The Evolutions Of Superconductivity,Charge Density Wave And Morphology-dependent Electrical Transport Behavior Of Materials Under High Pressure

Pressure can adjust the distance between atoms and strengthen the coupling of electron orbitals,thus tune the electron spin state,electron correlation,charge density distribution and band structure of materials.These microscopic changes of electronic structure can be reflected in macroscopic physical properties.Pressure introduces averagely five new phases in materials,which can greatly enrich the field of material science.It is also of great significance to solve some core issues in condenced mater physics,such the understanding of superconductivity,topology,and order structure of charge or spin in materials.This work focuses on the effect of pressure on several special properties related to the electrical transport process of materials,including superconductivity,charge density wave and morphology dependent electrical transport properties.The electrical transport preperties and structural evolution of several typical materials were investigated by in-situ X-ray diffraction(XRD),Raman scattering,electrical transport measurements and SEM/TEM characterization.The detailed contents are concluded as followed:1.The evolution of spin order and superconductivity of spin-ladder compound Ta Fe_1+yTe₃ under pressure has been studied.When pressure is applied,Ta Fe_1+yTe₃undergoes a spin flip from antiferromagnetic to ferromagnetic order at?3 GPa.The strong spin fluctuation induces superconductivity.Ferromagnetic phase and superconductivity coexist until the pressure reaches 10 GPa,then the superconductivity is strongly enhanced after the collapse of spin order.The maximum T_sc=6.1 K appears at 26 GPa.The pressure independently adjusts the spin state and superconducting state of the material without causing structural changes.The relationship between the spin order and superconductivity of the material is complex combineing coexistance and competition.Our findings deepen the understanding of the interaction of superconductivity,magnetism,and crystal structure in iron-based ladder compounds.2.The evolution of charge density wave and structure of layered charge density wave material 1T”-Nb Te₂ under pressure has been studied.The collapse of charge density wave is observed at about 20 GPa,which weakens the electron interaction and changes the band topology of the material.The collapse of the charge density wave is accompanied by the change of the polymerization state of the metal atoms in the layer from trimer to dimer,which is a new structural transformation mechanism different from other similar materials.Our researches are helpful to deepen the understanding of modulation of pressure on charge density waves in two-dimensional materials and the coupling between charge density waves and crystal structure.3.The morphology-dependent electrical and mechanical properties of Cu₂O polyhedron with submicron size under pressure were studied.Electrical properties of cubic,truncated octahedral and octahedral Cu₂O show different pressure dependence,which are caused by the combined interation of selective adsorption of oxygen on different crystal planes of Cu₂O and the modulation of pressure on the material surface/interface states.The anomalous changes in resistivity at 0.7-2.2,8.5,10.3 and21.6 GPa are caused by pressure-induced structural phase transitions.Pressure-induced nanotization results in sharp decreases in the resistivity of cubic and octahedral samples at 15 GPa,and of truncated octahedral sample at 20 GPa,which revealed the beneficial effect of lattice defects on electrical conductivity.The mechanical properties of the truncated octahedron are better than those of cube and octahedron,which provides guidance for the practical application of Cu₂O under relevant conditions.