Flux-Periodicity Crossover from hc/e in Normal Metallic to hc/2e in Superconducting Loops

Abstract

The periodic response of a metallic or a superconducting ring to an external magnetic flux is one of the most evident manifestations of quantum mechanics. It is generally understood that the oscillation period hc/2e in the superconducting state is half the period hc/e in the metallic state, because the supercurrent is carried by Cooper pairs with a charge 2e. On the basis of the Bardeen-Cooper-Schrieffer theory we discuss, in which cases this simple interpretation is valid and when a more careful analysis is needed. In fact, the knowledge of the oscillation period of the current in the ring provides information on the electron interactions. In particular, we analyze the crossover from the hc/e periodic normal current to the hc/2e periodic supercurrent upon turning on a pairing interaction in a metal ring. Further, we elaborate on the periodicity crossover when cooling a metallic loop through the superconducting transition temperature Tc.

The generalized Lang–Firsov transformation and the three- and the four-fermion interactions in a lattice fermion model

“…It turns out that signals suggesting the presence of sextets and even clusters made of eight fermions were observed in some cuprates. [15][16][17] Of course, one can imagine that instead of two tercets a bipolaron and a quartet can form a bound sextet whereas two quartets bind themselves into an eight-fermion cluster. If g cc < 0 and g dd < P. Tarasewicz 0, then the four-polaron potential is attractive whereas the three-polaron one is repulsive.…”

“…However, fermion sextets were observed in some superconducting materials. [15][16][17] A fermion tercet has a total spin different from zero and in order to compensate it, two tercets with opposite spins might attract each other to form a sextet. The four-fermion term stands for the attraction between two local bipolarons from different bands and can lead to the formation of on-site fermion quartets.…”