Incommensurate dynamic correlations in the quasi-two-dimensional spin liquid BiCu₂PO₆

We report detailed inelastic neutron-scattering measurements on single crystals of the frustrated two-leg ladder BiCu₂PO₆, whose ground state is described as a spin liquid phase with no long-range order down to 6 K. Two branches of steeply dispersing long-lived spin excitations are observed with excitation gaps of Δ₁=1.90(9) meV and Δ₂=3.95(8) meV. Significant frustrating next-nearest-neighbor interactions along the ladder leg drive the minimum of each excitation branch to incommensurate wave vectors ζ₁=0.574π and ζ₂=0.553π for the lower and upper energy branches, respectively. The temperature dependence of the excitation spectrum near the gap energy is consistent with thermal activation into singly and doubly degenerate excited states. The observed magnetic excitation spectrum as well as earlier thermodynamic data could be consistently explained by the presence of strong anisotropic interactions in the ground-state Hamiltonian.