We have studied high-temperature superconducting cuprates samples throughout the doping-temperature phase diagram. We find that the low energy excitations of these systems have a surprising, universal character at low doping. We have developed a procedure to extract the angular dependence of the excitations near the node of the d-wave order parameter from scanning tunneling spectroscopy (STS) data. Using this procedure, we show that the universal character of low energy excitations stems from a universal angular dependence of the gap as a function of angle near the nodes. We find that the reduction in Tc with reduced doping for underdoped samples is correlated with a reduction in the angular range over which we find universal nodal excitations. By contrast, reduction in Tc with increased doping for overdoped samples is correlated with a collapse of the universal nodal behavior and a nodal gap that reduces in strength.
Published in Science:
Extending Universal Nodal Excitations Optimizes Superconductivity in Bi2Sr2Ca2CuO8+δ, A. Pushp, C. V. Parker, A. N. Pasupathy, K. K. Gomes, S. Ono, J. Wen, Z. Xu, G. Gu, and A. Yazdani, Science 324, 1689-1693 (2009). (Article)