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 signal s at high energies from the ladders of La6Ca8Cu24O41. These signals constit ute the magnon branch (similar to 30 less than or equal to E less than or e qual to similar to 190 meV) for the lowest triplet excitation. A fit to a t heory including a coupling along the leg (J(leg)) and rung (J(rung)) alone gives the value J(leg)/J(rung) similar to 2 that is not consistent with exp ectations from a geometrical consideration and the exact diagonalization ca lculation. This inconsistency is removed when our data are fitted with J(le g) = J(rung) = 110 meV and a four-spin exchange interaction, J(ring) = 16.5 meV.