High-Order Coupled Cluster Method (CCM) Formalism 2 -- "Generalised" Expectation Values: Spin-Spin Correlation Functions for Frustrated and Unfrustrated 2D Antiferromagnets

Abstract

Recent developments of high-order CCM have been to extend existing formalism and codes to $s \ge \frac 12$ for both the ground and excited states. In this article, we describe how "generalised" expectation values for a wide range of one- and two-body spin operators may also be determined using existing the CCM code for the ground state. We present new results for the spin-spin correlation functions of the spin-half square- and triangular-lattice antiferromagnets by using the LSUB$m$ approximation. We show that the absolute values of the spin-spin correlation functions $| < \bf{s}(0).\bf{s}(r) > |$ converge with increasing approximation level for both lattices. We believe that the LSUB$m$ approximation provides reasonable results for the correlation functions for lattice separations roughly of order $r \approx m$ for the square lattice. We compare qualitatively our results for the square lattice to those results of quantum Monte Carlo (QMC) and we see that good correspondence is observed. Indeed as seen by QMC, the spin-spin correlation function initially decays strongly with $|r|$ before becoming constant for larger values of $|r|$. CCM results are also compared to results of exact diagonalisations for both lattices. ED results demonstrate a strong finite-size effects at lattice separations $r=L/2$ (where $N=L \times L$) for both lattices. The behaviour of the correlation function for the triangular lattice is qualitatively similar to that of the square lattice, namely, that it decays strongly at first before becoming constant. This is in keeping with the behaviour of both models, which are believed strongly to be N\'eel-ordered from approximate studies.