Typically the theory of low-dimensional magnetism precedes experimental testing, largely because of the difficulties in creating real materials described by theoretical models. However, in some cases experimental work uncovers physics not previously predicted. An example is the spin chain copper-benzoate where the local environment around the spins alternates along the chain. In this case, experiments revealed that a magnetic energy gap forms when a magnetic field is applied. This was entirely perplexing until it was found that the behaviour could be described by the sine-Gordon model of quantum-field theory, discovery that inspired a great deal more activity. In this paper, we highlight a new material, which at first glance could be a sine-Gordon chain, but with an added twist: a four-fold chiral structure. Using heat capacity, magnetometry and electron-spin resonance measurements, we show that the size and field-dependence of the energy gap, as well as the complex excitation spectrum, do not fit with the existing theories. We offer a qualitative explanation for the observations, however theoretical effort is called for to quantitatively account for the results. The study also raises the possibility of combining different chiral symmetries with anisotropic interactions to create new ground states and exotic excitations.

• Caption to figure: The structure of the spin-1/2 chiral chain [Cu(pyrimidine)(H2O)4]SiF6.H2O

• Publication: Physical Review Letters 122, 057207 (2019)
• DOI: https://doi.org/10.1103/PhysRevLett.122.057207