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

BaCdVO(PO4)2

Chemical formula:

BaCdVO(PO4)2

Lattice type:

Orthorhombic, space group Pbca

How to grow:

Bridgman furnace

Magnetic model:

Square lattice Heisenberg antiferromagnet with frustrating diagonal interactions

Why is it cool:

Very close to realizing the elusive quantum bond-nematic state

BaCdVO(PO4)2

Atom legend

The spin nematic is one of the longest-standing quarries in quantum magnetism: a state that breaks spin-rotational symmetry without any dipolar magnetic order, a quantum condensate of bound magnon pairs. The baseline host model is the S = 1/2 square lattice with ferromagnetic nearest-neighbor exchange frustrated by antiferromagnetic diagonal coupling, where the nematic is predicted to appear in magnetic fields just below saturation. BaCdVO(PO4)2 is the most promising candidate material [1], and our crystals of it are truly unique: no other group has grown it in single-crystal form, and for neutron work the natural cadmium had to be replaced by the isotope 114Cd. The effort paid off almost immediately: just below saturation we discovered a "presaturation" phase with no dipolar order whatsoever, precisely where the magnon-pair condensate was expected [1,2].

Is it the real thing? To find out, we first measured the spin Hamiltonian outright: a global fit of over 150 inelastic neutron scans in the fully polarized phase pinned down all eight symmetry-allowed exchange constants, revealing a far more intricate network of couplings than the idealized square lattice model [3]. The final word came from NMR spectroscopy: the nematic phase is narrowly avoided [4]. Yet the near miss is written all over the spin dynamics: the low-energy fluctuations just below saturation are dominated by two-magnon processes, the unmistakable signature of a system on the very brink of magnon-pair condensation.

Spin waves and global Hamiltonian fit in BaCdVO(PO4)2

Magnetic excitations in the fully polarized phase of BaCdVO(PO4)2, measured by inelastic neutron scattering along ten different trajectories in reciprocal space [3]. The white lines are a single global fit that determines all eight exchange constants of the spin Hamiltonian.