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  Tassel, C.; Kang, J.; Lee, C.; Hernandez, O.; Qiu, Y.; Paulus, W.; Collet, E.; Lake, B.; Guidi, T.; Whangbo, M.-H.; Ritter, C.; Kageyama, H.; Lee, S.-H.: Ferromagnetically Coupled Shastry-Sutherland Quantum Spin Singlets in (CuCl)LaNb₂O₇. Physical Review Letters 105 (2010), p. 167205/1-4

10.1103/PhysRevLett.105.167205

Abstract:
A thorough crystal structure determination at very low temperature of ðCuClÞLaNb2O7, originally proposed as a spin-1=2 square-lattice antiferromagnet, is reported thanks to the use of single-crystal x-ray diffraction and powder neutron diffraction. State-of-the-art calculations (maximum entropy method) reveal that ðCuClÞLaNb2O7 is orthorhombic with Pbam symmetry. First-principles calculations demonstrate that the dominant magnetic interactions are antiferromagnetic between fourth nearest neighbors 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 ðCuClÞLaNb2O7 the first system of ferromagnetically coupled Shastry-Sutherland quantum spin singlets.