Transport measurements, structure and Mössbauer investigations of FeSe-related novel superconductors under high pressure up to 100GPa

本项目主要研究FeSe及其相关化合物在高压下的输运性质。FeSe是最近发现的超导体中最简单的一种，它可以作为研究铁磷属元素化物、铁硫属元素化物及其相关化合物的超导电性机制和结构与性质关系的模型体系，FeSe在母相中是超导的，不需要掺杂，而且超导电性强烈依赖于精确的化学计量。与砷化物不同，四方晶系到正交晶系的转变与超导相有关。当FeSe可逆转变为NiAs型结构时，超导电性在压力下消失。用~（57）Fe-Mössbauer谱测量了整个压力-温度相图，观察到了非静态磁性。然而，77 Se-NMR光谱提供了短距离磁波动的证据。为了解决磁性、自旋涨落、量子临界性和/或声子在FeSe中是否重要的问题，我们想研究结构、磁性、声子态密度和电阻率作为压力、磁场和温度的函数，所研究的系统将是简单的模型系统，如FeSe，FeSe_（1-x）Tex，FeSe_（1-x）Sx，CuxFe_（1-x）Se，RuxFe_（1-x）Se，FeSe-CoAs，TlFe_2Se_（2-x）Asx。本论文的目标是完成FeSe的相图，并解决上述问题。

This project focuses on the investigation of the transport properties of FeSe and related compoundsunder high pressure. FeSe is the simplest of the recently discovered superconductors andcan serve therefore as a model system to gain insight into the mechanism of superconductivityand the structure to property relation of the iron pnictides, iron chalcogenides and related compounds.FeSe is superconducting in the parent phase, no doping is necessary; even more the superconductivity depends strongly on the exact stoichiometry. Different from the arsenides, atetragonal to orthorhombic transition is connected with the superconducting phase. Superconductivitydisappears under pressure, when FeSe transforms reversible into a NiAs-type structure. Nostatic magnetism is observed over the whole pressure vs. temperature phase diagram, measuredby 57Fe-Mössbauer spectroscopy. However, 77Se-NMR spectroscopy gives evidence for shortrange magnetic fluctuations. To solve the question whether magnetism, spin fluctuations, quantumcriticality and/or phonons are important in FeSe we want to investigate structure, magnetism,density of phonon states and resistivity as a function of pressure, magnetic field and temperature.The systems under investigation will be simple model systems such as FeSe, FeSe1-xTex, FeSe1-xSx, CuxFe1-x Se, RuxFe 1-x Se, FeSe–CoAs, TlFe2Se 2-x Asx. The goal is to complete thephase diagram of FeSe and to solve the above addressed questions.