# Novel magnetic field tuning of quantum spin excitations in a weakly coupled $S=$1/2 Heisenberg spin chain as seen from NMR

## TL;DR

This study uses NMR to explore how weak magnetic fields influence quantum spin excitations in a quasi-1D $S=1/2$ Heisenberg magnet with DM interactions, revealing a transition from spinon to magnon-like behavior.

## Contribution

It demonstrates magnetic field tuning of quantum spin excitations in a weakly coupled $S=1/2$ Heisenberg chain with Dzyaloshinskii-Moriya interactions, highlighting a crossover from spinon to magnon scattering.

## Key findings

- Spinon excitations observed above $T_N$ from constant $1/T_1$.
- Below $T_N$, $1/T_1$ flattens and then shows power-law dependence with increasing field.
- Transition from two-magnon to three-magnon scattering as field increases.

## Abstract

We report our NMR study of the spin excitations in the quasi-one dimensional (1D) $S=1/2$ quantum magnet CH$_3$NH$_3$Cu(HCOO)$_3$ lying in the 1D-3D dimensional crossover regime with Dzyaloshinskii-Moriya(DM) interactions. Above $T_N$, the spinon excitation is observed from the constant $1/T_1$ at low temperatures contributed from the staggered spin susceptibility. At low temperatures well below $T_N$, the $1/T_1$ begins to flatten out under weak magnetic fields. With the increasing field intensity, $1/T_1$ tends to show a power-law temperature dependence gradually, with the index increasing from zero to $\sim3$, and finally $\sim5$, which are the respective typical characteristic for the dominating two-magnon Raman process and three-magnon scattering contributions to the nuclear relaxation in a conventional 3D magnet. A possible physical mechanism for this novel magnetic field tuning of quantum spin excitations related with the enhanced effective staggered field created by the DM interactions under magnetic field is discussed.