  The Fermi surface in the state of cuprates is highly unusual because it
appears to consist of disconnected segments called arcs. Their very existence
challenges the traditional concept of a Fermi surface as closed contours of
gapless excitations in momentum space. The length of the arcs in the pseudogap
state was thought to linearly increase with temperature, pointing to the
existence of a nodal liquid state below T*. These results were interpreted as
an interplay of a d-wave pairing gap and strong scattering. Understanding the
properties of the arcs is a pre-requisite to understanding the origin of the
pseudogap and the physics of the cuprates. Here we use a novel approach to
detect the energy gaps based on the temperature dependence of the density of
states. With a significantly improved sensitivity, we demonstrate that the arcs
form rapidly at T* and their length remains surprisingly constant over an
extended temperature range between T* and Tarc, consistent with the presence of
an ordered state below T*. These arcs span fixed points in the momentum space
defining a set of wave vectors, which are the fingerprints of the ordered state
that causes the pseudogap.
