PUB - Publikationen an der Universität Bielefeld

We consider the strongly anisotropic spin-1/2XXZmodel on the sawtooth-chain lattice with ferromagnetic longitudinal interactionJ(zz)= Delta Jand aniferromagnetic transversal interactionJ(xx)=J(yy)=J> 0. At Delta = -1/2 the lowest one-magnon excitation band is dispersionless (flat) leading to a massively degenerate set of ground states. Interestingly, this model admits a three-coloring representation of the ground-state manifold [H.J. Changlani et al., Phys. Rev. Lett.120, 117202 (2018)]. We characterize this ground-state manifold and elaborate the low-temperature thermodynamics of the system. We illustrate the manifestation of the flat-band physics of the anisotropic model by comparison with two isotropic flat-band Heisenberg sawtooth chains. Our analytical consideration is complemented by exact diagonalization and finite-temperature Lanczos method calculations.