Cs₂CoBr₄

Chemical formula:

Cs₂CoBr₄

Lattice type:

Orthorhombic, space group Pnma

How to grow:

Bridgman furnace

Magnetic model:

XYZ-anisotropic zigzag spin ladders with both geometric and anisotropy frustration

Why is it cool:

A complex and unique "Zeeman ladder" excitation spectrum

A "lost" quantum magnet of the generic Cs₂BX₄ family, Cs₂CoBr₄ was rediscovered in our group. The primary antiferromagnetic interactions run along the legs and rungs of zigzag spin ladders, making the spin subsystem highly frustrated [1]. There is an additional twist: the cobalt ions carry S = 3/2 subject to extreme easy-plane single-ion anisotropy, so that only pseudospin-1/2 degrees of freedom remain active at low temperatures. The orientation of the easy plane alternates from site to site, and the effective S = 1/2 Hamiltonian is therefore full of strongly anisotropic, direction-dependent interactions [1].

In applied magnetic fields the material has a very complicated phase diagram [2]. First comes a longitudinal spin-density-wave phase, whose incommensurate propagation vector scales with the magnetization of the sample [3]. At higher fields the ordering vector locks in at the value 1/3, in an up-up-down magnetization-plateau phase. The system then goes through a spin-flop transition. Finally, at still higher fields, there is an incommensurate fan phase, with a propagation vector set not by the magnetization but by the exchange frustration ratio.

The most exciting physics is in the magnetic excitations. In zero field these form a hierarchy of dispersive two-kink bound states known as a "Zeeman ladder" [4]. Ours is arguably the most complex specimen of this physics on record: at least nine sharp bound states, sorted into two distinct sequences polarized longitudinally and transverse to the ordered moment. They remain confined to individual chains only at special wave vectors and propagate as genuinely two-dimensional objects elsewhere. Remarkably, theory shows that confinement survives here even though the frustrated XYZ Hamiltonian conserves no spin component whatsoever [5]. In higher magnetic fields, more conventional spin-wave-like excitations take over [4].

A "Zeeman ladder" of dispersive two-kink bound states observed in Cs₂CoBr₄ by inelastic neutron scattering.

• [1] L. Facheris, S. D. Nabi, K. Yu. Povarov, Z. Yan, A. Glezer Moshe, U. Nagel, T. Rõõm, A. Podlesnyak, E. Ressouche, K. Beauvois, J. R. Stewart, P. Manuel, D. Khalyavin, F. Orlandi, A. Zheludev, Magnetic field-induced phases and spin Hamiltonian in Cs₂CoBr₄ , Phys. Rev. B 109, 104433 (2024) (Editors' Suggestion); arXiv:2311.10535.

• [2] K. Yu. Povarov, L. Facheris, S. Velja, D. Blosser, Z. Yan, S. Gvasaliya, A. Zheludev, Magnetization plateaux cascade in the frustrated quantum antiferromagnet Cs₂CoBr₄ , Phys. Rev. Research 2, 043384 (2020); arXiv:2004.09893.

• [3] L. Facheris, K. Yu. Povarov, S. D. Nabi, D. G. Mazzone, J. Lass, B. Roessli, E. Ressouche, Z. Yan, S. Gvasaliya, A. Zheludev, Spin Density Wave versus Fractional Magnetization Plateau in a Triangular Antiferromagnet , Phys. Rev. Lett. 129, 087201 (2022); arXiv:2204.10682.

• [4] L. Facheris, S. D. Nabi, A. Glezer Moshe, U. Nagel, T. Rõõm, K. Yu. Povarov, J. R. Stewart, Z. Yan, A. Zheludev, Confinement of fractional excitations in a triangular lattice antiferromagnet , Phys. Rev. Lett. 130, 256702 (2023); arXiv:2301.13596.

• [5] C.-M. Halati, V. Romerio, P. Steffens, J. R. Stewart, A. Zheludev, T. Giamarchi, Zeeman Ladders in Frustrated XYZ Spin Chains , arXiv:2508.17834.