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

本项目从化合物Fe1/3TaS2开始，探索通过取代改变二维过渡金属二硫族化合物性质的范围和可能性。在项目工作中，首先处理MxTaS2和MxNbS2类型的化合物（M = Cr, Mn, Fe， Co和Ni），然后处理替代系列MxM' ytas2（主要是M = Fe和M' = Co）以及MxTaS2- ysey，重点将放在磁性和输输性质对化学成分的依赖上。这一复杂的研究课题将通过准备、深远的分析和物理表征以及理论的紧密结合以及相关工作组之间的强烈互动来处理。合成工作将通过化学传递反应提供足够大小的单晶，用于计划的物理性质表征。样品将用x射线衍射方法进行表征，在某些情况下也用电子显微镜进行表征。关于这一点，重点将放在超结构的存在、各种晶体位置的占据、热处理的影响以及由此产生的有序程度上。超结构的稳定性将用温度相关的原位x射线衍射来研究。所有化合物都将以详细的方式研究其与温度相关的磁和电子输运性质，特别是在低温和高磁场下测量磁输运性质。这些测量提供了有关磁有序和各向异性、磁阻（MR）和霍尔效应以及电荷密度波和超导性的信息。利用高压进一步改变结构参数和电子性能。为此，重点将放在寻找压力诱导的超导性以及研究结构、电子结构与超导性的相互作用上。对实验结果的详细理解将通过基于dft的电子结构计算来实现。亚化学计量学和取代学将使用相干电位近似合金理论来解释，该理论在研究亚晶格上的优先占位或短程顺序时被其他理论所利用。计算首先提供了基态和磁序的信息。有限温度下的性质将通过蒙特卡罗模拟进行研究。为了讨论输运性质，电导率张量将通过线性响应形式确定。在计算磁流变率时，相对论公式考虑了自旋-轨道耦合，并以温度相关的方式给出了霍尔电阻率。

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.