Atomic-Scale Quasi-Particle Scattering Resonances in Bi$_2$Sr$_2$CaCu$_2$O_8+\delta$
Science, 285 (5424):
88--91 (July 1999)Abstract
Low-temperature scanning tunneling spectroscopy of the high transition
temperature (high-Tc) cuprate Bi2Sr2CaCu2O8+ reveals the existence
of large numbers of identical regions with diameters of about 3 nanometers
that have a relatively high density of low-energy quasi-particle
states. Their spatial and spectroscopic characteristics are consistent
with theories of strong quasi-particle scattering from atomic-scale
impurities in a d-wave superconductor. These characteristics include
breaking of local particle-hole symmetry, a diameter near twice the
superconducting coherence length, and an inverse square dependence
of their local density-of-states on distance from the scattering
center. In addition to the validation of d-wave quasi-particle scattering
theories, these observations identify a source for the anomalously
high levels of low-energy quasi-particles in Bi2Sr2CaCu2O8+ at low
temperatures.