Kitaev honeycomb magnets and alpha-RuCl3

7 problems

• Are Kitaev couplings significant in cobalt honeycomb magnets?

  Contested

  Theory in 2018 proposed that cobalt honeycomb compounds could realize the Kitaev model with weaker competing couplings than the ruthenium and iridium ones. Neutron studies of BaCo2(AsO4)2 instead fit simpler models without large Kitaev terms, and the magnetic pattern of $\mathrm{Na_2Co_2TeO_6}$ is itself debated.

• Is the half-quantized thermal Hall plateau in $\alpha$-RuCl3 reproducible?

  Contested

  One group reported that the sideways heat flow stops changing with field at exactly half of a fundamental quantum value, the signature of a single Majorana edge channel. Other groups saw values near it without a flat plateau, or a strongly temperature-dependent signal, so it is unclear whether it is a real property of the material.

• Is there a field-induced spin-liquid phase in $\alpha$-RuCl3?

  Contested

  An in-plane magnetic field destroys the zigzag magnetic order of $\alpha$-RuCl3 near 70 to 80 kOe, and at higher field the spins line up with the field. Whether a separate spin-liquid phase exists between these regimes, or the observed features come from a smooth crossover and from structurally different pieces of the crystal, is unresolved.

• Nature of the intermediate phase of the antiferromagnetic Kitaev model in a field

  Contested

  Numerical studies of the ideal antiferromagnetic Kitaev model in a field along the cube diagonal find an extra phase between the low-field spin liquid and the high-field polarized state. Its identity, possibly a gapless spin liquid with a Fermi surface of neutral particles, is disputed.

• The minimal spin Hamiltonian of $\alpha$-RuCl3

  Contested

  Interpreting any $\alpha$-RuCl3 experiment requires its spin couplings, yet published fits disagree even on which coupling is largest. Without an agreed set, claims of spin-liquid behavior cannot be checked against theory.

• What carries the thermal Hall signal in $\alpha$-RuCl3?

  Contested

  Heat flowing through $\alpha$-RuCl3 in a magnetic field is deflected sideways, an effect called the thermal Hall effect. It is unclear whether the sideways heat is carried by Majorana fermions of a spin liquid, by magnons (spin waves) or by phonons (lattice vibrations).

• Find a material with a Kitaev spin-liquid ground state in zero field

  Open

  All known Kitaev candidates either order magnetically at low temperature or are too disordered to tell. The goal is a real crystal whose ground state without any applied field is the Kitaev spin liquid.