Evidence for magnons and solitons in the one-dimensional S = 1/2 antiferromagnet Yb₄As₃

By means of low-temperature measurements of the specific heat, C, and thermal expansion, α = 1^-1∂l/∂T, the magnetic properties of the quasi-one-dimensional (quasi-1D), S = 1/2 Heisenberg antiferromagnet Yb₄As₃ were investigated. In finite magnetic fields distinct anomalies were found which are pronounced differently strongly in both quantities: (i) the in-T linear contribution dominating the low-T specific heat becomes reduced with field and (ii) a peak is induced at slightly higher temperatures. The latter feature being much more pronounced in α(T, B) is well described by the classical sine-Gordon (SG) soliton solution of a ID Heisenberg antiferromagnet with a weak easy-plane anisotropy.