Seminar: Ilya Eremin – University of Copenhagen

Condensed Matter Theory > Calendar > 2016 > Seminar: Ilya Eremin

Seminar: Ilya Eremin

Superconductivity vs bound state formation in a two-band superconductor with small Fermi energy -- applications to Fe-based superconductors

In my talk I will consider the interplay between superconductivity and formation of bound pairs of fermions in multi-band 2D fermionic systems (BCS-BEC crossover) in application to iron-based superconductors. In two spatial dimensions a bound state develops already at weak coupling, and BCS-BEC crossover can be analyzed already at weak coupling, when calculations are fully under control. I will consider two different two-band models, one with one hole and one electron band and the other with two hole or two electron bands. The first model is relevant to experiments on Fe-pnictides and Fe-chalcogenides, particularly on FeSe, the second one is used to describe Nb-doped SrTiO3. We found that the behavior of the compensated metal with one electron and one hole bands is different in several aspects from that in the one-band model. There is a crossover from BCS-like behavior at EF>>E0 (E0 being the bound state energy formation in a vacuum) to BEC-like behavior at EF<< E0 with Tins > Tc. However, the actual Tc, below which long-range superconducting order develops, remains finite and of order Tins even when EF = 0 on both bands. The reason for a finite Tc is that the filled hole band acts as a reservoir of fermions. The pairing reconstructs fermionic dispersion and transforms some spectral weight into the newly created hole band below the original electron band and electron band above the original hole band. A finite density of fermions in these two bands gives rise to a finite Tc even when the bare Fermi level is exactly at the bottom of the electron band and at the top of the hole band.We also considered the model with two hole/two electron bands. We found that the behavior in this model is similar to that in the one-band model. Namely, BCS-BEC crossover occurs when the largest of the two EF ’s becomes comparable to E0. When the ratio EF =E0 is large, the system displays BCS-like behavior, when it is small, the system displays the same BEC-type behavior as in the one-band model, namely Tc scales with EF and is parametrically smaller than Tins.