ИСТИНА

In 2001, Siddharthan and Georges introduced a two-dimensional network of corner-sharing triangles with square lattice symmetry. In variance with kagomé lattice, which is a two-dimensional network of corner-sharing triangles with hexagonal voids, square kagome lattice (SKL) is a two-dimensional network of corner-sharing triangles with alternative square and octagonal voids. There are two non-equivalent positions for the magnetic ions, α and β, being in ratio one to two. It has been shown numerically that the properties of a square kagomé antiferromagnet should be similar to those on a kagomé lattice. A finite temperature phase transition into a phase with ordered resonance loops and broken translational symmetry has been predicted for SKL. This transition is not forbidden by Mermin-Wagner theorem since it stems from the breaking of a translational symmetry. Recently, it has been shown that the model of geometrically frustrated SKL possesses just single thermodynamically stable solution for arbitrary values of model parameters. The very existence of this solution means that the model cannot exhibit either first or second order phase transitions at non-zero temperatures. The temperature dependence of the specific heat of spin-1/2 Heisenberg SKL antiferromagnet has been analyzed by Tomczak and Richter who predicted the broad peak located at T ∼ 0.85J which is a correlation one typical for any low-dimensional magnetic systems, and the second one, at T ∼ 0.10J, which indicates an additional energy scale relevant to SKL being ascribed to the energy of the lowest spin triplet excitation. For a long time, the studies of the thermodynamic properties of compounds with SKL or its derivatives were carried out exclusively by theoretical methods, albeit the nature provided such patterns in some rare minerals. Among these minerals are nabokoite KCu7(SO4)5(TeO3)OCl, atlasovite KCu6FeBiO4(SO4)5Cl, elasmochloite Na3Cu6BiO4(SO4)5 and favreauite PbCu6BiO4(SeO3)4(OH)H2O. The intrinsic properties of SKL in nabokoite and atlasovite are masked by the presence of magnetic ions extraneous to this network. Recently, the iron-free sibling of atlasovite, KCu6AlBiO4(SO4)5Cl, has been synthesized. It has been established that down to 58 mK this compound persists in a gapless quantum spin liquid state. Additionally, a novel sodium bismuth oxo-cuprate phosphate chloride, Na6Cu7BiO4(PO4)4Cl3, containing both square kagomé layers and interlayer Cu2+ ions has been synthesized. This material shows no magnetic ordering down to 50 mK forming quantum spin liquid state similar to KCu6AlBiO4(SO4)5Cl.