Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Optical dynamic nuclear polarization of $^{13}$C spins in diamond at a
  low field with multi-tone microwave irradiation   Most of dynamic nuclear polarization (DNP) has been requiring helium
cryogenics and strong magnetic fields for a high degree of polarization. In
this work, we instead demonstrate an optical hyperpolarization of naturally
abundant $^{13}$C nuclei in a diamond crystal at a low magnetic field and an
ambient temperature. It exploits continuous irradiations of pump laser for
polarizing electron spins of nitrogen vacancy centers and microwave for
transferring the induced polarization to $^{13}$C nuclear spins. Triplet
structures corresponding to $^{14}$N hyperfine splitting were clearly observed
in the spectrum of $^{13}$C polarization. The powers of microwave irradiation
and pump laser were optimized. By simultaneously irradiating three microwave
frequencies matching to the peaks of the triplet, we achieved a $^{13}$C bulk
polarization of 0.113 %, leading to an enhancement of about a factor of 90,000
over the thermal polarization at 17.6 mT. We believe that the multi-tone
irradiation can be universally adopted to further enhance the $^{13}$C
polarization at a low magnetic field.

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The categories of this paper are: plasm-ph
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The categories of this paper are: ao-sci
29
The categories of this paper are: funct-an
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The categories of this paper are: atom-ph
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The categories of this paper are: chem-ph
21
The categories of this paper are: adap-org
24
The categories of this paper are: dg-ga
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The categories of this paper are: q-alg
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The categories of this paper are: q-fin
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The categories of this paper are: supr-con
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The categories of this paper are: patt-sol
4
The categories of this paper are: cond-mat
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The categories of this paper are: mtrl-th
6
The categories of this paper are: astro-ph
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The categories of this paper are: alg-geom
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The categories of this paper are: math-ph