Nontrivial triplet band-topology in the spin gap materials, KCuCl₃ and TlCuCl₃ (Part 1)

We investigate the spin gap system, XCuCl₃ (X=K, Tl), where the magnetic copper ions form a 3-dimensional lattice of two inequivalent dimers. The dominant antiferromagnetic intra-dimer interaction leads to a dimer-singlet ground state. Inelastic neutron scattering[1] and electron spin resonance measurements[2] revealed dispersive triplet excitations. The spectrum has been studied using isotropic Heisenberg models, which include further neighbor exchange paths.

We construct the symmetry-allowed anisotropies and explore their effects on the triplet band-topology. Inversion symmetry excludes the intra-dimer antisymmetric exchange anisotropy preventing the mixing of singlets and triplets in the ground state. This leads to pure triplet excitations limiting the variety of nontrivial band-topology, rendering the Zeeman gaps between the S_z=1, 0, and -1 bands trivial. Nonetheless, inter-dimer anisotropies lead to nontrivial topology within the S_z subspaces formed by the triplets of the two different dimers. We investigate the originating surface triplet modes and their magneto-electric effects.

[1] T. Kato et al, J. Phys. Soc. Jpn. 67, pp. 752-754 (1998), N. Cavadini et al, Physica B 276-278, 540 542 (2000), N. Cavadini et al, Phys. Rev. B 63, 172414 (2001), A. Oosawa et al, Phys. Rev. B 65, 094426 (2002)

[2] H.Tanaka et al Physica B 246–247, 545(1998), M. Motokawa et al J. Phys. Soc. Jpn. 72, 1-11 (2003), S. Kimura et al Physica B 346–347, 15–18 (2004).