ARPES The Phase Diagram of the Cuprates

The superconducting phase transition in the underdoped high temperature superconductors is rather unusual, in that it is not a mean-field transition as other superconducting transitions are. Instead, it is observed that a pseudo-gap in the electronic excitation spectrum appears at temperatures T* higher than Tc, while phase coherence, and superconductivity, are established at Tc. One would then wish to understand if T* is just a crossover, controlled by fluctuations in order which will set in at the lower Tc, or whether some symmetry is spontaneously broken at T*. Using angle-resolved photoemission with circularly polarized light, we find that, in the pseudogap state, left-circularly polarized photons give a different photocurrent than right-circularly polarized photons, and therefore the state below T* is rather unusual, in that it breaks time reversal symmetry. On the other hand, in the overdoped region, we find evidence for a new crossover line in the phase diagram between a coherent metal phase for lower temperatures and higher doping, and an incoherent metal phase for higher temperatures and lower doping. The former is characterized by two well-defined spectral peaks in ARPES due to coherent bilayer splitting, whereas the latter is characterized by a single broad spectral feature in ARPES. In this talk we will discuss how the psedogap and the coherent to incoherent crossover manifest themselves throughout the phase diagram.