Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Tuning the spin Hamiltonian of NENP by external pressure: a neutron
  scattering study   We report an inelastic neutron scattering study of antiferromagnetic spin
dynamics in the Haldane chain compound Ni(C2H8N2)2NO2ClO4 (NENP) under external
hydrostatic pressure P = 2.5 GPa. At ambient pressure, the magnetic excitations
in NENP are dominated by a long-lived triplet mode with a gap which is split by
orthorhombic crystalline anisotropy into a lower doublet centered at
$\Delta_\perp\approx$ 1.2meV and a singlet at $\Delta_\parallel\approx$ 2.5meV.
With pressure we observe appreciable shifts in these levels, which move to
$\Delta_\perp{(2.5GPa)}\approx$ 1.45 meV and $\Delta_\parallel(2.5GPa)\approx$
2.2meV. The dispersion of these modes in the crystalline c-direction
perpendicular to the chain was measured here for the first time, and can be
accounted for by an interchain exchange J'_c approximately 3e-4*J which changes
only slightly with pressure. Since the average gap value $\Delta_H\approx$ 1.64
meV remains almost unchanged with P, we conclude that in NENP the application
of external pressure does not affect the intrachain coupling J appreciably, but
does produce a significant decrease of the single-ion anisotropy constant from
D/J = 0.16(2) at ambient pressure to D/J = 0.09(7) at P = 2.5 GPa.

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The categories of this paper are: atom-ph
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The categories of this paper are: acc-phys
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The categories of this paper are: gr-qc
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The categories of this paper are: dg-ga
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The categories of this paper are: stat
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The categories of this paper are: chao-dyn
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The categories of this paper are: cmp-lg
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The categories of this paper are: alg-geom
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The categories of this paper are: hep-ph
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The categories of this paper are: q-alg
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The categories of this paper are: patt-sol
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The categories of this paper are: adap-org
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The categories of this paper are: quant-ph
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The categories of this paper are: mtrl-th
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The categories of this paper are: plasm-ph
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The categories of this paper are: hep-ex