Bound spinon excitations in the spin-1/2 anisotropic triangular antiferromagnet Ca3ReO5Cl2

The spin excitations of the S = 1/2 anisotropic triangular antiferromagnet Ca3ReO5Cl2 were investigated by inelastic neutron-scattering experiments. The spin excitation spectrum exhibits sharp dispersive modes in addition to a spinonlike continuum. The sharp modes exhibit large and small dispersions along the intrachain (K) and interchain directions (L), respectively, reflecting a one-dimensional character modified by interchain interactions. The dispersion intensity along the interchain direction is enhanced at the wave vectors around (K, L) = (0.25, +/- 1.0) and (0.75, 0.0) (in reciprocal lattice units, r.l.u.). This enhancement is well explained by the formation of bound spinon pairs. These features are reminiscent of the spin excitations observed in the prototypical compound Cs2CuCl4 [Coldea, et al., Phys. Rev. Lett. 86, 1335 (2001); Phys. Rev. B 68, 134424 (2003)]. The consistency with the simulated spectrum based on the random-phase approximation is better for Ca3ReO5Cl2 than for Cs2CuCl4, indicating that the spin system in the former remains closer to a Tomonaga-Luttinger liquidlike disordered state. In addition, the low-energy modes exhibit a wave-vector shift along the +/- K direction, suggesting that uniform intrachain Dzyaloshinskii-Moriya interactions are more relevant in Ca3ReO5Cl2.