Experimental confirmation of spin gap in antiferromagnetic alternating   spin-3/2 chain substances RCrGeO5 (R = Y or 154Sm) by inelastic neutron   scattering experiments
  A spin-singlet ground state with a spin gap has been discovered in antiferromagnetic spin chain substances when the spin value is 1/2, 1 or 2. To find spin gap (singlet-triplet) excitations in spin-3/2 chain substances, we performed inelastic neutron scattering and magnetization measurements on {\it R}CrGeO$_5$ ({\it R} = Y or Sm) powders. As expected, we observed spin gap excitations and the dispersion relation of the lowest magnetic excitations. We proved that the spin system of Cr$^{3+}$ was an antiferromagnetic alternating spin-3/2 chain. 
M. Hase, M. Soda, T. Masuda, D. Kawana, T. Yokoo, S. Itoh, A. Matsuo,   K. Kindo, M. Kohno
cond-mat.str-el