Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Limits on the Superconducting Order Parameter in NdFeAsO$_{1-x}$F$_y$
  from Scanning SQUID Microscopy   Identifying the symmetry of the superconducting order parameter in the
recently-discovered ferro-oxypnictide family of superconductors,
RFeAsO$_{1-x}$F$_{y}$, where $R$ is a rare earth, is a high priority. Many of
the proposed order parameters have internal $\pi$ phase shifts, like the d-wave
order found in the cuprates, which would result in direction-dependent phase
shifts in tunnelling. In dense polycrystalline samples, these phase shifts in
turn would result in spontaneous orbital currents and magnetization in the
superconducting state. We perform scanning SQUID microscopy on a dense
polycrystalline sample of \NdFeAsO$_{0.94}$F$_{0.06}$ with $T_c=48$ K and find
no such spontaneous currents, ruling out many of the proposed order parameters.

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The categories of this paper are: quant-ph
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The categories of this paper are: plasm-ph
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The categories of this paper are: q-alg
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The categories of this paper are: econ
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The categories of this paper are: comp-gas
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The categories of this paper are: q-fin
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The categories of this paper are: ao-sci
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The categories of this paper are: adap-org
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The categories of this paper are: chao-dyn
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The categories of this paper are: funct-an
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The categories of this paper are: patt-sol
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The categories of this paper are: stat
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The categories of this paper are: solv-int
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The categories of this paper are: supr-con
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The categories of this paper are: nucl-ex
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The categories of this paper are: math-ph