A zoo of presaturation phases in novel quantum ferro-antiferromagnets.

The standard paradigm of magnetic-saturation-by-BEC-of-magnons breaks down close to a classical ferro-antiferromagnetic transition [1]. Upon decreasing the external magnetic field the fully polarized state collapses before the single magnon instability is reached. New quantum presaturation phases with unusual long range orders may emerge, including spin density waves, nematics and chiral states.

Strong frustration of FM and AFM interactions is uncommon in materials. Among the most interesting candidates are the layered vanady phosphates ABVO(PO₄)₂ (proximate J₁-J₂ square lattices) and linear-chain molybdates (Rb, Cs)₂Cu₂Mo₃O₁₂ (J₁-J₂ spin chains). Both families are known for quite some time, but their investigation were hampered by a lack of single crystals. In recent breakthoughs, large crystals of BaCdVO(PO₄)₂ , Pb₂VO(PO₄)₂ , SrZnVO(PO₄)₂ were synthesized. The spin Hamiltonians was determined in neutron spectroscopy experiments [2] and the behavior in applied fields was studied in detail [3-5]. Indeed, novel presaturation phases were found in each of these materials, but seem to vary considerably between species.
Smaller single crystals of the molybdates are now also available [6,7]. The Cs system shows an entire cascade of field-induced transitions with a very distinctive presaturation phase [7]. The Rb material has a presaturation phase as well. It is more subtle and is only revealed in highly sensitive dielectric experiments.

It is a good moment in time for theorists to step in and help classify the animals in the zoo of new quantum presaturation phases in these compounds.

[1] H. T. Ueda and T. Momoi, Phys. Rev. B 87, 144417 (2013).
[2] Bettler et al., Phys. Rev. B 99, 184437(2019).
[3] Povarov et al., Phys. Rev. B 99 024413 (2019).
[4] Bhartiya et al., Phys. Rev. Research 1, 033078 (2019).
[5] Landolt et al., Phys. Rev. B 102, 094414 (2020).
[6] Hayashida et al., Phys. Rev. B 100, 134427 (2019).
[7] Flavian et al., Phys. Rev. B 101, 224408 (2020).