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

Chlorothionite

Chemical formula:

K2CuSO4Cl2 and K2CuSO4Br2

Lattice type:

Orthorhombic, space group Pnma

How to grow:

Wet chemistry synthesis

Magnetic model:

Heisenberg S = 1/2 chains

Why is it cool:

A unique pattern of Dzyaloshinskii-Moriya interactions

Chlorothionite crystal

Atom legend

The mineral chlorothionite, K2CuSO4Cl2, was first found in small quantities among the products of the 1906 eruption of Vesuvius; the synthetic crystals of it and of its bromide analog K2CuSO4Br2 grown in our lab capture attention by their size alone. Both are excellent realizations of the Heisenberg S = 1/2 chain, with the exchange constant almost an order of magnitude larger in the bromide. The real prize, however, is a perturbation that very few structures allow: an antisymmetric Dzyaloshinskii-Moriya interaction that is uniform along each chain, rather than alternating as in virtually all other chain materials. Such a term does not merely decorate the Hamiltonian: it twists the spin correlations, effectively frustrates the chain, and splits its excitations in ways that we mapped in detail by magnetometry in fields up to full saturation [1].

These materials also served us in a different role: as a testbed for finite-temperature scaling of spin dynamics at a field-induced quantum critical point. Saturation of a Heisenberg chain is a genuine quantum phase transition, and just below it the system is a strongly interacting quantum liquid with no energy scale other than temperature itself. In K2CuSO4Cl2, with its conveniently low saturation field, we measured the finite-temperature spin correlations near the critical field outright [2]. The experiment also taught us a hard lesson, namely that in a chain the universal critical fluctuations are masked by non-universal longitudinal scattering, and directly inspired the "symmetry filter" trick with spin ladders described on our quantum criticality page.

Chlorothionite scaling

Finite-temperature evolution of the excitation spectrum of the chlorothionite spin chain at magnetic saturation, measured by neutron spectroscopy [2]. At the quantum critical field the spectrum has no intrinsic energy scale: warming does not simply broaden it but reshapes it entirely, with temperature itself setting the only scale.