Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: First- and Second-Order Transitions between Quantum and Classical
  Regimes for the Escape Rate of a Spin System   We have found a novel feature of the bistable large-spin model described by
the Hamiltonian H = -DS_z^2 - H_xS_x.The crossover from thermal to quantum
regime for the escape rate can be either first (H_x<SD/2) or second
(SD/2<H_x<2SD) order, that is, sharp or smooth, depending on the strength of
the transverse field. This prediction can be tested experimentally in molecular
magnets like Mn_12Ac.

30
The categories of this paper are: q-alg
5
The categories of this paper are: gr-qc
8
The categories of this paper are: hep-ex
20
The categories of this paper are: acc-phys
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The categories of this paper are: eess
10
The categories of this paper are: q-bio
21
The categories of this paper are: adap-org
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The categories of this paper are: supr-con
31
The categories of this paper are: ao-sci
29
The categories of this paper are: funct-an
9
The categories of this paper are: nlin
28
The categories of this paper are: alg-geom
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The categories of this paper are: bayes-an
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The categories of this paper are: nucl-th
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The categories of this paper are: stat
25
The categories of this paper are: patt-sol