High-frequency high-field ESR of quantum double spin chain systems KCuCl3 and TlCuCl3

ESR measurements have been performed on KCuCl3 and TlCuCl3 in the frequency range up to 762 GHz and field range up to 30 T at helium temperatures. The exchange network in the present systems is described as an S = 1/2 Heisenberg spin ladder along the a-axis with an additional diagonal interaction. Both compounds have a singlet ground state with an excitation gap. In KCuCl3, two kinds of transitions are observed between the ground state and the excited triplet states. They correspond to the excitations at Q = (1, 0, 1) and (0.5, 0, 1) with the excitation energies Delta approximate to 640 and 1080 GHz, respectively. When the external field is increased, the frequency of the lowest branch with the zero-field gap Delta approximate to 640 GHz decreases linearly with a tendency toward zero at H-c approximate to 22 T. In TlCuCl3, a transition from the singlet ground state to the excited triplet state is observed. The zero-field gap is determined as Delta approximate to 160 GHz. (C) 1998 Elsevier Science B.V. All rights reserved.