The large-$m$ limit, and spin liquid correlations in kagome-like spin models

Condens. Matter Phys., 2017, vol. 20, No. 1, 13701 It is noted that the pair correlation matrix $\hat{\chi}$ of the nearest neighbor Ising model on periodic three-dimensional ($d=3$) kagome-like lattices of corner-sharing triangles can be calculated partially exactly. Specifically, a macroscopic number $1/3 \, N+1$ out of $N$ eigenvalues of $\hat{\chi}$ are degenerate at all temperatures $T$, and correspond to an eigenspace $\mathbb{L}_{-}$ of $\hat{\chi}$, independent of $T$. Degeneracy of the eigenvalues, and $\mathbb{L}_{-}$ are an exact result for a complex $d=3$ statistical physical model. It is further noted that the eigenvalue degeneracy describing the same $\mathbb{L}_{-}$ is exact at all $T$ in an infinite spin dimensionality $m$ limit of the isotropic $m$-vector approximation to the Ising models. A peculiar match of the opposite $m=1$ and $m\rightarrow \infty$ limits can be interpreted that the $m\rightarrow\infty$ considerations are exact for $m=1$. It is not clear whether the match is coincidental. It is then speculated that the exact eigenvalues degeneracy in $\mathbb{L}_{-}$ in the opposite limits of $m$ can imply their quasi-degeneracy for intermediate $1 \leqslant m < \infty$. For an anti-ferromagnetic nearest neighbor coupling, that renders kagome-like models highly geometrically frustrated, these are spin states largely from $\mathbb{L}_{-}$ that for $m\geqslant 2$ contribute to $\hat{\chi}$ at low $T$. The $m\rightarrow\infty$ formulae can be thus quantitatively correct in description of $\hat{\chi}$ and clarifying the role of perturbations in kagome-like systems deep in the collective paramagnetic regime. An exception may be an interval of $T$, where the order-by-disorder mechanisms select sub-manifolds of $\mathbb{L}_{-}$.