From a single-band metal to a high-temperature superconductor via two   thermal phase transitions
  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. 
Rui-Hua He, M. Hashimoto, H. Karapetyan, J. D. Koralek, J. P. Hinton,   J. P. Testaud, V. Nathan, Y. Yoshida, Hong Yao, K. T...
cond-mat.supr-con cond-mat.mtrl-sci cond-mat.str-el