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Electron–Phonon Mechanism for Superconductivity in Na 0.35 CoO 2 : Valence-Band Suhl–Kondo Effect Driven by Shear Phonons

2006; Physical Society of Japan; Volume: 75; Issue: 3 Linguagem: Inglês

10.1143/jpsj.75.033705

1347-4073

Keiji Yada, Hiroshi Kontani,

Advanced Thermoelectric Materials and Devices

To study the possible mechanism of superconductivity in Na 0.35 CoO 2 , we examine the interaction between all the relevant optical phonons (breathing and shear phonons) and t 2 g ( a 1 g + 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 g ' 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 Na 0.35 CoO 2 . 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.