# Unconventional field-induced spin gap in an $S = 1/2$ chiral staggered chain

## TL;DR

This study uncovers an unconventional field-induced spin gap in a chiral S=1/2 spin chain, revealing complex magnetic behavior influenced by the chain's chiral structure and proposing modifications to existing theoretical models.

## Contribution

It demonstrates that chiral structure introduces additional Hamiltonian terms affecting the spin gap, extending understanding beyond non-chiral staggered chains.

## Key findings

- Observation of a field-induced spin gap in a chiral spin chain
- Discrepancy between experimental data and sine-Gordon theory predictions
- Proposal of additional Hamiltonian terms due to chirality

## Abstract

We investigate the low-temperature magnetic properties of the molecule-based chiral spin chain [Cu(pym)(H$_2$O)$_4$]SiF$_6\cdot$H$_2$O (pym = pyrimidine). Electron-spin resonance, magnetometry and heat capacity measurements reveal the presence of staggered $g$ tensors, a rich low-temperature excitation spectrum, a staggered susceptibility and a spin gap that opens on the application of a magnetic field. These phenomena are reminiscent of those previously observed in non-chiral staggered chains, which are explicable within the sine-Gordon quantum-field theory. In the present case, however, although the sine-Gordon model accounts well for the form of the temperature-dependence of the heat capacity, the size of the gap and its measured linear field dependence do not fit with the sine-Gordon theory as it stands. We propose that the differences arise due to additional terms in the Hamiltonian resulting from the chiral structure of [Cu(pym)(H$_2$O)$_4$]SiF$_6\cdot$H$_2$O, particularly a uniform Dzyaloshinskii-Moriya coupling and a four-fold periodic staggered field.