Role of anisotropy in the spin-dimer compound BaCuSi2O6

We present results of magnetization and electron paramagnetic resonance experiments on the spin-dimer system $\mathrm{Ba}\mathrm{Cu}{\mathrm{Si}}_{2}{\mathrm{O}}_{6}$. Evidence indicates that the origin of anisotropic terms in the spin Hamiltonian lies in magnetic dipolar interactions. Axial symmetry breaking is on a very small energy scale of $\ensuremath{\leqslant}11\phantom{\rule{0.3em}{0ex}}\mathrm{mK}$, confirming Bose-Einstein condensation critical scaling over an extended temperature range in the vicinity of the quantum critical point.