constants and high degree of magnetic isotropy allow for a fairly accurate determination of several of the largest Hamiltonian parameters, by comparison with the measured single magnon dispersion relation above an approximately fully-polarized state. The approximate Hamiltonian, including a magnetic field, determined by the experimentalists in this way is $$\begin{aligned}
  \label{eq:Hexpt}
  H & = & \frac{1}{2} \sum_{ij} \left[ J_{ij} {\bm S}_i \cdot
    {\bm S}_j - {\bm D}_{ij} \cdot {\bm S}_i \times {\bm S}_j \right]
  - {\bm h}\cdot \sum_i {\bm S}_i.~~~\end{aligned}$$ Here $i,j$ are sites of a stack of triangular lattices (see Figs. \[fig: planar\_lattice\] and \[fig: DM\_distribution\]). The principle exchange interactions determined in Ref. are $J=0.374 ~{\rm meV}$ on nearest-neighbor bonds parallel to the $b$ axis, significantly smaller $J'=0.128~{\rm meV}\approx 0.34 J$ on diagonal bonds in the $b$-$c$ plane, and quite small $J''=0.045 J$ along vertical bonds between adjacent triangular layers. A Dzyaloshinskii-Moriya (DM) coupling was also measured along the diagonal bonds, with ${\bm D}_{ij}=-{\bm D}_{ji}=\pm D \hat{a}$ where the sign in specific directions are as indicated in Fig. \[fig: DM\_distribution\], and $D=0.020$meV$=0.05J$.

![Magnetic sites and exchange couplings in a 2D triangular layer in [[Cs$_2$CuCl$_4$]{}]{}: on-chain bonds $J$ (thick lines $\parallel b$), frustrating diagonal bonds $J'$ (dotted lines). Stacked layers 