Low frequency spin dynamics in the quantum magnet copper pyrazine dinitrate

TL;DR

This study investigates low frequency spin dynamics in the quantum magnet copper pyrazine dinitrate using NMR spectroscopy and quantum Monte Carlo calculations, revealing divergence of low energy excitations near the saturation field.

Contribution

It provides the first combined experimental and theoretical analysis of low frequency spin dynamics in CuPzN near the quantum critical point.

Findings

Strong divergence of low energy excitations at low temperatures.

Disentanglement of transverse and longitudinal spin fluctuations.

Transfer of delocalized spin moments from copper to nitrogen atoms.

Abstract

The S=1/2 antiferromagnetic Heisenberg chain exhibits a magnetic field driven quantum critical point. We study the low frequency spin dynamics in copper pyrazine dinitrate (CuPzN), a realization of this model system of quantum magnetism, by means of $^{13}$C-NMR spectroscopy. Measurements of the nuclear spin-lattice relaxation rate $T_1^{-1}$ in the vicinity of the saturation field are compared with quantum Monte Carlo calculations of the dynamic structure factor. Both show a strong divergence of low energy excitations at temperatures in the quantum regime. The analysis of the anisotropic $T_1^{-1}$-rates and frequency shifts allows one to disentangle the contributions from transverse and longitudinal spin fluctuations for a selective study and to determine the transfer of delocalized spin moments from copper to the neighboring nitrogen atoms.