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

Cs/RbFeCl3

Chemical formula:

Cs1−xRbxFeCl3

Lattice type:

Hexagonal, space group P63/mmc

How to grow:

Bridgman furnace

Magnetic model:

Triangular lattice of S = 1 ferromagnetic chains with strong easy-plane anisotropy

Why is it cool:

"Chemical pressure" tunes it between two entirely different ground states: a gapped quantum paramagnet and a gapless 120° helimagnet

Cs/RbFeCl3 crystals

Atom legend

In this family the ground state itself is a matter of chemical composition. The Fe2+ pseudospins S = 1 are pulled in opposite directions: strong easy-plane anisotropy favors a nonmagnetic singlet on every site, while ferromagnetic exchange along the chains, aided by a weak frustrated coupling on the triangular lattice, favors magnetic order. In CsFeCl3 anisotropy wins and the result is a gapped quantum paramagnet; in the isostructural RbFeCl3 exchange wins and the moments order into a gapless 120° helimagnetic structure. The smaller Rb ion squeezes the lattice, so substituting it for Cs acts as "chemical pressure" that continuously tunes this competition: the spin gap closes at x ≈ 0.35, at a quantum critical point where chemical disorder visibly broadens all magnetic excitations [1]. Even the seemingly conventional ordered phase holds a surprise: the measured spin waves defy standard spin-wave theory, and their anomalous long-wavelength intensities betray long-range magnetic dipolar interactions [2].

XFeCl3 spectra

Evolution of the magnetic excitation spectrum across the composition-induced quantum critical point, measured by inelastic neutron scattering at 0.1 K [1]: the spin gap of CsFeCl3 (left) closes near the critical composition x = 0.3 (center); spin waves of the ordered 120° structure emerge in RbFeCl3 (right).