Angle resolved photoemission on underdoped Bi2Sr2CaCu2O8 reveals that the magnitude and d-wave anisotropy of the superconducting state energy gap are independent of temperature all the way up to Tc. This lack of T variation of the entire k-dependent gap is in marked contrast to mean field theory. At Tc the point nodes of the d-wave gap abruptly expand into finite length ``Fermi arcs''. This change occurs within the width of the resistive transition, and thus the Fermi arcs are not simply thermally broadened nodes but rather a unique signature of the pseudogap phase.
@article{arxiv.0708.4099,
title = {Protected nodes and the collapse of the Fermi arcs in high Tc cuprates},
author = {A. Kanigel and U. Chatterjee and M. Randeria and M. R. Norman and S. Souma and M. Shi and Z. Z. Li and H. Raffy and J. C. Campuzano},
journal= {arXiv preprint arXiv:0708.4099},
year = {2009}
}