Polarization dependence of spin excitations in BaCu2Si2O7
  The polarization dependence of magnetic excitations in the quasi one-dimensional antiferromagnet BaCu2Si2O7 is studied as a function of momentum and energy transfer. The results of inelastic neutron scattering measurements are directly compared to semi-analytical calculations based on the chain-Mean Field and Random Phase approximations. A quantitative agreement between theoretically calculated and experimentally measured dynamic structure factors of transverse spin fluctuations is obtained. In contrast, substantial discrepancies are found for longitudinal polarization. This behavior is attributed to intrinsic limitations of the RPA that ignores correlation effects. 
A. Zheludev, S. Raymond, L.-P. Regnault, F.H.L. Essler, K. Kakurai, T.   Masuda, K. Uchinokura