Staggered Field Effect on the One-DimensionalS=12AntiferromagnetYb4As3

Inelastic neutron scattering measurements of magnetic excitations in the charge-ordered state of ${\mathrm{Yb}}_4{\mathrm{As}}_3$ have been performed under magnetic field up to about 6 T. By applying a magnetic field, the spectrum at the one-dimensional wave vector $q=1\mathrm{}$ [ $\pi /d$] changes drastically from a broad one corresponding to the spinon excitation continuum of the one-dimensional $S=\frac{1}{2}$ spin system to a sharp one at a finite energy, indicating the opening of an energy gap in the system. The magnetic field dependence of the gap is well fitted by the power law $H^{2/3}$. The experimental result gives strong evidence for the existence of a staggered field alternating along ${\mathrm{Yb}}^{3+}$ chains induced by the Dzyaloshinsky-Moriya interaction.