Low Energy Excitations of Yb4As3 in a Magnetic Field

We discuss the effects of an applied magnetic field on the low energy excitations in the low temperature phase of Yb4As3. We show also why the magnetic interaction of the Yb{3+} ions is nearly of an isotropic Heisenberg spin-1/2 type. A small anisotropy due to an intrachain dipolar interaction leads to the opening of a gap when a magnetic field is applied. The model agrees with available experimental data. Simple experiments are suggested in order to further test the present theory.