Magnetic-Field Induced Gap in One-Dimensional Antiferromagnet KCuGaF${}_6$

Magnetic susceptibility and specific heat measurements in magnetic fields were performed on an $S=1\mathrm{/}2$ one-dimensional antiferromagnet KCuGaF${}_6$. Exchange interaction was evaluated as $J\mathrm{/}{k}_{\mathrm{B}}\simeq 100$ K. However, no magnetic ordering was observed down to 0.46 K. It was found that an applied magnetic field induces a staggered magnetic susceptibility obeying the Curie law and an excitation gap, both of which should be attributed to the antisymmetric interaction of the Dzyaloshinsky-Moriya type and/or the staggered $g$-tensor. With increasing magnetic field $H$, the gap increases almost in proportion to $H^{2\mathrm{/}3}$.