High Pressure Synthesis And Pressure Effect Of New Quantum Functional Materials

The thesis focuses on the high pressure synthesis of cuprate superconductor and high pressure effect of new quantum functional materials,such as iron based superconductor and topological insulators.The main results include:(?)A Sr2CuO3+? superconductor with a new modulated structure has been synthesized using high pressure technique.Two superconducting transitions with Tc~75K and~48K respectively were found in Sr2CuO3+? superconductor.A new type of modulated phase with a periodicity of 2?2ap×2?2ap×cp of Pmmm symmetry is found in the sample by using transmission electron microscopy.Our experimental results suggest that the new Pmmm modulated phase is responsible for superconductivity with Tc at 48 K,while C2/m modulated phase for that with Tc at 75 K found previously.(?)we conduct systematic high-pressure transport and structural characterizations of the newly discovered superconductor FeS.It is found that superconductor FeS(tetragonal)partly transforms to a hexagonal structure at 0.4 GPa,and then completely transforms to an orthorhombic phase at 7.4 GPa and finally to a monoclinic phase above 9.0 GPa.The superconducting transition temperature of tetragonal FeS was gradually depressed by pressure,different from the case in tetragonal FeSe.With pressure increasing,the S-Fe-S angles only slightly change but the anion height deviates farther from 1.38 ?.This change of anion height,together with the structural instability under pressure,should be closely related to the suppression of superconductivity.We also observed an anomalous metal-semiconductor transition at 6.0 GPa and an unusual increased resistance with further compression above 9.6 GPa.The former can be ascribed to the tetragonal-orthorhombic structural phase transition,and the latter to the electronic changes of the high-pressure monoclinic phase.Finally,a phase diagram of tetragonal FeS as functions of pressure and temperature was mapped out for the first time,which will shed new light on understanding of the structure and physics of the superconducting FeS.(?)We found pressure induced phase evolution in both electronic & crystal structures for HfTe5 with a culmination of pressure induced superconductivity.Our experiments indicated that the temperature for anomaly resistance peak(Tp)due to Lifshitz transition decreases first before climbs up to a maximum with pressure while the Tp minimum corresponds to the transition from a weak TI to strong TI.The HfTe5 crystal becomes superconductive above ~5.5 GPa where the Tp reaches maximum.The highest superconducting transition temperature(Tc)around 5 K was achieved at 20 GPa.Crystal structure studies indicate that HfTe5 transforms from a Cmcm phase across a monoclinic C2/m phase then to a P-1 phase with increasing pressure.Based on transport,structure studies a comprehensive phase diagram of HfTe5 is constructed as function of pressure.The work provides valuable experimental insights into the evolution on how to proceed from a weak TI precursor across a strong TI to superconductors.