Solitary Magnetic Excitations in the Low-Carrier Density, One-DimensionalS=12AntiferromagnetYb4As3

We report low-temperature measurements of the specific heat, thermal expansion, and thermal conductivity on the quasi-one-dimensional, effective $S=\frac{1}{2}$ Heisenberg antiferromagnet ${\mathrm{Yb}}_4{\mathrm{As}}_3$. Distinct field-induced anomalies were found in the above quantities which are well described by the classical sine-Gordon soliton solution for an easy-plane Heisenberg antiferromagnet. Our findings strongly suggest that ${\mathrm{Yb}}_4{\mathrm{As}}_3$ represents the first example of an antiferromagnetic $S=\frac{1}{2}$ spin-chain system where this type of nonlinear excitation could be identified.