Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Effects of biquadratic exchange on the spectrum of elementary
  excitations in spin ladders   We investigate the influence of biquadratic exchange interactions on the
low-lying excitations of a S=1/2-ladder using perturbation theory, numerical
diagonalization of finite systems and exact results for ladders with matrix
product ground states. We consider in particular the combination of biquadratic
exchange interactions corresponding to ring exchange on the basic ladder
plaquette. We find that a moderate amount of ring exchange reduces the spin gap
substantially and makes equal bilinear exchange on legs and rungs consistent
with experimentally observed spectra.

37
The categories of this paper are: supr-con
34
The categories of this paper are: chem-ph
2
The categories of this paper are: math
6
The categories of this paper are: astro-ph
8
The categories of this paper are: hep-ex
29
The categories of this paper are: funct-an
7
The categories of this paper are: hep-th
31
The categories of this paper are: ao-sci
19
The categories of this paper are: eess
27
The categories of this paper are: comp-gas
10
The categories of this paper are: q-bio
16
The categories of this paper are: stat
12
The categories of this paper are: nucl-th
30
The categories of this paper are: q-alg
13
The categories of this paper are: quant-ph
35
The categories of this paper are: mtrl-th