Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Direct measurement of the temperature dependence of the magnetic
  penetration depth in Ba(Fe$_{1-x}$Ni$_x$)$_2$As$_2$ superconductors   The temperature dependence of the in-plane magnetic penetration depth
$\lambda_{ab}$ of Ba(Fe$_{1-x}$Ni$_x$)$_2$As$_2$ single crystals is determined
directly from the shielding magnetic susceptibility, measured in the Meissner
region with the field parallel to the ab layers. The doping levels studied
cover the underdoped, optimally-doped and overdoped regimes. At temperatures
below 0.5Tc a well-defined power-law behavior
$\lambda_{ab}(T)-\lambda_{ab}(0)=AT^n$ (with n approximately 2.5) is observed.
At lower temperatures (T<0.3Tc) the data are still consistent with n=2 and
$A\propto T_c^{-3}$, as predicted by the strong pair-breaking scenario proposed
by Gordon et al., Phys. Rev. B 81, 180501(R) (2010). The temperature dependence
of the superfluid density $\rho_s\propto\lambda_{ab}^{-2}$ presents a marked
positive curvature just below Tc which is a sign of two-gap superconductivity.
The analysis of $\rho_s(T)$ in terms of a two-gap model allowed to estimate
parameters like the in-band and inter-band couplings, the relative weight of
each band, and their dependence with the doping level. A comparison with
$\rho_s(T)$ data obtained by using other techniques in compounds with a similar
composition is also presented.

27
The categories of this paper are: comp-gas
37
The categories of this paper are: supr-con
0
The categories of this paper are: hep-ph
14
The categories of this paper are: math-ph
9
The categories of this paper are: nlin
11
The categories of this paper are: hep-lat
12
The categories of this paper are: nucl-th
10
The categories of this paper are: q-bio
24
The categories of this paper are: dg-ga
6
The categories of this paper are: astro-ph
15
The categories of this paper are: nucl-ex
1
The categories of this paper are: cs
35
The categories of this paper are: mtrl-th
7
The categories of this paper are: hep-th
31
The categories of this paper are: ao-sci
22
The categories of this paper are: chao-dyn