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 (similar to 30 less than or equal to E less than or equal to similar to 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) similar to 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.