High Pressure Probing Of Potential Superconductivity In Topological Semimetals And Electronic Correlated Metallic Phosphides

This Ph.D thesis mainly focuses on three semimetals(Cd3As2,Na3 Bi and PtBi2)with nontrivial topology and electronic correlated metals(CeAuBi2 and Eu(Fe0.82Ir0.18)2As2).By adopting high-pressure measurements of resistance,magnetoresistance,Hall resistance,specific heat,X-ray diffraction and absorption,the physical properties or the potential superconductivity of these materials have been studies.These results are expected to offer information for better understanding these materials.The main contents are as follows:In Chapter I,the theoretical and experimental progress of topological semimetals is reviewed.Simultaneously,the history and developments in heavy fermion superconductors and iron-based superconductors achieved at ambient pressure and high pressure are addressed.In Chapter II,high pressure instruments and methods which are used in this thesis are reported,including the high pressure-low temperature-magnetic systems,high-pressure transport,ac susceptibility,X-ray diffraction and absorption measurements in a diamond anvil cell and high-pressure specific heat measurement.In Chapter III,high pressure studies on three exotic semimetals are investigated.These semimetals are Cd3As2,Na3 Bi and PtBi2.In the studies on Cd3As2,it is found that applying pressure drives the sample to undergoes a structural phase transition from a tetragonal phase in space group I41/acd to a monoclinic phase in space group P21/c at 2.57 GPa.This structural phase transition breaks the Dirac semimetal state of Cd3As2,which is evidenced by the change of resistance behavior from a metallic state to a semiconducting state and the dramatic change in its Hall resistance and mobility at the phase boundary.In the high-pressure studies on Na3 Bi,it was found that the Na3 Bi also undergoes a structural phase transition from a hexagonal phase(space group P63/mmc)to a mixed phase of cubic(space group Fm3m)and another unknown structure below 2.19 GPa.No superconductivity was observed in the pressure range studied.In the high-pressure studies on PtBi2,it was found that its large magnetoresistance can be gradually suppressed by pressure,but still sustains at 36.77 GPa which is the highest pressure investigated.Moreover,the dependence of Hall resistance under different magnetic fields indicates that the electronic structure of PtBi2 is sensitive to pressure.In Chapter IV,high pressure studies on two kinds of electronic correlated metals,CeAuBi2 and Eu(Fe0.82Ir0.18)2As2,are addressed.The studies on heavy fermion compound CeAuBi2 found pressure-induced two possible magnetic phase transitions.No superconductivity was observed in the pressure range studied.Our high pressure X-ray diffraction and absorption results reveal that the two pressure-induced possible magnetic phase transitions are not caused by the structural phase transition or the change in valance.We proposed that it may be associated with the pressure-driven spin rotation of cerium ions.In the high-pressure studies of iridium doped Eu(Fe0.82Ir0.18)2As2 superconductor,it was found that the superconductivity and the ferromagnetic(FM)phase originated from Eu layers can coexist below 3.4 GPa,in which the TC is higher than Tm.When the Tm is higher than TC at pressure above 3.4 GPa,the superconductivity disappears and the FM phase prevails.Furthermore,applying magnetic field in the low temperature range at 0.16 GPa reveals that the superconductivity and the FM phase can coexist below 0.4 T,but the FM phase is completely suppressed when the magnetic field is applied above 0.4 T.These results have been discussed from the viewpoint of the interaction among electrons.