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Condensed Matter Seminar: Time-reversal symmetry breaking superconductivity in Fe-pnictides with only electron or only hole pockets

Event Type
190 Engineering Sciences Bldg - corner Goodwin & Springfield 
Mar 1, 2013   1:00 pm  
Prof Andrey Chubukov - Dept of Physics - University of Wisconsin
Originating Calendar
Physics - Condensed Matter Seminar

The pairing in moderately doped Fe-pnictides and Fe-chalcogenides is generally understood as  being due to magnetically enhanced  interaction between hole and electron pockets.
Recently,  superconductivity has been observed in AFe2Se2 (A = K, Rb, Cs), which contain only electron pockets, and in KFe2As2, which contains only hole pockets.
In this talk  I  l review different (and sometimes conflicting) scenarios for the pairing in these systems and propose my own one.  I argue that the pairing condensate in systems with only electron pockets necessary contains an  inter-pocket component, made of fermions belonging to different electron pockets. 
I  analyze the interplay between intra-pocket and inter-pocket pairing, depending on the ellipticity of the electron pockets and the strength of their hybridization,
 and show that, with increasing hybridization, the system undergoes a transition from a d-wave state to a novel  s+- state, in which the gap changes sign between hybridized pockets.
This s+- state has the full gap and at the same time supports spin resonance, in agreement with the  experimental data.
Near the boundary between d and s+- states the system develops s+id state which breaks time-reversal symmetry. 
 For systems with only hole pockets, I  present arguments  for an  s+- state, in which the gap changes sign between hole pockets.  I show that this state is qualitatively different from a "conventional" s+- state
in which the gap on hole pockets has the same sign.   I further show that the transition from one s-wave state to the other involves highly unusual s+is state, which again breaks time reversal symmetry.


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