s -wave superconductivity due to Suhl-Kondo mechanism in Na x CoO 2 ⋅ y H 2</mml:mn…
2008; American Physical Society; Volume: 77; Issue: 18 Linguagem: Inglês
10.1103/physrevb.77.184521
ISSN1550-235X
Autores Tópico(s)Magnetic and transport properties of perovskites and related materials
ResumoTo study the electron-phonon mechanism of superconductivity in ${\text{Na}}_{x}{\text{CoO}}_{2}\ensuremath{\cdot}y{\text{H}}_{2}\text{O}$, we perform semiquantitative analysis of the electron-phonon interaction (EPI) between relevant optical phonons (breathing and shear phonons) and ${t}_{2g}$ electrons (${a}_{1g}$ and ${e}_{g}^{\ensuremath{'}}$ electrons) in the presence of trigonal distortion. We consider two kinds of contributions to the EPI: the EPI originating from the Coulomb potential of O ions and that originating from the $d\text{\ensuremath{-}}p$ transfer integral between Co and O in ${\text{CoO}}_{6}$ octahedron. We find that the EPI for shear phonons, which induces the interorbital hopping of electrons, is large in ${\text{Na}}_{x}{\text{CoO}}_{2}\ensuremath{\cdot}y{\text{H}}_{2}\text{O}$ because of the trigonal distortion of ${\text{CoO}}_{2}$ layer. For this reason, ${T}_{c}$ for $s$-wave pairing is prominently enlarged owing to interorbital hopping of Cooper pairs induced by shear phonons, even if the top of ${e}_{g}^{\ensuremath{'}}$ electron band is close to but below the Fermi level as suggested experimentally. This mechanism of superconductivity is referred to as the valence-band Suhl-Kondo (SK) mechanism. Since the SK mechanism is seldom damaged by the Coulomb repulsion, $s$-wave superconductivity is realized irrespective of large Coulomb interaction $U\ensuremath{\sim}5\text{ }\text{eV}$ at Co sites. We also study the oxygen isotope effect on ${T}_{c}$ and find that it becomes very small due to strong Coulomb interaction. Finally, we discuss the possible mechanism of anisotropic $s$-wave superconducting state in ${\text{Na}}_{x}{\text{CoO}}_{2}\ensuremath{\cdot}y{\text{H}}_{2}\text{O}$, resulting from the coexistence of strong EPI and the antiferromagnetic fluctuations.
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