Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Doping - dependent anisotropic superconducting gap in
  Na$_{1-\delta}$(Fe$_{1-x}$Co$_x$)As   The London penetration depth was measured in single crystals of self-doped
Na$_{1-\delta}$FeAs (from under - to optimal - doping, $T_c$ from 14 to 27 K)
and electron-doped Na(Fe$_{1-x}$Co$_{x}$)As with $x$ ranging from undoped,
$x=0$ to overdoped $x= 0.1$. In all samples, the low - temperature variation of
the penetration depth exhibits a power-law dependence, $\Delta \lambda (T) =
AT^n$, with the exponent that varies in a dome - like fashion from $n \sim 1.1$
in the underdoped, reaching a maximum of $n \sim 1.9$ in the optimally doped
and decreasing again to $n \sim 1.3$ on the overdoped side. While the
anisotropy of the gap structure follows universal dome-like evolution, the
exponent at the optimal doping, $n \sim 1.9$, is lower than in other charge -
doped Fe-based superconductors (FeSCs). The full - temperature range superfluid
density, $\rho_s(T) = (\lambda(0)/\lambda(T))^2$, at the optimal doping is also
distinctly different from other charge - doped FeSCs but is similar to
isovalently - substituted BaFe$_2$(As$_{1-x}$P$_x$)$_2$, believed to be a nodal
pnictide at the optimal doping. These results suggest that the superconducting
gap in Na(Fe$_{1-x}$Co$_{x}$)As is highly anisotropic even at the optimal
doping.

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The categories of this paper are: eess
33
The categories of this paper are: plasm-ph
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The categories of this paper are: stat
8
The categories of this paper are: hep-ex
26
The categories of this paper are: bayes-an
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The categories of this paper are: quant-ph
2
The categories of this paper are: math
0
The categories of this paper are: hep-ph
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The categories of this paper are: q-alg
31
The categories of this paper are: ao-sci
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The categories of this paper are: comp-gas
10
The categories of this paper are: q-bio
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The categories of this paper are: math-ph
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The categories of this paper are: patt-sol
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The categories of this paper are: funct-an
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The categories of this paper are: q-fin