Variation of the magnetic and transport properties of two-dimenional compounds by substitution

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Starting from the compound Fe1/3TaS2 the project explores the range and possibility to vary the properties of two-dimensional transition metal dichalcogenides by substitution. Within the project work, dealing first with compounds of the type MxTaS2 and MxNbS2 (M = Cr, Mn, Fe, Co and Ni) and later on also with the substitutional series MxM'yTaS2 (primarily M = Fe and M' = Co) as well as MxTaS2-ySey, the focus will be on the dependency of the magnetic and transport properties on the chemical composition. This complex research topic will be dealt with by a tight combination of preparation, far-reaching analytical and physical characterization as well as theory with a strong interaction between the involved working groups.The synthetic work will supply via chemical transport reactions single crystals of sufficient size for the planned characterization of the physical properties. The samples will be characterized by X-ray diffraction methods and in selected cases also by electron microscopy. Concerning this, emphasis will be put on the presence of super structures, the occupation of the various crystallographic sites, the influence of a heat treatment and the resulting degree of ordering. The stability of super structures will be studied using temperature dependent in-situ X-ray diffraction.All compounds will be investigated in a detailed way with respect to their temperature dependent magnetic and electronic transport properties, with the magneto-transport properties being measured in particular at low temperatures and high magnetic fields. These measurements deliver information on the magnetic order and anisotropy, on the magneto-resistance (MR) and the Hall effect as well as on the occurrence of charge density waves and superconductivity. Application of high pressure is used to further vary the structural parameters and electronic properties. Concerning this, the focus will be on the search for pressure induced superconductivity and the investigation on the interplay of structure, electronic structure and superconductivity. A detailed understanding of the experimental results will be achieved by DFT-based electronic structure calculations. Substoichiometry and substitution will be accounted for using the Coherent Potential Approximation alloy theory, exploited among others when investigating the preferential occupation or short-range order on sub lattices. The calculations deliver first of all information on the ground state and magnetic ordering. Properties at finite temperatures will be investigated by means of Monte Carlo simulations. For the discussion of the transport properties the conductivity tensor will be determined via the linear response formalism. A relativistic formulation accounts for spin-orbit-coupling when calculating the MR and gives access to the Hall resistivity in a temperature dependent way.