Microscopic theory of pseudogap phenomena and unconventional Bose-liquid superconductivity and superfluidity in high-$T_c$ cuprates and other systems

A consistent microscopic theory of pseudogap phenomena and novel Bose-liquid superconductivity (superfluidity) is presented, based on the fact that in high-$T_c$ cuprates and related systems the energy $\varepsilon_A$ of the effective attraction between fermions is comparable with their Fermi energy $\varepsilon_F$ and the bosonic Cooper pairs are formed above $T_c$ and then a part of such Cooper pairs condense into a Bose superfluid at $T_c$. High-$T_c$ cuprates and other systems with low Fermi energies ($\varepsilon_F\sim\varepsilon_A$) are bosonic superconductors/superfluids and exhibit pseudogap phases above $T_c$, $λ$-like superconducting transition at $T_c$ and Bose-liquid superconductivity below $T_c$. The relevant charge carriers in high-$T_c$ cuprates are polarons which are bound into bosonic Cooper pairs above $T_c$. Polaronic and pseudogap effects weaken with increasing the doping and disappear at a quantum critical point. The modified BCS-like theory describes another pseudogap regime but the superfluid transition in high-$T_c$ cuprates and other systems is neither BCS-like transition nor usual Bose-Einstein condensation. The criteria for bosonization of Cooper pairs are formulated. The mean-field theory of the coherent single particle and pair condensates of bosonic Cooper pairs describes fairly well the novel superconducting states and properties of high-$T_c$ cuprates in full agreement with the experimental findings. The unusual superconducting/superfluid states and properties of other exotic systems (e.g., heavy-fermion and organic compounds, $\rm{Sr_2RuO_4}$, $^3$He, $^4$He and atomic Fermi gases) are explained more clearly by the theory of Bose superfluids. Finally, the new criteria and principles of unconventional superconductivity and superfluidity are formulated.

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