Spin-1/2 antiferromagnetic chiral chains: the sine-Gordon model and beyond.

The dramatic effect of an alternating local spin environment on the properties of the spin-1/2 antiferromagnetic chain was first discovered through high-field neutron scattering and heat capacity experiments on copper-benzoate, which revealed the development of an energy gap on application of magnetic field. This was perplexing until it was found that the behaviour of this system, and a handful of others, could be described by the sine-Gordon model of quantum-field theory. Under the influence of the applied field, the gap emerges thanks to the presence of internal staggered fields and DM interactions that are a direct result of the staggered Cu(II) octahedra. Here, we report on the molecule-based chiral spin chain [Cu(pym)(H₂O)₄]SiF₆.H₂O (pym = pyrimidine), which at first glance could be a sine-Gordon chain, but with an added twist: a 4₁ screw. 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 sine-Gordon materials. In the present case, however, the size of the gap and its measured linear field dependence do not fit with the sine-Gordon model as it stands. We propose that the differences arise due to additional terms in the Hamiltonian resulting from the chiral structure.

This project has received funding from the European Research Council (Grant No. 681260). We also thank the NHMFL, EPSRC, STFC, Royal Society, NSF (Grant No. DMR-1703003, DMR-1157490), DoE and the State of Florida.