  The quantum antiferromagnet Cu2Te2O5Br2 was investigated by NMR and NQR. The
125Te NMR investigation showed that there is a magnetic transition around 10.5K
at 9T, in agreement with previous studies. From the divergence of the
spin-lattice relaxation rate, we ruled out the possibility that the transition
could be governed by a one-dimensional divergence of the spin-spin correlation
function. The observed anisotropy of the 125Te shift was shown to be due to a
spin polarization of the 5s2 "E" doublet of the [TeO3E] tetrahedra,
highlighting the importance of tellurium in the exchange paths. In the
paramagnetic state, Br NQR and NMR measurements led to the determination of the
Br hyperfine coupling and the electric field gradient tensor, and to the the
spin polarization of Br p orbitals. The results demonstrate the crucial role of
bromine in the interaction paths between Cu spins.
