Spin pseudogap in the $S=\frac{1}{2}$ chain material Sr$_2$CuO$_3$ with   impurities
  The low energy magnetic excitation spectrum of the Heisenberg antiferromagnetic $S = 1/2$ chain system Sr$_2$CuO$_3$ with Ni- and Ca-impurities is studied by neutron spectroscopy. In all cases, a defect-induced spectral pseudogap is observed and shown to scale proportionately to the number of scattering centers in the spin chains. 
G. Simutis, S. Gvasaliya, N. S. Beesetty, T. Yoshida, J. Robert, S.   Petit, A. I. Kolesnikov, M. B. Stone, F. Bourdarot, H....
cond-mat.str-el