Neutron Scattering and Magnetism
Laboratory for Solid State Physics · ETH Zurich

K2Co(SeO3)2

Chemical formula:

K2Co(SeO3)2

Lattice type:

Trigonal, space group R3̄m

How to grow:

Flux method

Magnetic model:

S = 1/2 almost-Ising antiferromagnet on a triangular lattice

Why is it cool:

A spin supersolid emerging in a sequence of three topological Berezinskii-Kosterlitz-Thouless transitions

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Some models are important enough that finding their material realization becomes a quest in itself. The S = 1/2 triangular lattice antiferromagnet near the Ising limit is one of them: theory predicts a magnetic supersolid ground state, in which "solid" spin order along the easy axis coexists with transverse "superfluid" coherence, a Bose-Einstein condensate of spin flips. K2Co(SeO3)2 realizes that Hamiltonian with remarkable fidelity: the Co2+ pseudospins S = 1/2 occupy a geometrically perfect triangular lattice, and the measured exchange anisotropy of α = 0.08 places the material deep in the almost-Ising regime. The supersolid duly emerges, through a sequence of three topological Berezinskii-Kosterlitz-Thouless transitions: the first two, at about 11 K and 0.8 K, bracket an extended critical phase and establish the three-sublattice "solid" order, while the third, at 0.35 K, marks the onset of the "superfluid" [2]. The excitations are thoroughly unconventional as well: in place of sharp spin waves, ultra-high-resolution neutron spectroscopy reveals broad dispersive continua, a pseudo-Goldstone mode with a gap of a mere 60 μeV, and a roton-like minimum at the M point [1,2]. Better still, the underlying model is amenable to large-scale quantum Monte Carlo simulations, which reproduce the measured thermodynamics and spin dynamics in complete quantitative detail [2].

Constant-energy maps of the excitation continuum in K2Co(SeO3)2

Constant-energy maps of the zero-field magnetic excitations in K2Co(SeO3)2 at 70 mK, from the lowest energies (top left) to the highest (bottom right) [1]. The sharp cones emanating from the corners of the Brillouin zone progressively dissolve into a broad continuum that floods all of reciprocal space. Note the ×5 intensity scale in the last two panels.