Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Crossover from spin-waves to diffusive spin excitations in underdoped
  Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$   Using inelastic neutron scattering, we show that the onset of
superconductivity in underdoped Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ coincides
with a crossover from well-defined spin waves to overdamped and diffusive spin
excitations. This crossover occurs despite the presence of long-range stripe
antiferromagnetic order for samples in a compositional range from x=0.04-0.055,
and is a consequence of the shrinking spin-density wave gap and a corresponding
increase in the particle-hole (Landau) damping. The latter effect is captured
by a simple itinerant model relating Co doping to changes in the hot spots of
the Fermi surface. We argue that the overdamped spin fluctuations provide a
pairing mechanism for superconductivity in these materials.

29
The categories of this paper are: funct-an
3
The categories of this paper are: physics
24
The categories of this paper are: dg-ga
14
The categories of this paper are: math-ph
0
The categories of this paper are: hep-ph
11
The categories of this paper are: hep-lat
16
The categories of this paper are: stat
10
The categories of this paper are: q-bio
17
The categories of this paper are: q-fin
30
The categories of this paper are: q-alg
28
The categories of this paper are: alg-geom
1
The categories of this paper are: cs
35
The categories of this paper are: mtrl-th
36
The categories of this paper are: cmp-lg
5
The categories of this paper are: gr-qc
22
The categories of this paper are: chao-dyn