N‐band Hubbard models. III. Boson–fermion and interaction–boson models for high‐T_c superconductivity

Abstract

ABSTRACT:In this series of articles (I, II), N-band Hubbard models have been considered for strongly correlated electron systems, which are realized in d-p, -d, -R, and -R conjugated systems. The magnetism and superconductivity of these systems have been elucidated in terms of effective exchange integrals (J), which are calculated by first-principle methods. In part III of this series, the BCS-BEC crossover theory, has been introduced to elucidate the physical foundation of our J and JP model for the high-T c superconductivity (HTSC). The boson-fermion (BF) model for this theory is found useful for a reasonable explanation of the experimental phase diagrams of HTSC. The underlying physics of the BF model is different from that of the slave boson fieldtheoretical model assuming spinon-holon condensations in the low dimension. The interaction boson model (IBM) for nuclear matter is also employed to describe the cooperative mechanisms of electron-phonon (EP), spin fluctuation (SF), charge fluctuation (CF), and many-bands (MB) effects. This phenomenological model is useful for pictorial understanding and for the theoretical explanation of the cooperative mechanisms: (EP ϩ SF), (SF ϩ CF), (EP ϩ SF ϩ MB), etc. These are also investigated in analogy to BF model of fermionic gases, where the Feshbach resonance between boson and fermion is responsible for their coupling. The implications of these theoretical results are discussed in relation to recent ALPES and STM experiments for HTSC, which suggest the contributions of SF (J) and EP (P) interactions. The recently discovered superconductivity of boron-doped diamond is examined as an example of two-band sigma-radical (-R) conjugated systems. Finally, the bipolaron model is briefly discussed in relation to boson-fermion model via EP interaction to superconductivity.

N‐bands Hubbard models. IV. Comparisons of electron‐ or hole‐doped quaternary oxypictides LaOMPn superconductors with cuprates

“…(16) [35]), supporting theoretical proposals based on strong electron correlations such as magnetic excitations. The LaOFeAs family seem very interesting even from the scientific view point since medium coherence length does not suffer the disorder problem in cuprates, but electron correlation is still crucial for superconductivity.…”

“…The spin-charge separation has been confirmed for 1D system such as SrCuO 2 [35]. These exotic particles exist in the confinement state because of the following condition [21] h j ϩ h j ϩ f j ϩ f j ϭ 1 (29) There are a lot of theoretical studies based on the t-J model [18].…”

“…According to Yamada [84], the triplet superconductivity of Sr 2 RuO 4 cannot be explained by the second-order (FLEX) theory, though third-order perturbation theory works well even for the species. The key point is that spin (SF)-fluctuation is not so effective for Sr 2 RuO 4 , leading to a electroncorrelation (EC) mechnism [35,84,85]. It is noteworthy that the magnitude of antiferromagnetic J values by the spin-polarized UHF is often too small as compared with the experiments, indicating the necessity of correlation corrections by UMP and UCC treatments in Eq.…”

“…Then one of possible phenomenological theories is to extrapolate results beyond their regime of validity. We have constructed a phenomenological phase diagram for qualitative purpose, which exhibits characteristics even at both limits [28,33,35]. To this end, available experimental results [96 -99] have been used as several key constraints.…”

“…Indeed, several kinds of SF models have been proposed on the experimental and theoretical grounds in Part II [33]. However, a unified theory describing phase diagrams in an intermediate (near Mott transition) regime as well as both weak and strong correlation limits is not established yet [54], partly because serious disorder and defects in each CuO 2 plane (BEC-BCS crossover problem in part III [35]). …”