Angle resolved photoemission on underdoped ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_8$ reveals that the magnitude and $d$-wave anisotropy of the superconducting state energy gap are independent of temperature all the way up to $T_c$. This lack of $T$ variation of the entire $\mathbf{k}$-dependent gap is in marked contrast to mean field theory. At $T_c$ 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.

• Received 11 January 2007

DOI:https://doi.org/10.1103/PhysRevLett.99.157001