Nov 22, 2002
ARPES View of the Phase Diagram of the High Temperature Superconductors
Juan Campuzano, University of Illinois
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 and superlinear behavior in the resistivity, whereas the
latter is characterized by a single broad spectral feature in ARPES and a
linear temperature dependence of the resistivity.
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