Spin-triplet excitons in the $S=\frac{1}{2}$ gapped antiferromagnet $\mathrm{Ba}\mathrm{Cu}{\mathrm{Si}}_2{\mathrm{O}}_6$: Electron paramagnetic resonance studies

$\mathrm{Ba}\mathrm{Cu}{\mathrm{Si}}_2{\mathrm{O}}_6$, a $S=\frac{1}{2}$ quantum antiferromagnet with a double-layer structure of ${\mathrm{Cu}}^{2+}$ ions in a distorted planar-rectangular coordination and with a dimerized spin singlet ground state, has been studied by means of the electron paramagnetic resonance technique. It is argued that multiple absorptions observed at low temperatures are intimately related to a thermally activated spin-triplet exciton superstructure, which appears to be a characteristic feature of low-dimensional anisotropic spin-$\frac{1}{2}$ systems with the dimerized spin-singlet ground state. We showed that analysis of the angular dependence of exciton modes could be used for accurate estimation of anisotropy parameters in $\mathrm{Ba}\mathrm{Cu}{\mathrm{Si}}_2{\mathrm{O}}_6$; the procedure can be applied for a large number of $S=\frac{1}{2}$ quantum antiferromagnets. In addition, the temperature dependence of EPR intensity and linewidth in $\mathrm{Ba}\mathrm{Cu}{\mathrm{Si}}_2{\mathrm{O}}_6$ has been studied and discussed.

Figure 1

(Color online) Schematic views of the room-temperature crystal structure of $\mathrm{Ba}\mathrm{Cu}{\mathrm{Si}}_2{\mathrm{O}}_6$, composed of $\mathrm{Si}{\mathrm{O}}_4$ tetrahedra (Si atoms are located in tetrahedron centers), $\mathrm{Cu}{\mathrm{O}}_4$ distorted squares, and Ba atoms. The ${\mathrm{Cu}}^{2+}$ ions in the ${\mathrm{Cu}}_2{\mathrm{Si}}_4{\mathrm{O}}_{12}$ layers form a quasi-two-dimensional system of weakly interacting dimers.

Figure 2

Room-temperature angular dependence of the $g$ factor and the peak-to-peak EPR linewidth in $\mathrm{Ba}\mathrm{Cu}{\mathrm{Si}}_2{\mathrm{O}}_6$, $\Delta B$, in the $ac$ plane (close squares and circles, respectively), and in the $ab$ plane (open squares and circles, respectively). The line is a fit of the angular dependence of the EPR linewidth in the $ac$ plane.

Figure 3

The evolution of the low-temperature EPR spectra of $\mathrm{Ba}\mathrm{Cu}{\mathrm{Si}}_2{\mathrm{O}}_6$; the magnetic field is applied along the $a$ axis. Note that points at lower temperatures are taken with an increased amplification rate. The lines A, B1 and B2 correspond to spin-triplet excitons. The hyperfine structure C1-4 is related to interactions within unpaired doublet ${\mathrm{Cu}}^{2+}$ sites, caused by defects. The line F (Fig. 4) is outside the shown field range. The inset shows a schematic view of the energy-level diagram for dimerized $S=\frac{1}{2}$ systems in the tetragonal configuration.

Figure 4

Angular dependence of the EPR spectrum in $\mathrm{Ba}\mathrm{Cu}{\mathrm{Si}}_2{\mathrm{O}}_6$ in the $ac$ plane, taken at a temperature of $6\mathrm{K}$; $\gamma$ is the angle between the direction of the applied field and the $a$ axis. Triangles and circles correspond to experimental data, while lines correspond to stimulation results.

Figure 5

Temperature dependencies of the integrated EPR intensity $I$ (circles) and peak-to-peak linewidth $\Delta B$ (squares), measured with magnetic field $H$ parallel to the $a$ axis at temperatures down to $10\mathrm{K}$. The solid line is a fit using the isolated-dimer model with an energy gap $\Delta =53\mathrm{K}$. The dashed line is a guide for the eye.