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Anomalous superconductivity and superfluidity in repulsive fermion systems

“…We note that a very interesting physics appears in the so-called 'difficult' comer of the phase diagram of the generalized t − J model, namely, in the case of small J (J/t << 1) and low doping δ = (1 − n e ) << 1 (this region is frequently called the pseudogap). For this part of the phase diagram, in accordance with the ideas of Laughlin [144,145] on the spin-charge confinement of spin and charge in two-dimensional and threedimensional strongly correlated electron systems (see also [146,147]), a concept of a strongly interacting Fermi-Bose mixture of spinons and holons has been proposed. The spinons and holons in the mixture form the composite holes in the confinement potential of an antiferromagnetic (AFM) string [131,148]).…”

“…The spinons and holons in the mixture form the composite holes in the confinement potential of an antiferromagnetic (AFM) string [131,148]). In this case, according to the assumption made in [96,147], the phase diagram of cuprate superconductors in the region of low doping can be considered in the framework of the scenario of the BCS-BEC crossover between the local and extended pairs for pairing of two composite holes (two spin polarons or two AFM strings) in the d x 2 −y 2 -wave channel. Certainly, the transition from the region of optimal doping with a large Fermi surface and extended Cooper pairs to the region of low doping with local pairs and hole-type pockets (small Fermi surface) can be realized in a very nontrivial way and can contain a singularity in the middle, such as a quantum critical point (QCP) (see, e.g., [149][150][151]), or even certain intermediate phases.…”