Ferromagnetically coupled Shastry-Sutherland quantum spin singlets in (CuCl)LaNb$_2$O$_7$

TL;DR

This study reveals that (CuCl)LaNb$_2$O$_7$ hosts ferromagnetically coupled Shastry-Sutherland quantum spin singlets due to its unique orthorhombic structure and dominant fourth nearest neighbor antiferromagnetic interactions.

Contribution

It is the first system identified with ferromagnetically coupled Shastry-Sutherland quantum spin singlets, combining experimental and first-principles methods.

Findings

Orthorhombic crystal structure with tilted CuCl$_4$O$_2$ octahedra.

Dominant fourth nearest neighbor antiferromagnetic interactions.

Ferromagnetic coupling between spin singlets.

Abstract

Using inelastic neutron scattering, x-ray, neutron diffraction, and the first-principle calculation techniques, we show that the crystal structure of the two-dimensional quantum spin system (CuCl)LaNb$_2$O$_7$ is orthorhombic with $Pbam$ symmetry in which CuCl$_4$O$_2$ octahedra are tilted from their high symmetry positions and the Cu$^{2+} (s = 1/2)$ ions form a distorted square lattice. The dominant magnetic interactions are the fourth nearest neighbor antiferromagnetic interactions with a Cu-Cl--Cl-Cu exchange path, which lead to the formation of spin singlets. The two strongest interactions between the singlets are ferromagnetic, which makes (CuCl)LaNb$_2$O$_7$ the first system of ferromagnetically coupled Shastry-Sutherland quantum spin singlets.