Microscopic theory of novel pseudogap phenomena and Bose-liquid superconductivity and superfluidity in high-Tc cuprates and other systems

A unified and empirically adequate microscopic theory of novel pseudogap phenomena and Bose- liquid superconductivity and superfluidity in high-Tc cuprates and other systems is developed based on the original ideas of the pseudogap state and unusual superconducting/superfluid states of matter. This theory establishes the following laws: (i) the high-Tc cuprates and other systems with low Fermi energies εF ∼ εA (where εA is the energy of the attractive interaction between fermionic quasiparticles) are bosonic superconductors and superfluids exhibiting pseudogap behavior above the superconducting/superfluid transition temperature Tc and a λ-like phase transition at Tc, (ii) the pseudogap state and bosonic Cooper pairs in such systems (with εF 2εA and Bardeen– Cooper–Schrieffer (BCS)-like gap F 0.17εF ) are formed above Tc, (iii) only a minority of preformed bosons condenses into a Bose superfluid at Tc and (iv) only the systems with εF >> εA >> F become BCS-type conventional or topological fermionic superconductors and superfluids. A modified BCS-like model describes the precursor Cooper pairing of fermionic quasiparticles and the formation of bosonic Cooper pairs above Tc. The criteria for the bosonization of Cooper pairs and fermion–boson transitions are formulated. The mean-field theory describing new laws of condensation of attracting bosons into Bose superfluids below Tc is presented. The proposed microscopic theory explains all the emerging pseudogap behaviors and unusual superconducting/superfluid states and properties of high-Tc materials and other systems. In high-Tc cuprates, the unconventional electron–phonon interactions and polaronic effects give rise to in-gap states, Fermi-surface reconstruction, two distinct pseudogaps and unusual normal-state properties, a quantum critical point and crossover from BCS superconductivity to Bose-liquid superconductivity. The theory of three-dimensional (3D) and two-dimensional (2D) Bose superfluids describes fairly well the novel superconducting states (i.e., the so-called A and B phases below Tc and an extended A phase and related vortex-like state above Tc) and properties of high-Tc cuprates (e.g., λ-like transition at Tc, first-order phase transition at lower temperatures and other unusual features) in accordance with the experimental data. The reasons for suppression and enhancement of superconductivity by disorders in high-Tc cuprates are discussed. Strongly enhanced 2D Bose-liquid superconductivity emerging within a 3D cuprate superconductor (with the highest bulk Tc) persists up to room temperature in multi-lamellar blocks and at grain boundaries and interfaces. Most enhanced 3D Bose-liquid superconductivity can emerge at room temperature in high-Tc hydrides under high pressures. Superconducting/superfluid states and properties of heavy-fermion and organic compounds, ruthenate (Sr₂RuO₄) and possibly high-Tc hydrides, quantum liquids ( 3 He and 4 He) and atomic Fermi gases are also well explained by the proposed theory of Bose superfluids. Finally, new criteria and principles of unconventional superconductivity and superfluidity are formulated. (author)