Weakly coupled alternating $S=1/2$ chains in the distorted honeycomb lattice compound Na$_2$Cu$_2$TeO$_6$

TL;DR

This study uses neutron scattering, DFT, and RPA to analyze the exchange interactions in Na$_2$Cu$_2$TeO$_6$, confirming it as a weakly-coupled alternating antiferromagnetic-ferromagnetic chain compound with a robust gapped ground state.

Contribution

The paper provides a comprehensive understanding of exchange interactions in Na$_2$Cu$_2$TeO$_6$, confirming its realization as a weakly-coupled alternating AF-FM chain system.

Findings

Dominant intrachain AF and FM couplings identified

Interchain couplings are weak and antiferromagnetic

Gapped ground state remains stable under weak interchain interactions

Abstract

Spin-1/2 chains with alternating antiferromagnetic (AF) and ferromagnetic (FM) couplings exhibit quantum entanglement like the integer-spin Haldane chains and might be similarly utilized for quantum computations. Such alternating AF-FM chains have been proposed to be realized in the distorted honeycomb-lattice compound Na$_2$Cu$_2$TeO$_6$, but to confirm this picture a comprehensive understanding of the exchange interactions including terms outside of the idealized model is required. Here we employ neutron scattering to study the spin dynamics in Na$_2$Cu$_2$TeO$_6$ and accurately determine the coupling strengths through the random phase approximation and density functional theory (DFT) approaches. We find the AF and FM intrachain couplings are the dominant terms in the spin Hamiltonian, while the interchain couplings are AF but perturbative. This hierarchy in the coupling strengths and the alternating signs of the intrachain couplings can be understood through their different exchange paths. Our results establish Na$_2$Cu$_2$TeO$_6$ as a weakly-coupled alternating AF-FM chain compound and reveal the robustness of the gapped ground state in alternating chains under weak interchain couplings.