Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Effective actions for spin ladders   We derive a path-integral expression for the effective action in the
continuum limit of an AFM Heisenberg spin ladder with an arbitrary number of
legs. The map is onto an $O(3)$ nonlinear $\sigma$-model (NL$\sigma$M) with the
addition of a topological term that is effective only for odd-legged ladders
and half-odd integer spins. We derive the parameters of the effective
NL$\sigma$M and the behaviour of the spin gap for the case of even-legged
ladders.

5
The categories of this paper are: gr-qc
28
The categories of this paper are: alg-geom
36
The categories of this paper are: cmp-lg
9
The categories of this paper are: nlin
29
The categories of this paper are: funct-an
8
The categories of this paper are: hep-ex
18
The categories of this paper are: econ
34
The categories of this paper are: chem-ph
17
The categories of this paper are: q-fin
37
The categories of this paper are: supr-con
30
The categories of this paper are: q-alg
22
The categories of this paper are: chao-dyn
6
The categories of this paper are: astro-ph
13
The categories of this paper are: quant-ph
27
The categories of this paper are: comp-gas
19
The categories of this paper are: eess