期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2022
卷号：119
期号：37
DOI：10.1073/pnas.2207449119
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：Significance
Charge-transfer superexchange interactions between electrons on adjacent Cu sites have long been hypothesized to generate the intense spin-singlet electron-pair formation in cuprate superconductors. But this concept is unproven, partly because there existed no analogue isotope effect in which one could controllably vary the charge-transfer energy
E
(
r
)
and measure the changes in the electron-pair condensate
Ψ
. Our concept is to visualize both
E
(
r
)
and
n
P
(
r
)
=
|
Ψ
|
2
directly at atomic scale and as a function of varying apical oxygen displacements
δ
(
r
)
that occur in Bi
2Sr
2CaCu
2O
8+x. These data provide access to controllable variations in
E
(
r
)
and resultant effects on
n
P
(
r
)
, yielding
d
n
P
/
d
E
≈
−
0.81
±
0.17
e
V
−
1
. This compares with recent prediction
d
n
P
/
d
E
≈
−
0.9
e
V
−
1
for superexchange-mediated electron pairing in Bi
2Sr
2CaCu
2O
8+x, indicating that charge-transfer superexchange is the electron-pairing mechanism in hole-doped superconductor Bi
2Sr
2CaCu
2O
8+x.
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
E
, generate “superexchange” spin–spin interactions of energy
J
≈
4
t
4
/
E
3
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
E
. To explore this hypothesis directly at atomic scale, we combine single-electron and electron-pair (Josephson) scanning tunneling microscopy to visualize the interplay of
E
and the electron-pair density
n
P
in Bi
2Sr
2CaCu
2O
8+x. The responses of both
E
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
2Sr
2CaCu
2O
8+x.
