Optical spectroscopy of (La,Ca)14Cu24O41 spin ladders: comparison of experiment and theory

Abstract Transmission and reflectivity of LaxCa14−xCu24O41 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 σ1 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.