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

by Paul Goddard (University of Warwick, UK)

Friday Jul 5, 2024, 2:00 PM → 4:00 PM Europe/Berlin

D5-153 (UHG)

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 [1]. 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 [2,3]. 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.

 

I will present data 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 4_1 screw [4]. 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. 

 

If time permits, I will also present new low-temperature data on the heat capacity, magnetization and magnetocaloric effect measured in high magnetic fields of up to 73 T, which is sufficient to close the gap seen at lower fields. The data reveal several unusual features, including an apparent field-induced quantum-critical phase transition in the chiral chain close to saturation that appears absent in more conventional sine-Gordon chains.

 

[1] D. C. Dender et al. Phys. Rev. Lett. 79, 1750 (1997)

[2] M. Oshikawa and I. Affleck, Phys. Rev. Lett. 79, 2883 (1997)

[3] I. Affleck and M. Oshikawa, Phys. Rev. B 60, 1038 (1999)

[4] J. Liu et al. Phys. Rev. Lett. 122, 57207 (2019)