Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Formation of the density wave energy gap in Na$_2$Ti$_2$Sb$_2$O: an
  optical spectroscopy study   We performed optical spectroscopy measurement on single crystals of
Na$_2$Ti$_2$Sb$_2$O, which is the parent compound for the newly discovered
Ti-based superconductors. The study reveals significant spectral change across
the phase transition at 114 K and formation of a density-wave type energy gap.
The opening of the gap removes most part of the free carrier spectral weight
and causes a dramatic reduction of the carrier scattering rate. The ratio of
2$\Delta/k_BT_s\approx$14 is considerably larger than the mean-field value
based on the weak-coupling BCS theory. The study also reveals a weak
correlation effect in the titanium oxypnictides.

21
The categories of this paper are: adap-org
2
The categories of this paper are: math
31
The categories of this paper are: ao-sci
8
The categories of this paper are: hep-ex
33
The categories of this paper are: plasm-ph
0
The categories of this paper are: hep-ph
3
The categories of this paper are: physics
10
The categories of this paper are: q-bio
26
The categories of this paper are: bayes-an
5
The categories of this paper are: gr-qc
13
The categories of this paper are: quant-ph
6
The categories of this paper are: astro-ph
20
The categories of this paper are: acc-phys
14
The categories of this paper are: math-ph
24
The categories of this paper are: dg-ga
16
The categories of this paper are: stat