| Bose-Einstein condensation (BEC) of magnons is one of the fascinating quantum critical phenomena. Recently, feasible experimental access to the BEC in quantum magnets facilitated extensive studies of several model compounds, such as TlCuCl3 and BaCuSi2O6, and initiated an ongoing discussion regarding the role of magnetic frustration and possible dimensional reduction of the BEC at a quantum critical point. Since magnon BEC is a field-induced long-range magnetic ordering, the bosonic model of the specific compound is fully determined by underlying exchange couplings. In this contribution, we present the results of an experimental and computational study of two novel BEC materials, Pb2V3O9 and AgVOAsO4. We evaluate individual exchange couplings, simulate the BEC, and compare our results to the experiment. Using a combination of density functional theory calculations and quantum Monte Carlo simulations, we achieve a remarkably accurate description of the quasi-2D spin system in Pb2V3O9 and explore the exchange couplings inducing the BEC. Further on, we will present recent experimental results on AgVOAsO4 that is rather similar to Pb2V3O9 with respect to the underlying spin lattice but different in the high-field behavior. In particular, AgVOAsO4 shows an exotic example of two subsequent BEC-like transitions. Experimental and computational results on the temperature-vs-field phase diagram of AgVOAsO4 will be presented. |
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