Dynamics of an anisotropic spin dimer system in a strong magnetic field

Recently measured high-field ESR spectra of the spin dimer material ${\mathrm{TlCuCl}}_3$ are described within the framework of an effective field theory. A good agreement between the theory and experiment is achieved, for all geometries and in a wide field range, under the assumption of a weak anisotropy of the interdimer as well as intradimer exchange interaction.

Figure 1

Schematic picture of the anisotropy axes in ${\mathrm{TlCuCl}}_3$.

Figure 2

ESR frequency-field dependencies (Ref. 10) taken at $T=1.5\mathrm{K}$ for different field orientations, in comparison with the present theory. Solid lines show the lowest magnon gap, and the dashed ones correspond to the thermally activated transitions between three branches of the magnon triplet at $q=0$.

Figure 3

INS results of Rüegg et al. (Refs. 7, 8) taken at $T=1.5\mathrm{K}$, compared with the present theory. In this geometry [$\mathit{H}$ is perpendicular to the plane defined by $\left(010\right)$ and $\left(104\right)$ vectors] the predicted lowest gap stays below $0.09\mathrm{meV}$ at $H>H_c$, i.e., beyond the available experimental resolution.