Magnetic and transport properties of ternary iron chalcogenides with reduced dimensionality

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The proposed research project is focused on the investigation of one-dimensional (1D) linear-chain ternary iron chalcogenides AFeX2 (A = K, Rb, Cs, Tl; X = S, Se) and two-dimensional (2D) planar ternary iron chalcogenides A(1-x)Fe(2−y)X2 (A = K, Rb; X = S, Se). The former undergo an antiferromagnetic phase transition at elevated temperatures, the latter are superconductors with coexisting antiferromagnetic order at stoichiometries close to 2:4:5 with superconducting transition temperatures above 30 K. The project supposes the whole cycle of studies: (i) synthesis of high-quality single crystals, (ii) detailed investigation of their physical properties by means of a wide range of complementary experimental methods, (iii) extended conventional analysis of the collected data, disclosure of the artifacts related to the reduced dimensionality of the studied materials, and (iv) development of a self-consistent unified approach based on modern ab initio calculations of band structures, electron densities of states, spin- and charge distributions, electric-field gradients, phonon dispersions and element specific phonon density of states. The set of experimental data from heat capacity, magnetic moment, infrared absorption, electron spin resonance and Mössbauer spectroscopy measurements will be examined in a consistent way based on results of the ab initio studies. This extended analysis will provide a solid basis for building of spin-chain models for interpreting the unusual quasi-linear increase of magnetic susceptibility with increasing temperature, observed in many of linear-chain iron chalcogenides in a wide range of temperatures above the Néel temperature. Quantum statistical models of the magnetic susceptibility of spin-chain compounds will be developed and verified on the database of our measurements. The common building blocks of FeX4 tetrahedra motivate the comparative investigation of 1D antiferromagnetic spin chains and 2D layered superconductors with the focus on correlations between magnetism and superconductivity. The project support from DFG and RFBR will strengthen the existing collaboration between Augsburg and Kazan teams and unlock the synergy of the two groups working together.