Magnetic excitations in two-leg spin 1/2 ladders: experiment and theory

Abstract 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 σ ( ω ) 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 ∥ / J ⊥ ≤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.