Raman scattering from magnetic excitations in the spin-ladder compounds CaV2O5 and MgV2O5

We present the Raman-scattering spectra of CaV2O5 and MgV2O5. The magnetic contribution in the Raman spectra of CaV2O5 is found in the form of a strong asymmetric line, centered at 2 Delta(S), with a tail on the high-energy side. Our analysis of its spectral shape shows that the magnetic ordering in CaV2O5 can be described using a S=1/2 two-leg ladder Heisenberg antiferromagnetic model with J(parallel to)//J(perpendicular to)similar to 0.1, and a small interladder exchange. The spin-gap and exchange constant an estimated to be Delta(s)similar to 400 cm(-1) (570 K) and J(perpendicular to)similar to 640 K. No magnon bound states are found. In contrast to CaV2O5 the existence of the spin gap is not confirmed in MgV2O5, since we found no feature in the spectra which could be associated with the onset of the two-magnon continuum. Instead, we observe two-magnon excitation at 340 cm(-1), presumably related to the top of the two-magnon branch.