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 Δ ≈ 640 and 1080 GHz, respectively. When the external field is increased, the frequency of the lowest branch with the zero-field gap Δ ≈ 640 GHz decreases linearly with a tendency toward zero at H_c ≈ 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 Δ ≈ 160 GHz.