Visualizing Pair Formation on the Atomic Scale in the High-Tc Superconductor Bi2Sr2Ca2CuO8+δ

We take an atom-by-atom look at where pairing gaps form in the cuprate superconductor Bi2Sr2Ca2CuO8+δ. When pairing occurs in a superconductor, a gap forms in the density of states (DOS) that can be probed by STM. In Bi2Sr2Ca2CuO8+δ, the gap measured at low temperature shows large variations on the atomic scale. In this experiment, we raise the temperature of the sample through Tc while imaging and measuring the same sample area. We find that the larger gaps disappear at higher temperatures leading to a patchwork of gapped areas above Tc. By studying samples at different doping levels, we find that overdoped samples have a simple linear relationship between the size of the gap and the temperature at which it disappears. This linear relationship breaks down for underdoped samples.

These measurements are performed in our specially designed variable-temperature STMs where we are able to track the same area of the sample as the temperature is raised through the superconducting transition temperature Tc. In order to perform such measurements, we have achieved exceptional STM stability over month-long periods at elevated temperatures.

Published in Nature:
Visualizing Pair Formation on the Atomic Scale in the High-Tc Superconductor Bi2Sr2Ca2CuO8+δ, K. K. Gomes, A. N. Pasupathy, A. Pushp, S. Ono, Y. Ando, A.  Yazdani, Nature 447, 569 (2007). (Letter)