Magnetic excitations in the 1/3 plateau state in InCu${}_3$(OH)${}_6$Cl${}_3$

Magnetic dynamics in InCu${}_3$(OH)${}_6$Cl${}_3$ was investigated from the NMR relaxation rate measurement. In InCu${}_3$(OH)${}_6$Cl${}_3$, the magnetization isotherm shows a plateau at the 1/3 of full-saturation magnetization, characterizing the 1/3 plateau state. As the 1/3 plateau state appears above 7 T upto 14 T, the microscopic magnetic properties were investigated with the NMR measurement in steady fields. The temperature and field dependence of $1\mathrm{/}{T}_1$ measurement reveals a gap in the magnetic excitation spectrum and its evolution with field in the 1/3 plateau state. The field dependence of spin gap provides an important information to understand the microscopic origin of 1/3 plateau state in the kagome antiferromagnets.