Magnetic excitations and exchange interactions in the spin-1/2 two-leg ladder compound La₆Ca₈Cu₂₄O₄₁

Using a pulsed neutron source we have been able to observe inelastic signals at high energies from the ladders of La₆Ca₈Cu₂₄O₄₁. These signals constitute the magnon branch (∼ 30≤E≤ ∼ 190 meV) for the lowest triplet excitation. A fit to a theory including a coupling along the leg (J_leg) and rung (J_rung) alone gives the value J_leg/J_rung ∼ 2 that is not consistent with expectations from a geometrical consideration and the exact diagonalization calculation. This inconsistency is removed when our data are fitted with J_leg = J_rung = 110 meV and a four-spin exchange interaction, J_ring = 16.5 meV.