Thermal conductivity of pure and Mg-doped CuGeO_3 in the incommensurate phase
The thermal conductivity κof Cu_1-xMg_xGeO_3 is measured in magnetic fields up to 16 T. At the transition field H_c to the high-field incommensurate (I) phase, κabruptly decreases. While κof the pure CuGeO_3 is enhanced with the application of higher magnetic fields, an anomalous plateau feature shows up in the κ(H) profile in the I phase of the Mg-doped samples which includes antiferromagnetic ordering (I-AF phase). With the help of specific heat data, taken supplementally for the identical samples, the above features are well understood that phonons are strongly scattered by spin solitons and that κin the I phase is governed by their spatial distribution. In particular, the plateau feature in the doped samples suggests "freezing" of the solitons, in which antiferromagnetism must be playing an essential role because this feature is absent at temperatures above T_N.