Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The origin of strong correlations and superconductivity in Na$_x$CoO$_2$   We propose a minimal model resolving a puzzle of enigmatic correlations
observed in sodium-rich Na$_x$CoO$_2$ where one expects a simple, free motion
of the dilute $S=1/2$ holes doped into a band insulator NaCoO$_2$. The model
also predicts singlet superconductivity at experimentally observed
compositions. The model is based on a key property of cobalt oxides -- the
spin-state quasidegeneracy of CoO$_6$ octahedral complex -- leading to an
unusual physics of, {\it e.g.}, LaCoO$_3$. We show that correlated hopping
between $t_{2g}$ and $e_g$ states leads to the spin-polaron physics at $x\sim
1$, and to an extended s-wave pairing at larger doping when coherent fermionic
bands are formed.

15
The categories of this paper are: nucl-ex
2
The categories of this paper are: math
30
The categories of this paper are: q-alg
16
The categories of this paper are: stat
28
The categories of this paper are: alg-geom
36
The categories of this paper are: cmp-lg
37
The categories of this paper are: supr-con
5
The categories of this paper are: gr-qc
24
The categories of this paper are: dg-ga
1
The categories of this paper are: cs
6
The categories of this paper are: astro-ph
0
The categories of this paper are: hep-ph
31
The categories of this paper are: ao-sci
9
The categories of this paper are: nlin
35
The categories of this paper are: mtrl-th
8
The categories of this paper are: hep-ex