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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

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