Structural Transitions and Magnetic Structure in NH4CuCl3 via 14N-NMR

TL;DR

This study uses 14N-NMR to investigate structural and magnetic properties of NH4CuCl3, revealing a structural phase transition at 70 K and insights into the magnetic structure within the 1/4-magnetization plateau.

Contribution

It provides new NMR evidence showing the magnetic structure does not break crystal symmetry and suggests triplet dimers form across chains, not just nearest neighbors.

Findings

Quadrupole splitting indicates a structural phase transition below 70 K.

Eight distinct N sites identified at 4.2 K in the 1/4-plateau.

Magnetic structure preserves crystal symmetry in the plateau.

Abstract

We report results of 14N-NMR experiments on NH4CuCl3 at the magnetic field of 7 T, where the 1/4-magnetization plateau is observed at low temperatures. The quadrupole splitting parameter $\nu_{z}$ splits below 70 K, indicating a structural phase transition. At 4.2 K, eight N sites with distinct values of both $\nu_{z}$ and the magnetic hyperfine shift $K_{z}$ are resolved in the NMR spectrum for general field directions. We then conlude that the magnetic structure in the 1/4-plateau does not break the symmetry of the crystal. Based on the NMR and the recent neutron scattering results by Ruegg et al. [Phys. Rev. Lett. 93 (2004) 037207], we propose that triplet dimers in the 1/4-plateau is formed not between the nearest neighbor pairs but over different chains.