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Magnetic and transport properties of ternary iron chalcogenides with reduced dimensionality

Subject Area Experimental Condensed Matter Physics
Term from 2019 to 2023
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 410409371
 
Final Report Year 2024

Final Report Abstract

The project focused on the investigation of one-dimensional linear-chain antiferromagnetic ternary iron chalcogenides AFeX2 (A = K, Rb, Cs, Tl; X = S, Se), which are highly interesting due to their unusual linear increase of the magnetic susceptibility on increasing temperature in the paramagnetic regime. Moreover the results obtained in this project have also impact on the understanding of the quasi two-dimensional iron-based superconductors, because the chain compounds contain the same FeX4 building blocks. The studies started from the synthesis of high-quality single crystals followed by detailed investigations of their physical properties by a wide range of complementary experimental methods. For the analysis of the collected data a self-consistent unified approach was developed 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, nuclear inelastic scattering, and Mössbauer spectroscopy measurements was examined in a consistent way based on results of the ab initio studies. This extended analysis provided a solid basis to separate the magnetic contribution from the specific heat data, to determine the involved entropy and the corresponding spin value of iron in these chain compounds. Further studies concerned the question on possible one-dimensional electrical conductivity along the chains of edge-sharing FeX4 tetrahedra. Although experimentally the samples exhibit an allover semiconducting behavior, our density functional theory calculations reveal an insulatorto-metal transition on passing from the long-range ordered antiferromagnetic phase into the paramagnetic phase. Electron spin resonance measurements in the paramagnetic phase of the AFeX2 compounds indicate two-dimensional spin correlations on approaching the Néel temperature: here one observes a Berezinskii-Kosterlitz-Thouless scenario in the temperature dependence of the resonance linewidth. Comparative investigations have been performed in the layered iron-pnictide superconductor EuRbFe4As4, which exhibits coexisting superconducting and magnetically ordered phases including a Berezinskii-Kosterlitz-Thouless transition.

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