Electron-Phonon Interaction and Strong Correlations in High-Temperature Superconductors: One can not avoid the unavoidable

Abstract

Abstract. The important role of the electron-phonon interaction (EPI) in explaining the properties of the normal state and pairing mechanism in high-T c superconductors (HTSC) is discussed. A number of experimental results are analyzed such as: dynamical conductivity, Raman scattering, neutron scattering, ARPES, tunnelling measurements, isotope effect and etc. They give convincing evidence that the EPI is strong and dominantly contributes to pairing in HTSC oxides. It is argued that strong electronic correlations in conjunction with the pronounced (in relatively weakly screened materials) EPI are unavoidable ingredients for the microscopic theory of pairing in HTSC oxides. I present the well defined and controllable theory of strong correlations and the EPI. It is shown that strong correlations give rise to the pronounced forward scattering peak in the EPI -the FSP theory. The FSP theory explains in a consistent way several (crucial) puzzles such as much smaller transport coupling constant than the pairing one (λ tr ≪ λ ), which are present if one interprets the results in HTSC oxides by the old Migdal-Eliashberg theory for the EPI. The ARPES shift puzzle where the nodal kink at 70 meV is unshifted in the superconducting state, while the anti-nodal one at 40 meV is shifted can be explained at present only by the FSP theory. A number of other interesting predictions of the FSP theory are also discussed.

Competition of Superconductivity and Charge Density Waves in Cuprates: Recent Evidence and Interpretation

“…Prima facie subtle mathematical differences between descriptions of s-wave and d-wave CDW superconductors lead to conspicuous physical consequences. Indeed, the numerical dependences Δ(T) and Σ(T) found from (17) and (18) and shown in Figure 10 differ qualitatively from their counterparts Δ s (T) and Σ s (T) in a certain range of model parameters. (In this subsection, we do not introduce a natural subscript "d" for brevity.)…”

“…Weakcoupling values of the ratio Δ 0 /T c0 for other anisotropic order parameter symmetries do not differ much from the value of (Δ 0 /T c0 ) d [627,628]. Due to the different order parameter symmetry, readily seen from (17) and (18), the situation is mathematically more involved than for isotropic CDW superconductors, where a simple relationship (4) takes place. This was not recognized in a recent work [629], where the opposite wrong statement was made.…”

“…Therefore, T u CDW thus defined coincides with T CDW0 . Second, below the lower critical temperature of the CDW reentrance region, T l CDW , if any, (18) defines Δ(T) = dMü(Δ 0 , T), and we should use (17) with T = T l CDW and Δ(T l CDW ) = dMü(Δ 0 , T l CDW ) to determine T l CDW (Δ 0 , Σ 0 , μ) numerically. The crossover value of Σ cr 0 , when T l CDW = 0, corresponds to the separatrix on the order parameter-T plane, dividing possible Σ(T)-curves (see Figure 10(a)) into two types: reentrant and nonreentrant.…”

“…It is remarkable that, for hole-and electron-doped cuprates, there is still no clarity concerning the specific mechanisms of superconductivity [17,[107][108][109][110][111][112][113][114][115] and the order parameter symmetry [109,[116][117][118][119][120][121][122][123][124][125][126][127][128][129][130], contrary to the "official" viewpoint [131][132][133]) and even the very character of the phenomenon (in particular, there have been furious debates concerning the Cooper pairing versus boson condensation dilemma in cuprates [8,134,135]). The same seems to be true for other old and new "exotic" superconductors [46,107,108,[136][137][138][139][140][141][142][143][144][145][146]…”

The Role of Polarons in Cuprates Hi-Tc Superconductivity

“…One of the major points of disagreement is whether phonons contribute along with the accepted spin magnetic fluctuation to the high critical temperature of the cuprates. Many researchers are in support of the phonon mechanism as the only factor responsible for the cuprates high critical temperatures [4][5][6].…”