Superconductivity as a Bose-Einstein condensation?

Though commonly unrecognized, a superconducting BCS condensate consists of equal numbers of two-electron (2e) and two-hole (2h) Cooper pairs (CPs). A complete (in the sense that 2h-CPs are not ignored) boson-fermion statistical model, however, is able to depart from this perfect 2e-/2h-CP symmetry and yields robustly higher T_c's without abandoning electron-phonon dynamics. The complete model reduces to all the known statistical theories of superconductors, including the BCS-...

The tremendous efforts to unveil high temperature superconductivity (HTSC) have been devoted to the search of the mechanism underlying Cooper pairs which, however, remains a mysterious subject of vigorous debate, let alone many other mysteries like the pseudogap state, the peculiar Homes law, the unnegligible electron-phonon interaction, the stripe, the universal nodal Fermi velocity, etc. Most of subsequent works either bring in more controversies or swell the list of myster...

Cooper pair (CP) binding with both zero and nonzero center-of-mass momenta (CMM) is studied with a set of renormalized equations assuming a short-ranged (attractive) pairwise interfermion interaction. Expanding the associated dispersion relation in 2D in powers of the CMM, in weak-to-moderate coupling a term {\it linear} in the CMM dominates the pair excitation energy, while the quadratic behavior usually assumed in Bose-Einstein (BE)-condensation studies prevails for any cou...

We review efforts to unify both the Bardeen, Cooper and Schrieffer (BCS) and Bose-Einstein condensation (BEC) pictures of superconductivity. We have finally achieved this in terms of a "\textit{complete} boson-fermion (BF) model" (CBFM) that reduces in special cases to all the main continuum (as opposed to "spin") statistical theories of superconductivity. Our BF model is "complete" in the sense that not only two-electron (2e) but also two-hole (2h) Cooper pairs (CPs) are all...

The basic principle of superconductivity is suggested in this paper. There have been two vital wrong suggestions on the basic principle, one is the relation between superconductivity and the Bose-Einstein condensation (BEC), and another is the relation between superconductivity and pseudogap.

A new theory for Cooper pair formation and superconductivity is derived from quantum statistical mechanics. It is shown that zero momentum Cooper pairs have non-local permutations and behave as effective bosons with an internal weight close to unity when bound by a primary minimum in the potential of mean force. For a short-ranged, shallow, and highly curved minimum there is no thermodynamic barrier to condensation. The size of the condensing Cooper pairs found here is orders...

The Cooper pair binding energy {\it vs.} center-of-mass-momentum dispersion relation for Bose-Einstein condensation studies of superconductivity is found in two dimensions for a renormalized attractive delta interaction. It crosses over smoothly from a linear to a quadratic form as coupling varies from weak to strong.

It is shown that the temperature dependence of the value of energy gap in superconductors can be classified as the order-disorder transition with its characteristic features. The obtained relationship between the critical temperature and the critical magnetic field of the condensate of electrically charged particles is in accordance with measurement data of superconductors.

The long-range Froehlich electron-phonon interaction has been identified as the most essential for pairing in high-temperature superconductors owing to poor screening, as is now confirmed by optical, isotope substitution, recent photoemission and some other measurements. I argue that low energy physics in cuprate superconductors is that of superlight small bipolarons, which are real-space hole pairs dressed by phonons in doped charge-transfer Mott insulators. They are itinera...

The common wisdom that the phonon mechanism of electron pairing in the weak-coupling Bardeen-Cooper-Schrieffer (BCS) superconductors leads to conventional s-wave Cooper pairs is revised. An inevitable anisotropy of sound velocity in crystals makes the phonon-mediated attraction of electrons non-local in space providing unconventional Cooper pairs with a nonzero orbital momentum in a wide range of electron densities. As a result of this anisotropy quasi-two dimensional charge ...