Exchange Interactions Mediated by Non-Magnetic Cations in Double Perovskites

Abstract

Establishing the physical mechanism governing exchange interactions is fundamental for exploring exotic phases such as the quantum spin liquids (QSLs) in real materials. In this work, we address exchange interactions in Sr2CuTe${}_{1-x}$W${}_x$O, a series of double perovskites that realize the spin-1/2 square lattice and were suggested to harbor a QSL ground state arising from random distribution of non-magnetic ions. Our {\it ab initio} multi-reference configuration interaction calculations show that replacing Te atoms with W atoms changes the dominant couplings from nearest to next-nearest neighbor explained by the crucial role of unoccupied states of non-magnetic ions in the super-superexchange mechanism. Combined with spin-wave theory simulations, our calculated exchange couplings provide an excellent description of the inelastic neutron scattering spectra of the end compounds, as well as explain the magnetic excitations in Sr2CuTe${}_{0.5}$W${}_{0.5}$O as emerging from the bond-disordered exchange couplings. Our results provide crucial understanding of the role of non-magnetic cations in exchange interactions paving the way to further exploration of QSL phases in bond-disordered materials.

arXiv commentVersion accepted in Phys. Rev. Lett

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• Freezing of a disorder induced quantum spin liquid
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• Structure, spin correlations and magnetism of the $S=1/2$ square-lattice antiferromagnet Sr${}_2$CuTe${}_{1-x}$W${}_x$O${}_6$ ($0\le x\le 1$)
• Characterizing Random-singlet State in Two-dimensional Frustrated Quantum Magnets and Implications for the Double Perovskite Sr${}_2$CuTe${}_{1-x}$W${}_x$O${}_6$
• Phase diagram and magnetic excitations of $J_1$-$J_3$ Heisenberg model on the square lattice
• Electronic structure of the frustrated diamond lattice magnet NiRh${}_2$O${}_4$
• Frustrated network of indirect exchange paths between tetrahedrally coordinated Co in Ba2CoO4