Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Magnetic excitations in two-leg spin 1/2 ladders: experiment and theory   Magnetic excitations in two-leg S=1/2 ladders are studied both experimentally
and theoretically. Experimentally, we report on the reflectivity, the
transmission and the optical conductivity sigma(omega) of undoped La_x Ca_14-x
Cu_24 O_41 for x=4, 5, and 5.2. Using two different theoretical approaches
(Jordan-Wigner fermions and perturbation theory), we calculate the dispersion
of the elementary triplets, the optical conductivity and the momentum-resolved
spectral density of two-triplet excitations for 0.2 <=
J_parallel/J_perpendicular <= 1.2. We discuss phonon-assisted two-triplet
absorption, the existence of two-triplet bound states, the two-triplet
continuum, and the size of the exchange parameters.

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The categories of this paper are: hep-th
3
The categories of this paper are: physics
1
The categories of this paper are: cs
14
The categories of this paper are: math-ph
8
The categories of this paper are: hep-ex
17
The categories of this paper are: q-fin
31
The categories of this paper are: ao-sci
24
The categories of this paper are: dg-ga
25
The categories of this paper are: patt-sol
16
The categories of this paper are: stat
15
The categories of this paper are: nucl-ex
18
The categories of this paper are: econ
0
The categories of this paper are: hep-ph
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The categories of this paper are: hep-lat
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The categories of this paper are: adap-org
36
The categories of this paper are: cmp-lg