The underlying theory of the high-temperature superconductors is still - more than twenty years after their discovery - one of the great unsolved problems of modern physics. The central conceptual difficulty is the understanding of the enigmatic underdoped "pseudogapped" parts of the phase diagram. There is now a large body of evidence that this region shows strong evidence for superconducting signatures even in those regions of the phase diagram that do not exhibit bulk superconductivity.
We investigate these regions by use of a novel time-domain THz technique that can explicitly measure the temporal coherence of the relevant fluctuations and not merely their presence. We find, in strong contrast to the interpretation of other measurements, strong evidence that fluctuations are limited to temperatures on the order of 15K above Tc. In this range the superconducting fluctuation coherence time becomes of order the normal state electron coherence time. In the extremely underdoped part of the phase diagram, we find a regime of superconducting fluctuations in samples which are so underdoped that they are not themselves superconductors. As they persist to T=0, these fluctuations cannot be thermally activated and are presumably quantum in nature, which shows the principal role that superconductivity plays even in the nearby insulating or metallic groundstate(s). In particular it shows that superconductivity is destroyed on the underdoped side by quantum disordering the d-wave superconducting state. This is decidedly non-BCS-type behavior and cannot be described by any weakly interacting theory. Throughout this talk I will make extensive comparison to related measurements that we have also done on the amorphous superconductor InOx.
Host Haruo Kojima