Pressure-induced Quantum Order In Strong Spin-orbit Coupling Materials

The new quantum functional materials,Rashba spin-orbit coupling(SOC)semiconductors BiTeX(X=Cl,Br,I)and topologically trivial material LaB(B=Sb,Bi)with extreme magnetoresistance(XMR),are predicted to realize topological phase by pressure.Strong spin orbital interaction(SOI)can induce unique quantum phenomena such as topological insulators,the Rashba effect,or p-wave superconductivity.Combining these three quantum phenomena into a single compound has important scientific implications.As the semimetals LaB will show abundant physical properties at external field or pressure,an ideal model system to formulate a theoretical understanding of the exotic consequences of breaking time reversal symmetry in topological semimetals.The main content of the thesis focuses on high pressure effects of new quantum functional materials,Rashba semiconductors BiTeX(X=Cl,Br,I),and XMR semimetals La Sb.1.Here we report experimental observations of consecutive quantum phase transitions from a Rashba type topological trivial phase to topological insulator state then further proceeding to superconductivity in a SOI compound BiTeI tuned via pressures.The electrical resistivity measurement with V shape change signals the transition from a Rashba type topological trivial to a topological insulator phase at 2GPa,which is caused by an energy gap close then reopen with band inverse.Superconducting transition appears at 8 GPa with a critical temperature T_C of 5.3 K.Structure refinements indicate that the consecutive phase transitions are correlated to the changes in the Bi–Te bond and bond angle as function of pressures.The Hall Effect measurements reveal an intimate relationship between superconductivity and the unusual change in carrier density that points to possible unconventional superconductivity.2.The compound BiTeCl is expected to develop a topological nontrivial state via pressure based on the research on BiTeI material.Measurements of resistivity,Hall coefficient,synchrotron X-ray diffraction and Raman spectroscopy are performed on Rashba semiconductor BiTeCl crystal at high pressures up to 54.8 GPa.Our experiments indicate Rashba semiconductor BiTeCl has a pressure induced metal to insulator like transition at 2.8 GPa.The pressure induced structure phase transition is observed above 5.5 GPa,which is followed by a superconducting transition.The superconducting transition temperature(T_C)increases to a maximum 4.2 K with pressure up to 32 GPa.Compared with BiTeI,the structure phase transition of BiTeCl takes place at relative lower pressure as a result of the weaker Rashba effects,which will introduce the inner chemical pressure because of replacing I with Cl.The observed superconductivity induced at pressures from the insulator like precursor phase points to a possible unconventional nature.By comparing property of BiTeBr under high pressure,we find that the Rashba structural phase transition to orthorhombic Pnma with superconducting state is a universality in BiTeX(X=Cl,Br,I).The structure transition pressure increasing with halogens Cl,Br,I,the atomic radius.These properties can be summarized as the commonness and regularity of these materials.3.Here we report experimental observations of quantum phase transition from XMR to superconductivity,further structure transition in LaSb.By increasing pressure,we observed the suppressive XMR followed by the emergence of superconductivity at 5.2GPa.Above 13.9 GPa,we discovered a structure phase transition from cubic lattice to tetragonal lattice.(1)MR under 7 T can reach 10~5percent level,at 3.5 GPa pressure MR was inhibited to 10~0 level,the pressure induced XMR suppression is an electronic effect without drastic structural parameters changes.(2)After the XMR is inhibited,the semiconductor phase is induced by pressure in LaSb,accompanying with the superconducting phase transition.Superconducting transition temperature is gradually suppressed by pressure at ambient structure.(3)Above 33 GPa,the rate of resistance dropping below T_C may be related to the increasing of contents of superconductivity phase.