Dynamical measurements of superconducting fluctuations in underdoped cuprates and amorphous superconductors: Classical to quantum
N. Peter Armitage
Johns
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.