Spin-Spin Correlations in the CuAlCr₄S₈ Breathing Pyrochlore Lattice

In recent years, the breathing pyrochlore structure has been realized in a number of compounds which display complex spin-spin correlations arising from the many competing interactions that characterize them. Recently, a new breathing pyrochlore, CuAlCr₄S₈, has been synthesized and found to develop magnetic order below 20 K [Phys. Rev. B 106, 024407 (2022)]. In an attempt to theoretically understand the nature of the ordering in this system, superexchange couplings were determined using density-functional theory (DFT), and the resulting spin Hamiltonian was studied using large-N theory, pseudo-fermion functional renormalization group (PFFRG) calculations and classical Monte Carlo (MC) simulations. Interestingly, the DFT-based exchange model suggests that this system, and some other related Cr-based breathing pyrochlore materials, can be mapped onto an effective face-centered cubic (FCC) antiferromagnetic model which displays order-by-disorder, where an accidental degeneracy is lifted by thermal or quantum fluctuations. In this talk, we will present results using PFFRG, large-N and MC simulations that clarify when these breathing pyrochlore systems can be described as an effective FCC antiferromagnet and when they cannot.