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 stron g asymmetric line, centered at 2 Delta(S), with a tail on the high-energy s ide. 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 antiferrom agnetic model with J(parallel to)//J(perpendicular to)similar to 0.1, and a small interladder exchange. The spin-gap and exchange constant an estimate d to be Delta(s)similar to 400 cm(-1) (570 K) and J(perpendicular to)simila r to 640 K. No magnon bound states are found. In contrast to CaV2O5 the exi stence of the spin gap is not confirmed in MgV2O5, since we found no featur e 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), presum ably related to the top of the two-magnon branch.