ESR Study of the Opening and Closing of the Field-Induced Gap inNH4CuCl3

ESR measurements in the double-chain compound ${\mathrm{NH}}_4{\mathrm{CuCl}}_3$, which has magnetization plateaus at one-quarter and three-quarters of the saturation magnetization, have been performed in magnetic fields up to 30 T in the frequency range $\sim 95–762\mathrm{GHz}$. It is found that the frequency versus field diagrams for $H\parallel a$ and $H\parallel b$ coincide when normalized by the $g$ factors. In each plateau region, a pair of excitations expressed as $\omega =\gamma \left({H-H}_{\mathrm{lc}}\right)$ and $\omega =\gamma (H_{\mathrm{hc}}-H)$ are observed, where $H_{\mathrm{lc}}$ and $H_{\mathrm{hc}}$ are the lower and higher edge fields of the plateau, respectively. These two modes correspond to the quantum gap in the lower- and higher-field halves in the plateau region.