Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: When can localized spins interacting with conduction electrons in ferro-
  or antiferromagnets be described classically via the Landau-Lifshitz
  equation: Transition from quantum many-body entangled to quantum-classical
  nonequilibrium states   Experiments in spintronics and magnonics operate with macroscopically large
number of localized spins within ferromagnetic (F) or antiferromagnetic (AF)
materials, so that their nonequilibrium dynamics is standardly described by the
Landau-Lifshitz (LL) equation treating localized spins as classical vectors of
fixed length. However, spin is a genuine quantum degree of freedom, and even
though quantum effects become progressively less important for spin value
$S>1$, they exist for all $S < \infty$. While this has motivated exploration of
limitations/breakdown of the LL equation, by using examples of F insulators,
analogous comparison of fully quantum many-body vs. quantum (for
electrons)-classical (for localized spins) dynamics in systems where
nonequilibrium conduction electrons are present is lacking. Here we employ
quantum Heisenberg F or AF chains of $N=4$ sites, whose localized spins
interact with conduction electrons via $sd$ exchange interaction, to perform
such comparison by starting from unentangled pure (at zero temperature) or
mixed (at finite temperature) quantum state of localized spins as the initial
condition. This reveals that quantum-classical dynamics can faithfully
reproduce fully quantum dynamics in the F metallic case, but only when spin
$S$, Heisenberg exchange between localized spins and $sd$ exchange are
sufficiently small. Increasing any of these three parameters can lead to
substantial deviations, which are explained by the dynamical buildup of
entanglement between localized spins and/or between them and electrons. In the
AF metallic case, substantial deviations appear even at early times, despite
starting from unentangled N\'{e}el state, which therefore poses a challenge on
how to rigorously justify wide usage of the LL equation in phenomenological
modeling of antiferromagnetic spintronics experiments.

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The categories of this paper are: hep-ex
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The categories of this paper are: funct-an
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The categories of this paper are: nucl-ex
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The categories of this paper are: chao-dyn
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The categories of this paper are: alg-geom
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The categories of this paper are: dg-ga
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The categories of this paper are: astro-ph
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The categories of this paper are: hep-lat
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The categories of this paper are: chem-ph
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The categories of this paper are: atom-ph
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The categories of this paper are: nucl-th
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The categories of this paper are: cmp-lg
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The categories of this paper are: hep-ph
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The categories of this paper are: q-fin
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The categories of this paper are: econ
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The categories of this paper are: bayes-an