H. Tanaka et al., HIGH-FREQUENCY HIGH-FIELD ESR OF QUANTUM DOUBLE SPIN CHAIN SYSTEMS KCUCL3 AND TLCUCL3, Physica. B, Condensed matter, 246, 1998, pp. 545-548
ESR measurements have been performed on KCuCl3 and TlCuCl3 in the freq
uency range up to 762 GHz and field range up to 30 T at helium tempera
tures. The exchange network in the present systems is described as an
S = 1/2 Heisenberg spin ladder along the a-axis with an additional dia
gonal interaction. Both compounds have a singlet ground state with an
excitation gap. In KCuCl3, two kinds of transitions are observed betwe
en 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 wit
h a tendency toward zero at H-c approximate to 22 T. In TlCuCl3, a tra
nsition 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.