Observation of Field-Induced Transverse N{e}el Ordering in the Spin Gap System TlCuCl$_3$

Neutron elastic scattering experiments have been performed on the spin gap system TlCuCl$_3$ in magnetic fields parallel to the $b$-axis. The magnetic Bragg peaks which indicate the field-induced N{e}el ordering were observed for magnetic field higher than the gap field $H_{rm g}approx 5.5$ T at $Q=(h, 0, l)$ with odd $l$ in the $a^*-c^*$ plane. The spin structure in the ordered phase was determined. The temperature and field dependence of the Bragg peak intensities and the phase boundary obtained were discussed in connection with a recent theory which describes the field-induced N{e}el ordering as a Bose-Einstein condensation of magnons.