  The nature of the pseudogap phase of cuprate high-temperature superconductors
is a major unsolved problem in condensed matter physics. We studied the
commencement of the pseudogap state at temperature T* using three different
techniques (angle-resolved photoemission spectroscopy, polar Kerr effect, and
time-resolved reflectivity) on the same optimally-doped Bi2201 crystals. We
observed the coincident, abrupt onset at T* of a particle-hole asymmetric
antinodal gap in the electronic spectrum, a Kerr rotation in the reflected
light polarization, and a change in the ultrafast relaxational dynamics,
consistent with a phase transition. Upon further cooling, spectroscopic
signatures of superconductivity begin to grow close to the superconducting
transition temperature (Tc), entangled in an energy-momentum-dependent fashion
with the pre-existing pseudogap features, ushering in a ground state with
coexisting orders.
