Optical spectroscopy of (La,Ca)14Cu24O41 spin ladders: comparison of experiment and theory
M. Grueninger, M. Windt, T. Nunner, C. Knetter, K.P. Schmidt, G.S. Uhrig, T. Kopp, A. Freimuth, U. Ammerahl, B. Buechner, A. Revcolevschi
Abstract
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 12000 1/cm. This allows us to determine the optical conductivity sigma_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.