Complex 2D Oxide BaCuSi$_2$O$_6$: A NMR Study*

BaCuSi$_2$O$_6$ is a quasi-2D oxide composed of Cu$_2$Si$_4$O$_{12}$ layers where Cu$^{2+}$ ions are arranged in well-separated dimers perpendicular to 2D layers. It has a singlet ground state in zero magnetic field, with a large gap to the lowest excited triplet states. Magnetic fields in excess of $H_{c1}$ $\sim$ 23.5 T close the gap, cooling in $H \leq H_{c1}$ results in a state characterized by long-range magnetic order, the nature of which has not been determined yet. We present nuclear magnetic resonance (NMR) measurements of $^{29}$Si and $^{63,65}$Cu on single crystals of BaCuSi$_2$O$_6$ below as well as above $H_{c1}$. Our results prove that the system is less symmetric and more complicated than initially supposed. In the ``normal'' phase we confirmed an IC character of the phase below 100 K. Unexpectedly, two copper sites having strongly different spin polarizations have been observed, which can be associated by the presence of two different gaps and J (J$_1$ and J$_2$) values in the system. Analysis of these data provides a quantitative measure for the size of the perturbation of the ``ideal'' Hamiltonian, helping to decide whether a BEC-type description is still possible.