Pressure And Strain Effects In Layered Transition-Metal Chalcogenides

The layered transition metal chalcogenides MXn(M = transition metals;X = S,Se,Te)have attracted a lot of research interest,such as the the '11' iron-based super-conductor FeSe and the semiconductor MX2(M = Mo,W;X = S,Se).The FeSe superconductor is the structurally simplest one among the iron-based superconductors,and the superconducting Tc ? 9 K of FeSe sensitively depends on the structure and microstructure and can be significantly enhanced up to 37 K by physical pressure,or to above 100 K in single-layer films.In contrast to FeSe,MX2 is a semiconductor with layered hexagonal structure.It has promising properties for optoelectronic applications.The optoelectronic properties as well as the electronic structure sensitively depend on the structure and microstructure.Pressure is an effective method to tune the structure and microstructure of a con-densed mater system.By studying the effects of applied pressure,compressive strain,and chemical pressure induced by element substitutions on the structure,magnetism,and electronic properties of a material,one can achieve deep insight into the under-lying physics.In this thesis,we studied the biaxial compressive strain effect on the superconductivity in FeSe,the substitution effect of Sb for Te on the electrical transport properties,thermal dynamics,magnetism,and structure in FeTe,and the high pressure effect on the structure and electrical properties in WSe2.The main contents are included as follows:The first chapter is an introduction to the "11" family of iron-based superconduc-tors and transition metal diachalcogenides MX2(M = Mo,W;X = S,Se).The second chapter,the enhancement of superconductivity in FeSe thin crystals induced by in-plane biaxial compressive strain,with an underlying Scotch tape as an in-situ strain generator.It is found that,due to the compressive strain,the superconducting transition temperature Tc ? 9 K of FeSe is increased by 30%-40%and the upper critical field H,2(0)? 14.8 T is increased by?20%.In parallel,the T*,which characterizing an onset of enhanced spin fluctuations,is raised up from 69 K to 87 K.On the other hand,the structural transition temperature Ts ? 94 K,below which an orthorhombic structure and an electronic nematic phase settle in,is suppressed down by~5 K.These findings reveal clear evolutions of the orders/fluctuations under strain effect in FeSe,the structurally simplest iron-based superconductor where the lattice/spin/charge degrees of freedom are closely coupled to one another.The third chapter,the doping effects of Sb on the magnetic,transport and structural properties in FeTe1-xSbx single crystals.We synthesized a series of FeTe1-xSbx single crystals and investigated the doping effects of Sb on the magnetic and transport proper-ties.Structural analysis shows that Sb doping induces an expansion of the lattice along the a axis and a shrinkage along the c axis,equating to the uniaxial pressure effect.Re-sistivity,magnetic susceptibility and heat capacity experiments consistently reveal that the magnetic/structural transition temperature TN?70 K in undoped Fe1.05Te is grad-ually suppressed by Sb doping.Heat capacity and Hall coefficient results consistently indicate that hole carriers are successfully introduced,but no superconductivity is ob-served for x up to 10%.These results indicate that the antiferromagnetism in the parent Fe1+yTe might have a different microscopic origin than the itinerant SDW magnetism in the iron pnictides.The forth chapter,pressure-induced iso-structural phase transition and metalliza-tion in WSe2.We present in situ high-pressure synchrotron X-ray diffraction and Raman spectroscopy study,and electrical transport measurement of single crystal WSe2 in di-amond anvil cell up to 62.8 GPa.The XRD and Raman results show that the phase undergoes a pressure-induced iso-structural transition via layer sliding,beginning at 28.5 GPa and not being completed up to around 60 GPa.The Raman data also reveal-s a dominant role of the in-plane strain over the out-of-plane compression in helping achieve the transition.Consistently,the electrical transport experiments down to 1.8 K reveals a pressure-induced metallization for WSe2 through a broad pressure range of 28.2-61.7 GPa,where a mixed semiconducting and metallic feature is observed due to the coexisting low-and high-pressure structures.The fifth chapter,summary and prospect.