Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Elementary Excitations of Ferromagnetic Metal Nanoparticles   We present a theory of the elementary spin excitations in transition metal
ferromagnet nanoparticles which achieves a unified and consistent quantum
description of both collective and quasiparticle physics. The theory starts by
recognizing the essential role played by spin-orbit interactions in determining
the energies of ferromagnetic resonances in the collective excitation spectrum
and the strength of their coupling to low-energy particle-hole excitations. We
argue that a crossover between Landau-damped ferromagnetic resonance and
pure-state collective magnetic excitations occurs as the number of atoms in
typical transition metal ferromagnet nanoparticles drops below approximately
$10^4$, approximately where the single-particle level spacing, $\delta$,
becomes larger than, $\sqrt{\alpha} E_{\rm res}$, where $E_{\rm res}$ is the
ferromagnetic resonance frequency and $\alpha$ is the Gilbert damping
parameter. We illustrate our ideas by studying the properties of semi-realistic
model Hamiltonians, which we solve numerically for nanoparticles containing
several hundred atoms. For small nanoparticles, we find one isolated
ferromagnetic resonance collective mode below the lowest particle-hole
excitation energy, at $E_{\rm res} \approx 0.1$ meV. The spectral weight of
this pure excitation nearly exhausts the transverse dynamical susceptibility
spectral weight. As $\delta$ approaches $\sqrt{\alpha} E_{\rm res}$, the
ferromagnetic collective excitation is more likely to couple strongly with
discrete particle-hole excitations. In this regime the distinction between the
two types of excitations blurs. We discuss the significance of this picture for
the interpretation of recent single-electron tunneling experiments.

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The categories of this paper are: math
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The categories of this paper are: solv-int
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The categories of this paper are: eess
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The categories of this paper are: q-bio
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The categories of this paper are: hep-ph
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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: atom-ph
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The categories of this paper are: quant-ph
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The categories of this paper are: hep-th
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The categories of this paper are: chem-ph
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The categories of this paper are: gr-qc
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The categories of this paper are: nucl-ex
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The categories of this paper are: nucl-th
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The categories of this paper are: hep-lat
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The categories of this paper are: plasm-ph