Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Low-energy singlet and triplet excitations in the spin-liquid phase of
  the two-dimensional J1-J2 model   We analyze the stability of the spontaneously dimerized spin-liquid phase of
the frustrated Heisenberg antiferromagnet - the J1-J2 model. The lowest triplet
excitation, corresponding to breaking of a singlet bond, is found to be stable
in the region 0.38 < J2/J1 < 0.62. In addition we find a stable low-energy
collective singlet mode, which is closely related to the spontaneous violation
of the discrete symmetry. Both modes are gapped in the quantum disordered phase
and become gapless at the transition point to the Neel ordered phase
(J2/J1=0.38). The spontaneous dimerization vanishes at the transition and we
argue that the disappearance of dimer order is related to the vanishing of the
singlet gap. We also present exact diagonalization data on a small (4x4)
cluster which indeed show a structure of the spectrum, consistent with that of
a system with a four-fold degenerate (spontaneously dimerized) ground state.

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The categories of this paper are: funct-an
20
The categories of this paper are: acc-phys
35
The categories of this paper are: mtrl-th
3
The categories of this paper are: physics
9
The categories of this paper are: nlin
18
The categories of this paper are: econ
6
The categories of this paper are: astro-ph
13
The categories of this paper are: quant-ph
25
The categories of this paper are: patt-sol
21
The categories of this paper are: adap-org
10
The categories of this paper are: q-bio
23
The categories of this paper are: solv-int
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The categories of this paper are: stat
31
The categories of this paper are: ao-sci
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The categories of this paper are: chao-dyn
14
The categories of this paper are: math-ph