Optical spectroscopy of ( La , Ca ) 14 Cu 24 O 41 spin ladders : comparison of experiment and theory

Transmission and reflectivity of La x Ca 14−x Cu 24 O 41 two-leg spin-1/2 ladders were measured in the mid-infrared regime between 500 and 12 000 cm −1. This allows us to determine the optical conductivity σ 1 directly and with high sensitivity. Here we show data for x=4 and 5 with the electrical field polarized parallel to the rungs (E || a) and to the legs (E || c). Three characteristic peaks are identified as magnetic excitations by comparison with two different theoretical calculations. The quantum nature of magnetic excitations in spin-1/2 systems and in particular the role of quantum fluctuations in low dimensions are a fascinating subject. Antiferromagnetic (AF) S=1/2 Heisenberg ladders represent an intermediate step between one-dimensional (1D) chains and the 2D CuO 2 layers of undoped high-T c superconductors. The elementary excitations of the ladders can be described as triplets or as interacting spinons. Topics of current interest are theoretical predictions of 2-triplet bound states [1], the size of the exchange coupling along the rungs (J ⊥) and the legs (J) as well as the role of the ring exchange J cyc [2]. We address these issues in La x Ca 14−x Cu 24 O 41 which contains layers with Cu 2 O 3 two-leg AF S=1/2 ladders [3].