Microscopic model of (CuCl)LaNb2O7: coupled spin dimers replace a frustrated square lattice

We present a microscopic model of the spin-gap quantum magnet (CuCl)LaNb2O7 that was previously suggested as a realization of the spin-1/2 frustrated square lattice. Taking advantage of the precise atomic positions from recent crystal structure refinement, we evaluate individual exchange integrals and construct a minimum model that naturally explains all the available experimental data. Surprisingly, the deviation from tetragonal symmetry leads to the formation of spin dimers between fourth neighbors due to a Cu-Cl-Cl-Cu pathway with a leading antiferromagnetic exchange J4 ~ 25 K. The total interdimer exchange amounts to 12 - 15 K. Our model is in agreement with inelastic neutron scattering results and is further confirmed by quantum Monte-Carlo simulations of the magnetic susceptibility and the high-field magnetization. Our results establish (CuCl)LaNb2O7 as a non-frustrated system of coupled spin dimers with predominant antiferromagnetic interactions and provide a general perspective for related materials with unusual low-temperature magnetic properties.