Neutron inelastic scattering investigation of the magnetic excitations in ${\mathrm{Cu}}_2{\mathrm{Te}}_2{\mathrm{O}}_5{X}_2$ $(X=\mathrm{Br},\mathrm{Cl})$

Neutron inelastic scattering investigations have been performed on the spin tetrahedral system ${\mathrm{Cu}}_2{\mathrm{Te}}_2{\mathrm{O}}_5{X}_2$ $(X=\mathrm{Cl},\mathrm{Br})$. We report the observation of magnetic excitations with a dispersive component in both compounds, associated with the three-dimensional incommensurate magnetic order that develops below $T_N^{\mathit{Cl}}=18.2\mathrm{K}$ and $T_N^{\mathit{Br}}=11.4\mathrm{K}$. The excitation in ${\mathrm{Cu}}_2{\mathrm{Te}}_2{\mathrm{O}}_5{\mathrm{Cl}}_2$ softens as the temperature approaches $T_N^{\mathit{Cl}}$, leaving diffuse quasi-elastic scattering above the transition temperature. In the bromide, the excitations are present well above $T_N^{\mathit{Br}}$, which might be attributed to the presence of a degree of low dimensional correlations above $T_N^{\mathit{Br}}$ in this compound.

Figure 1

${\mathrm{Cu}}^{2+}$ ions are located at the vertices of ${\mathrm{Cu}}_4{\mathrm{O}}_8{X}_4$ tetrahedra (O and $X$ atoms not shown). The spin tetrahedra align in chains along the $c$ axis. Four exchange interaction paths are indicated; $J_1$ and $J_2$ are intra-tetrahedral while $J_a$ and $J_b$ are inter-tetrahedral.

Figure 2

(Color online) 2D map of the scattering intensity of (a) ${\mathrm{Cu}}_2{\mathrm{Te}}_2{\mathrm{O}}_5{\mathrm{Br}}_2$ and (b) ${\mathrm{Cu}}_2{\mathrm{Te}}_2{\mathrm{O}}_5{\mathrm{Cl}}_2$ as a function of energy transfer $\left(\hslash \omega \right)$ and momentum transfer $(\mid \mathbf{Q}\mid )$, obtained at $8\mathrm{K}$, with incident energy $17\mathrm{meV}$. The gray (color) scale denotes the scattering intensity ($S(\mid \mathbf{Q}\mid ,\hslash \omega )$, arb. units). These data were taken on MARI, and have not been corrected for phonon or multiple scattering.

Figure 3

Scattering intensity of ${\mathrm{Cu}}_2{\mathrm{Te}}_2{\mathrm{O}}_5{\mathrm{Br}}_2$ and ${\mathrm{Cu}}_2{\mathrm{Te}}_2{\mathrm{O}}_5{\mathrm{Cl}}_2$ as a function of momentum transfer, with phonon and multiple scattering corrections applied to the data. The data were taken over an energy range $3–7\mathrm{meV}$, at $T=8\mathrm{K}$. The solid line represents the square of the ${\mathrm{Cu}}^{2+}$ magnetic form factor (see Ref. 13), and the inset shows the raw data without corrections for phonon and multiple scattering. These data were taken on the MARI spectrometer, ISIS.

Figure 4

(Color online) 2D map of the scattering intensity of (a) ${\mathrm{Cu}}_2{\mathrm{Te}}_2{\mathrm{O}}_5{\mathrm{Br}}_2$ and (b) ${\mathrm{Cu}}_2{\mathrm{Te}}_2{\mathrm{O}}_5{\mathrm{Cl}}_2$ with corrections made for phonon and multiple scattering. These data were taken on the MARI spectrometer, ISIS.

Figure 5

Elastic neutron scattering of ${\mathrm{Cu}}_2{\mathrm{Te}}_2{\mathrm{O}}_5{X}_2$ $(X=\mathrm{Br},\mathrm{Cl})$, measured at the high flux D20 diffractometer, ILL. This is a difference plot, showing the high temperature scattering intensity $(T=25\mathrm{K})$ subtracted from the low temperature scattering intensity $(T=2\mathrm{K})$ as a function of momentum transfer.

Figure 6

Scattering intensity vs energy transfer for $X=\mathrm{Br}$ and Cl, summed over the low angle detector bank (scattering angle 3°–13°, which corresponds approximately to $0.5{\mathrm{\AA }}^{-1}<\mid \mathbf{Q}\mid <1.4{\mathrm{\AA }}^{-1}$), taken at $T=8\mathrm{K}$. These data are not corrected for multiple and phonon scattering, however, the solid (dotted) line shows the contribution of the non-magnetic background for $X=\mathrm{Br}$ $(X=\mathrm{Cl})$, as determined by the phonon and multiple scattering calculations. These data were taken on the MARI spectrometer, ISIS.

Figure 7

Scattering intensity vs energy transfer at 8, 15, and $50\mathrm{K}$ for ${\mathrm{Cu}}_2{\mathrm{Te}}_2{\mathrm{O}}_5{\mathrm{Br}}_2$ and ${\mathrm{Cu}}_2{\mathrm{Te}}_2{\mathrm{O}}_5{\mathrm{Cl}}_2$. The data are summed over the low angle detector bank (scattering angle 3°–13°, which corresponds approximately to $0.5{\mathrm{\AA }}^{-1}<\mid \mathbf{Q}\mid <1.4{\mathrm{\AA }}^{-1}$). These data are not corrected for phonon and multiple scattering, which give only a small contribution to the overall scattering in this low $\mid \mathbf{Q}\mid$ region. The data were collected on the MARI spectrometer at ISIS.