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Phase Coherence Experiments 

Even the “normal” state of the high-Tc  materials is unconventional and is not well understood.  In the early 1990’s researchers began noticing an energy-gap like feature well above Tc.  This feature was seen in many different types of experiments and is now called the pseudogap (PG).  Many experiments, namely tunneling spectroscopy, show that the pseudogap continuously and smoothly becomes more pronounced as the temperature is lowered, eventually merging into (perhaps creating) the superconducting (SC) gap. In addition, angle resolved photoemission measurements have shown the pseudogap to have the same d-wave anisotropy as the SC gap.  This created a large number of so called “pre-formed pairs” models.  In this models cooper pairs form at a temperature called T* which can be 100-200 K above Tc.  However at this temperatures there is no long-range phase coherence across the bulk sample.  However, phase coherence may exist and be observable on very short, nanoscale lengths.  By studying the resistivity fluctuations in nanobridges we may be able to observe large downard fluctuations due to local phase coherence.