Pressure Induced Superconductivity In Several Topological Materials

Recently,high-pressure techniques have been widely used in various fields,such as condensed matter physics,materials,chemistry,environmental and planetary sciences,etc.Many systems exhibit abundant physical properties under high-pressure.For instance,there is a metal-insulator transition in alkali metal sodium under high pressure;the topological material Bi2Se3 becomes superconducting under high-pressure In this thesis,combining high-pressure techniques and first-principles calculations,we have systematically studied the high-pressure induced superconductivity in three different topological materials:ZrTe5,BiI and Au2Pb.First of all,we studied the properties of ZrTes under high pressure.ZrTes is one of the typical thermoelectrical materials.In recent years,ZrTe5 is found to be a topological material.As a new type of topological materials,ZrTe5 shows many exotic properties under extreme conditions.Using resistance and ac magnetic susceptibility measurements under high pressure,while the resistance anomaly near 128 K is completely suppressed at 6.2 GPa,a superconducting transition emerges.The superconducting transition temperature Tc increases with applied pressure,and reaches a maximum of 4.0 K at 14.6 GPa,followed by a slight drop and then remaining almost constant value up to 68.5 GPa.At pressures above 21.2 GPa,a second superconducting phase with the maximum Tc of about 6.0 K appears and coexists with the original one up to the maximum pressure studied in this work.In situ high-pressure synchrotron X-ray diffraction and Raman spectroscopy combined with theoretical calculations indicate the observed two-stage superconducting behavior is correlated to the structural phase transition from ambient Cmcm phase to high-pressure C2/m phase around 6 GPa,and to a mixture of two high-pressure phases of C2/m and P-1 above 20 GPa.The combination of structure,transport measurement,and theoretical calculations enable a complete understanding of the emerging exotic properties in 3D topological materials under extreme environments.Then,using a machine-learning accelerated crystal structural search,and ab initio calculations together with high-pressure Raman measurements,we studied the crystal and electronic structures of bismuth iodide(BiI)systematically up to 50 GPa.We found that the ambient C2/m phase(β-Bi4I4)transforms across a tetragonal P42/mmc phase at 8.5 GPa and finally to a hexagonal P63/mmc phase at 28.2 GPa.Our high-pressure Raman experiments identified a phase transition at around 8.6 GPa,and the experimental Raman modes evolution agrees with our calculations reasonably.Band-structures calculations suggest the BiI system undergoes a pressure-induced topological phase transition from a topological metal(P42/mmc phase)to a trivial metal(the P63/mmc phase).Our electron-phonon coupling calculations show both the P42/mmc and P63/mmc phases are superconductors and the estimated superconducting critical temperature agrees with previous measurements.Our study shows the previously reported pressure-induced superconductivity in BiI should originate from structure phase transitions.Au2Pb is a candidate of natural topological superconductors that has attracted much attention in the last few years.Combining ab initio calculations with machine-learning accelerated crystal structure searches,we found two ground states of Au2Pb:Pca21 phase at ambient pressure and 1-42d phase at high pressure.The Pca21 phase is energetically more favorable than the known Pbcn structure and can be a candidate for x-ray diffraction refinement;our calculations suggest that high-pressure I-A2d is a conventional superconductor.By the high-pressure electric resistance measurements,we observed evidence of a Tc enhancement.Tc reaches a maximum value of around 4 K at 5 GPa,then decreases with further compression.The superconductivity can remain unchanged after pressure releasing,in line with our theoretical predictions.These results show that Au2Pb exhibits abundant behaviors under pressure and temperature,which can help to understand how to adjust its electronic properties by external conditions.