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

TL;DR

This study uses scanning tunnelling microscopy to visualize nanoscale electron pairing regions in a high-Tc superconductor, revealing their formation above Tc and providing insights into the nature of superconductivity in these materials.

Contribution

First spatially resolved measurements of pairing gap formation above Tc in a high-Tc superconductor, linking nanoscale regions to superconducting fluctuations.

Findings

Pairing gaps nucleate in nanoscale regions above Tc.

The temperature for gap development follows 2D/kBTp = 8.

Inhomogeneity in gap sizes persists in the superconducting state.

Abstract

Pairing of electrons in conventional superconductors occurs at the superconducting transition temperature Tc, creating an energy gap D in the electronic density of states (DOS). In the high-Tc superconductors, a partial gap in the DOS exists for a range of temperatures above Tc. A key question is whether the gap in the DOS above Tc 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-Tc superconductor, measured on Bi2Sr2CaCu2O8+d samples with different Tc 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 Tc. These regions proliferate as the temperature is lowered, resulting in a spatial distribution of gap sizes in the superconducting state. Despite the inhomogeneity, we find that every pairing gap develops locally at a temperature Tp, following the relation 2D/kBTp = 8. At very low doping (<0.14), systematic changes in the DOS indicate the presence of another phenomenon, 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 of fluctuating superconducting response above Tc in hole-doped high-Tc copper oxide superconductors.