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 transmittance and the optical conductivity sigma(omega) of undoped LaxCa14-xCu24O41 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 less than or equal to J(\\)/J(perpendicular to) less than or equal to 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. (C) 2002 Elsevier Science Ltd. All rights reserved.