Development of submillimeter wave ESR measurement system under high pressure

We have developed the submillimeter wave ESR measurement system under pressure up to 3.5 kbar using the transmission technique. This system has the advantages that the pulsed magnetic field up to 16 T and the wide frequency region up to 670 GHz are available. The pressure is generated by the clamped-type pressure cell whose inner parts are all made by the sapphire. We applied this system to the study of the quantum spin-gap system KCuCl/sub 3/ and obtained the direct evidence that the gap energy of this compound decreased under pressure.