Solitary magnetic excitations in the low-carrier density, one-dimensional S=1/2 antiferromagnet Yb4As3

We report low-temperature measurements of the specific heat, thermal expans ion, and thermal conductivity on the quasi-one-dimensional, effective S = 1 /2 Heisenberg antiferromagnet Yb4As3. Distinct field-induced anomalies were found in the above quantities which are well described by the classical si ne-Gordon soliton solution for an easy-plane Heisenberg antiferromagnet. Ou r findings strongly suggest that Yb4As3 represents the first example of an antiferromagnetic S = 1/2 spin-chain system where this type of nonlinear ex citation could be identified. [S0031-9007(99)09295-9].