Evidence for an unconventional magnetic instability in the spin-tetrahedrasystem Cu2Te2O5Br2 - art. no. 227201

Thermodynamic experiments as well as Raman scattering have been used to stu dy the magnetic instabilities in the spin-tetrahedra systems Cu2Te2O5X2, X = Cl and Br. While the phase transition observed in the Cl system at T-N = 18.2 K is consistent with 3D antiferromagnetic ordering, the phase transiti on at T-o = 11.4 K in the Br system has several unusual features. We propos e an explanation in terms of weakly coupled tetrahedra with a singlet-tripl et gap and low-lying singlets.