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 t2g (a1g+e'g) electrons of Co ions, and study the transition temperature for s-wave superconductivity. The obtained Tc is very low when the e'g valence bands are far below the Fermi level. However, Tc is strongly enhanced when the top of the e'g valence bands is close to the Fermi level (e.g., -50meV), 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∼3) due to optical phonons, is also explained. (author)