Magnetic order and spin-waves in the quasi-1D S=1/2 antiferromagnet   ${\bf BaCu_{2}Si_{2}O_{7}}$
  Elastic and inelastic neutron scattering were used to study the ordered phase of the quasi-one-dimensional spin-1/2 antiferromagnet ${\rm BaCu_{2}Si_{2}O_{7}}$. The previously proposed model for the low-temperature magnetic structure was confirmed. Spin wave dispersion along several reciprocal-space directions was measured and inter-chain, as well as in-chain exchange constants were determined. A small gap in the spin wave spectrum was observed and attributed to magnetic anisotropy effects. The results are discussed in comparison with existing theories for weakly-coupled quantum spin chain antiferromagnets. 
M. Kenzelmann, A. Zheludev, S. Raymond, E. Ressouche, T. Masuda, P.   B\"oni, K. Kakurai, I. Tsukada, K. Uchinokura, R. Coldea