Magnon and soliton excitations in the carrier-poor, one-dimensional S=1/2 antiferromagnet Yb4As3

The semimetallic quasi-one-dimensional S = 1/2 Heisenberg antiferromagnet Y b4As3 was studied by low-temperature measurements of the specific heat C(T, B), thermal expansion alpha(T, B), and thermal conductivity kappa(T, B). A t finite magnetic fields (B less than or equal to 12 T) we observed the fol lowing distinct anomalies: (1) the magnon contribution to C(T, 0), gamma T, with large coefficient gamma approximate to 200 mJ/(K-2 mol), becomes stro ngly reduced with field, and (2) a broad hump in C(T, B = const) is induced at slightly higher temperatures. (3) The latter corresponds to a pronounce d peak in alpha(T, B = const) as well as (4) to a broad minimum in kappa(T, B = const)/kappa(T, 0). These anomalies are well described by the classica l sine-Gordon solution of a. one-dimensional Heisenberg antiferromagnet wit h a weak easy-plane anisotropy. However, the soliton-rest energy deduced fr om the experimental results depends on the magnetic field like E-S similar to B-nu, with an exponent nu approximate to 0.66, while the classical sine- Gordon model requires nu = 1. Thus, our results suggest an alternative desc ription of soliton excitations in an antiferromagnetic S = 1/2 Heisenberg c hain in terms of the quantum sine-Gordon model, for which an exponent nu = 2/3 is appropriate.