Physical Review Letters ( IF 8.385 ) Pub Date : 2020-10-16 , DOI: 10.1103/physrevlett.125.167201
Ganesh Pokharel; Hasitha Suriya Arachchige; Travis J. Williams; Andrew F. May; Randy S. Fishman; Gabriele Sala; Stuart Calder; Georg Ehlers; David S. Parker; Tao Hong; Andrew Wildes; David Mandrus; Joseph A. M. Paddison; Andrew D. Christianson

We present a comprehensive neutron scattering study of the breathing pyrochlore magnet ${\mathrm{LiGaCr}}_{4}{\mathrm{S}}_{8}$. We observe an unconventional magnetic excitation spectrum with a separation of high- and low-energy spin dynamics in the correlated paramagnetic regime above a spin-freezing transition at 12(2) K. By fitting to magnetic diffuse-scattering data, we parametrize the spin Hamiltonian. We find that interactions are ferromagnetic within the large and small tetrahedra of the breathing pyrochlore lattice, but antiferromagnetic further-neighbor interactions are also essential to explain our data, in qualitative agreement with density-functional-theory predictions [Ghosh et al., npj Quantum Mater. 4, 63 (2019)]. We explain the origin of geometrical frustration in ${\mathrm{LiGaCr}}_{4}{\mathrm{S}}_{8}$ in terms of net antiferromagnetic coupling between emergent tetrahedral spin clusters that occupy a face-centered-cubic lattice. Our results provide insight into the emergence of frustration in the presence of strong further-neighbor couplings, and a blueprint for the determination of magnetic interactions in classical spin liquids.

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