temperature superconductivity occurs typically at the base of a periodic array of edge-sharing CuO 5 pyramids. Virtual transitions of electrons between adjacent planar Cu and O atoms, occurring at a rate t/ℏ and across the charge-transfer energy gap <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mi mathvariant="script">E</mml:mi> </mml:math> , generate “superexchange” spin–spin interactions of energy <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:mi mathvariant="bold-italic">J</mml:mi> <mml:mo>≈</mml:mo> <mml:mn>4</mml:mn> <mml:msup> <mml:mrow> <mml:mi mathvariant="bold-italic">t</mml:mi> </mml:mrow> <mml:mn>4</mml:mn> </mml:msup> <mml:mo>/</mml:mo> <mml:msup> <mml:mrow> <mml:mi mathvariant="script">E</mml:mi> </mml:mrow> <mml:mn>3</mml:mn> </mml:msup> </mml:mrow> </mml:math> in an antiferromagnetic correlated-insulator state. However, hole doping this CuO 2 plane converts this into a very-high-temperature superconducting state whose electron pairing is exceptional. A leading proposal for the mechanism of this intense electron pairing is that, while hole doping destroys magnetic order, it preserves pair-forming superexchange interactions governed by the charge-transfer energy scale <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mi mathvariant="script">E</mml:mi> </mml:math> . To explore this hypothesis directly at atomic scale, we combine single-electron and electron-pair (Josephson) scanning tunneling microscopy to visualize the interplay of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mi mathvariant="script">E</mml:mi> </mml:math> and the electron-pair density n P in Bi 2 Sr 2 CaCu 2 O 8+x . The responses of both <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mi mathvariant="script">E</mml:mi> </mml:math> and n P to alterations in the distance δ between planar Cu and apical O atoms are then determined. These data reveal the empirical crux of strongly correlated superconductivity in CuO 2 , the response of the electron-pair condensate to varying the charge-transfer energy. Concurrence of predictions from strong-correlation theory for hole-doped charge-transfer insulators with these observations indicates that charge-transfer superexchange is the electron-pairing mechanism of superconductive Bi 2 Sr 2 CaCu 2 O 8+x .