Freezing of the Lattice in the Kagome Lattice Heisenberg Antiferromagnet Zn-barlowite ZnCu$_3$(OD)$_6$FBr

TL;DR

This study reveals that in Zn-barlowite, the lattice completely freezes below 50 K, with structural distortions influencing the magnetic ground state, as shown by $^{79}$Br NQR measurements indicating lattice dynamics and dimer formation.

Contribution

We demonstrate the onset of ultra slow lattice dynamics and lattice freezing below 50 K in Zn-barlowite, linking structural distortions to the magnetic disordered ground state.

Findings

Lattice dynamics slow down below 160 K, freezing below 50 K.

$^{79}$Br NQR lines broaden due to increased distortions.

Evidence of structural dimers forming prior to spin singlet emergence.

Abstract

We use $^{79}$Br nuclear quadrupole resonance (NQR) to demonstrate that ultra slow lattice dynamics set in below the temperature scale set by the Cu-Cu super-exchange interaction $J$~($\simeq160$~K) in the kagome lattice Heisenberg antiferromagnet Zn-barlowite. The lattice completely freezes below 50~K, and $^{79}$Br NQR lineshapes become twice broader due to increased lattice distortions. Moreover, the frozen lattice exhibits an oscillatory component in the transverse spin echo decay, a typical signature of pairing of nuclear spins by indirect nuclear spin-spin interaction. This indicates that some Br sites form structural dimers via a pair of kagome Cu sites prior to the gradual emergence of spin singlets below $\sim30$~K. Our findings underscore the significant roles played by subtle structural distortions in determining the nature of the disordered magnetic ground state of the kagome lattice.