Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Competition between superconductivity and magnetic/nematic order as a
  source of anisotropic superconducting gap in underdoped
  Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$   The in-plane London penetration depth, $\Delta\lambda(T)$, was measured using
a tunnel diode resonator technique in single crystals of
Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ with doping levels $x$ ranging from heavily
underdoped, $x$=0.16 ($T_{c}$=7~K) to nearly optimally doped, $x$= 0.34
($T_{c}=$39 K). Exponential saturation of $\Delta\lambda(T)$ in the $T\to0$
limit is found in optimally doped samples, with the superfluid density
$\rho_{s}(T)\equiv(\lambda(0)/\lambda(T))^{2}$ quantitatively described by a
self-consistent $\gamma$-model with two nodeless isotropic superconducting
gaps. As the doping level is decreased towards the extreme end of the
superconducting dome at $x$=0.16, the low-temperature behavior of
$\Delta\lambda(T)$ becomes non-exponential and best described by the power-law
$\Delta\lambda(T)\propto T^{2}$, characteristic of strongly anisotropic gaps.
The change between the two regimes happens within the range of coexisting
magnetic/nematic order and superconductivity, $x<0.25$, and is accompanied by a
rapid rise in the absolute value of $\Delta\lambda(T)$ with underdoping. This
effect, characteristic of the competition between superconductivity and other
ordered states, is very similar to but of significantly smaller magnitude than
what is observed in the electron-doped Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$
compounds. Our study suggests that the competition between superconductivity
and magnetic/nematic order in hole-doped compounds is weaker than in
electron-doped compounds, and that the anisotropy of the superconducting state
in the underdoped iron pnictides is a consequence of the anisotropic changes in
the pairing interaction and in the gap function promoted by both magnetic and
nematic long-range order.

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The categories of this paper are: plasm-ph
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The categories of this paper are: acc-phys
3
The categories of this paper are: physics
9
The categories of this paper are: nlin
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The categories of this paper are: chem-ph
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The categories of this paper are: chao-dyn
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The categories of this paper are: gr-qc
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The categories of this paper are: alg-geom
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The categories of this paper are: funct-an
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The categories of this paper are: supr-con
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The categories of this paper are: hep-lat
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The categories of this paper are: nucl-ex
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The categories of this paper are: astro-ph
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The categories of this paper are: cs
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The categories of this paper are: bayes-an
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The categories of this paper are: dg-ga