Visualizing pair formation on the atomic scale in the high-Tc superconductor Bi2Sr2CaCu2O8+δ

Pairing of electrons in conventional superconductors occurs at the superconducting transition temperature T-c, creating an energy gap D in the electronic density of states (DOS)(1). In the high-T-c superconductors, a partial gap in the DOS exists for a range of temperatures above T-c (ref. 2). A key question is whether the gap in the DOS above T-c is associated with pairing, and what determines the temperature at which incoherent pairs form. Here we report the first spatially resolved measurements of gap formation in a high-T-c superconductor, measured on Bi2Sr2CaCu2O8+delta samples with different T-c values ( hole concentration of 0.12 to 0.22) using scanning tunnelling microscopy. Over a wide range of doping from 0.16 to 0.22 we find that pairing gaps nucleate in nanoscale regions above T-c. These regions proliferate as the temperature is lowered, resulting in a spatial distribution of gap sizes in the superconducting state(3-5). Despite the inhomogeneity, we find that every pairing gap develops locally at a temperature T-p, following the relation 2 Delta/k(B)T(p) = 7.9 +/- 0.5. At very low doping (<= 0.14), systematic changes in the DOS indicate the presence of another phenomenon(6-9), which is unrelated and perhaps competes with electron pairing. Our observation of nanometre-sized pairing regions provides the missing microscopic basis for understanding recent reports(10-13) of fluctuating superconducting response above T-c in hole-doped high-T-c copper oxide superconductors.