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Friday, 23. Aug. 2013 |
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14:00 – 15:25 |
Abstract
We review
the challenge of finding a unified mechanism for Cooper pairing
in
correlated superconductors e.g. copper-oxides, iron-arsenides
and heavy fermions.
Our
experimental context is the comparison of electronic structure imaging using
spectroscopic
imaging STM results for each of these systems.
Because
this technique can determine simultaneously the k-space structure
of the
superconductivity and the r-space structure of proximate broken symmetry
states,
it provides
some of the best indications of the gestalt of strongly correlated
superconductivity.
In copper-oxides
there are two well-known broken-symmetry states:
the Q≠0
density waves (Science 315, 1380 (2007)) and Q=0 intra-unit-cell nematic
(Nature 466, 374 (2010)) that can be
visualized directly.
Real-space
imaging of these two states shows that they are closely linked
(Science 333, 426 (2012). The concurrent k-space
imaging is achieved by
using
Fourier-transform STM and reveals the evolution of the Fermi arcs
with doping
(Nature 454, 1072 (2008)). Now we report that both these
broken-symmetry
states disappear at a critical doping, where that the
k-space
topology also undergoes an abrupt transition from arcs to closed
contours.
Similarly motivated SI-STM studies of iron-arsenides
discovered
the
electronic nematicity of the parent state (Science
327, 181 (2010)),
while k-space
imaging revealed anisotropy, magnitude, and relative
orientations
of the energy gaps of the superconducting state
(Science
336, 563 (2010)), and combined imaging shows a strong interplay
of these
two phenomena at the nanoscale (Nature Physics
9, 220
(2013)).
Finally,
corresponding SI-STM studies allowed the first imaging of heavy
fermions
(Nature 465, 570 (2010)), which presaged the first visualization
of k-space
structure of superconducting energy, gaps in a heavy fermion
system
(Nature Physics 9, 468 (2013)).
But could
these very disparate observations possibly be related
within a
unified mechanism?
A simple
synthesis is possible:
The strong,
on-site, repulsive electron- electron interactions
that are
the proximate cause of such superconductivity are more
typically
drivers of commensurate magnetism; suppression of
commensurate
antiferromagnetism (AF) usually allows unconventional
superconductivity
to emerge; it is between these AF and SC phases
that the
’intertwined’ electronic ordered phases (density waves,
nematic etc.) are
usually discovered. We discuss a logical basis,
motivated
by this analysis, for a unified explanation of the
relationship
between the antiferromagnetic electron-electron
interactions,
the intertwined ordered phases and the correlated
superconductivity
in all these systems.
--
Alexander Balatsky