Figure 1

High-energy resolution data from the Low Energy Transfer (LET) with the 66 mK, 7 T data set used as a background. Inelastic scattering integrated over $[\frac{1}{2}H,\frac{1}{2}H,H]=[-1.5,1.0]\mathrm{r}.\mathrm{l}.\mathrm{u}.$ (reciprocal lattice unit) and $[K,K,-K]=[-0.5,0.5]\mathrm{r}.\mathrm{l}.\mathrm{u}.$ is shown for (a) $T=66\mathrm{mK}$ and ${\mu }_{0}H=0$, (b) $T=66\mathrm{mK}$ and ${\mu }_{0}H=0.25\mathrm{T}$, (c) $T=1.5\mathrm{K}$ and ${\mu }_{0}H=0$, and (d) $T=60\mathrm{K}$ and ${\mu }_{0}H=0$. A spin gap of $\sim 0.06-0.08\mathrm{meV}$ opens up below 1.5 K, which was not resolved previously. The spin gap correlates strongly with the appearance of the $(\frac{1}{2},\frac{1}{2},\frac{1}{2})$ elastic magnetic peaks shown in Figs. 1–4. (e) Field and temperature dependences of the low-energy inelastic scattering as shown in panels (a)–(c), integrated over the full $[H,-H,0]$ range. The inset to (e) shows the same data corrected for the Bose factor, that is, ${\chi }^{\prime \prime }(\mathbf{Q},E)$ [see Eq. (1)].Reuse & Permissions