Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Restoration of long range order of Na ions in $Na_xCoO_2$ at high
  temperatures by sodium site doping   We have systematically investigated the $Na_xCoO_2$ system doped with Cu, Y,
Sn, W, Au and Bi for $x$ = 0:5; 0:75 and 1.00 using density functional theory.
Sn, W, and Bi always substitute a Co while Au always substitutes a Na
regardless of Na concentration. However, for Cu and Y, the substitution site
depends on Na concentration. When compared to the available experimental data,
we find that thermoelectric performance is enhanced when the dopants substitute
a Na site. In this case, surprisingly, resistivity decreases despite the
reduced hole concentration caused by carrier recombination. We propose improved
carrier mobility to be the cause of observed reduced resistivity.

7
The categories of this paper are: hep-th
37
The categories of this paper are: supr-con
12
The categories of this paper are: nucl-th
29
The categories of this paper are: funct-an
19
The categories of this paper are: eess
10
The categories of this paper are: q-bio
34
The categories of this paper are: chem-ph
32
The categories of this paper are: atom-ph
1
The categories of this paper are: cs
8
The categories of this paper are: hep-ex
20
The categories of this paper are: acc-phys
18
The categories of this paper are: econ
14
The categories of this paper are: math-ph
13
The categories of this paper are: quant-ph
6
The categories of this paper are: astro-ph
27
The categories of this paper are: comp-gas