Temperature-dependent spin gap and singlet ground state inBaCuSi2O6

Bulk magnetic measurements and inelastic neutron scattering were used to investigate the magnetic properties of ${\mathrm{BaCuSi}}_2$ ${\mathrm{O}}_6$, a quasi-two-dimensional antiferromagnet with a square-lattice bilayer structure. The compound was shown to have a spin-singlet dimerized ground state with a gap Δ≈4.5 meV in the magnetic excitation spectrum. The dispersion of the gap modes was measured and found to be temperature dependent and totally suppressed at temperatures higher than the characteristic gap energy. The observed excitations are analogous to magnetoexcitons in light rare-earth compounds, but are an intrinsic property of a simple Heisenberg Hamiltonian for the S=1/2 magnetic bilayer. The behavior is discussed in comparison to that of related systems, in particular, ${\mathrm{Cs}}_3$ ${\mathrm{Cr}}_2$ ${\mathrm{Br}}_9$.