Infrared spectroscopy of tetrahedral quantum spin systems Cu2Te2O5X2 (X=Br and Cl) A. Perucchi, L. Degiorgi, H. Berger and P. Millet Eur. Phys. J. B, 38 1 (2004) 65-70 Published online: 20 April 2004 DOI: 10.1140/epjb/e2004-00100-7