M2S - Phonons, electronic correlations and superconductivity in cobaltates

A. GRECO; A. FOUSSATS; M. BEJAS; A. MURAMATSU

Dresden

Congreso; M2S; 2006

We consider possible routes to superconductivity in hydratedcobaltates $Na_xCoO_2.yH_2O$ on the basis of the $t$-$J$-$V$ modelplus phonons on the triangular lattice. We studied the stabilityconditions for the homogeneous Fermi liquid (HFL) phase againstdifferent broken symmetry phases. Besides the $\sqrt{3}\times\sqrt{3}$-CDW phase, triggered by the nearest-neighbour Coulombinteraction $V$, we have found that the HFL is unstable, at verylow doping, against a bond-ordered phase due to $J$. We alsodiscuss the occurrence of phase separation at low doping and $V$.The interplay between the electron-phonon interaction andcorrelations near the $\sqrt{3}\times \sqrt{3}$-CDW leads tosuperconductivity in the unconventional next-nearest neighbour$f$-wave (NNN-$f$) channel with a dome shape for $T_c$ around $x\sim 0.35$, and with values of a few Kelvin as seen inexperiments. Near the bond-ordered phase at low doping we foundtendencies to superconductivity with $d$-wave symmetry forfinite $J$ and $x<0.15$. Contact with experiments is given along the paper.