Electron-phonon mechanism for superconductivity in Na0.35CoO2:: Valence-band Suhl-Kondo effect driven by shear phonons

To study the possible mechanism of superconductivity in Na0.35CoO2, we examine the interaction between all the relevant optical phonons (breathing and shear phonons) and t(2g) (a(lg) + e '(g)) electrons of Co ions, and study the transition temperature for s-wave superconductivity. The obtained T-c is very low when the e '(g) valence bands are far below the Fermi level. However, T-c is strongly enhanced when the top of the e' valence bands is close to the Fermi level (e.g., -50 meV), thanks to the interband hopping of Cooper pairs caused by shear phonons. This valence-band Suhl-Kondo mechanism due to shear phonons is important in understanding the superconductivity in Na0.35CoO2. By the same mechanism, the kink structure of the band dispersion observed by angle-resolved photoemission spectroscopy (ARPES) measurements, which indicates the strong mass enhancement (m*/m similar to 3) due to optical phonons, is also explained.