Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Delta chain with ferromagnetic and antiferromagnetic interactions at the
  critical point   We investigate the spin-1/2 Heisenberg model on the delta chain (sawtooth
chain) with ferromagnetic nearest-neighbor and antiferromagnetic next-neighbor
interactions. For a special ratio between these interactions there is a class
of exact ground states formed by localized magnons and the ground state is
macroscopically degenerate with a large residual entropy per spin
$s_0=\frac{1}{2}\ln 2$. An important feature of this model is a sharp decrease
of the gaps for excited states with an increase of the number of magnons. These
excitations give an essential contribution to the low-temperature
thermodynamics. The behavior of the considered model is compared with that of
the delta chain with both antiferromagnetic interactions.

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The categories of this paper are: dg-ga
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The categories of this paper are: solv-int
14
The categories of this paper are: math-ph
20
The categories of this paper are: acc-phys
35
The categories of this paper are: mtrl-th
32
The categories of this paper are: atom-ph
0
The categories of this paper are: hep-ph
13
The categories of this paper are: quant-ph
30
The categories of this paper are: q-alg
21
The categories of this paper are: adap-org
31
The categories of this paper are: ao-sci
36
The categories of this paper are: cmp-lg
37
The categories of this paper are: supr-con
19
The categories of this paper are: eess
9
The categories of this paper are: nlin
27
The categories of this paper are: comp-gas