Pairing from repulsive interactions

F.J.W. Hahne, Peter A. Ring

F. Guinea

Dimitri Pimenov, Andrey V. Chubukov

We study the pairing of fermions by an interaction consisting of a Hubbard repulsion, mimicking a screened Coulomb potential, and a dynamical phonon-mediated attraction. For such interaction, the gap equation allows even- and odd-frequency solutions ${\mathrm{\ensuremath{\Delta}}}_{e}$ and ${\mathrm{\ensuremath{\Delta}}}_{o}$. We show that odd-frequency pairing does not develop within the Eliashberg approximation due to over-critical pair breaking from the self-energy. When vertex corrections are included, the pairing interaction gets stronger, and ${\mathrm{\ensuremath{\Delta}}}_{o}$ can develop. We argue that even in this case keeping the self-energy is still a must as it cancels out the thermal piece in the gap equation. We further argue that ${\mathrm{\ensuremath{\Delta}}}_{o}$ is not affected by Hubbard repulsion and for strong repulsion is comparable to a reduced ${\mathrm{\ensuremath{\Delta}}}_{e}$. The resulting superconducting state is a superposition ${\mathrm{\ensuremath{\Delta}}}_{e}\ifmmode\pm\else\textpm\fi{}i{\mathrm{\ensuremath{\Delta}}}_{o}$, which spontaneously breaks the time-reversal symmetry, despite that the pairing symmetry is an ordinary $s$ wave.

J. Dukelsky, Stuart Pittel

We introduce an exactly solvable model for interacting bosons that extend up to high spin and interact through a repulsive pairing force. The model exhibits a phase transition to a state with almost complete sd dominance. The repulsive pairing interaction that underlies the model has a natural microscopic origin in the Pauli exclusion principle between constituent nucleons. As such, repulsive pairing between bosons seems to provide a new mechanism for the enhancement of sd dominance, giving further support for the validity of the sd interacting boson model.

Douglas J. Scalapino

T. Espinosa-Ortega, Oleksandr Kyriienko, Ivan Andreevich Shelykh

We consider a mixture of two neutral cold Fermi gases with repulsive interactions. We show that in some region of the parameter space of the system the effective attraction between fermions of the same type can appear due to the exchange of collective excitations. This leads to the formation of BCS pairing in the case where bare interatomic interactions are repulsive.