The Exploration And Research Of Superconductivity In Novel Metals Under High Pressure

Since the discovery of superconductvity in mercury by Kamerlingh Onnes in 1911,many people have devoted efforts in exploring new superconductors.J.G.Bednorz and K.A.Muller found the first high temperature superconductor family cuprate in 1986.Then in 2008,H.Hosono and his collaborators discovered the new iron-based high temperature superconducting family.The discovery and exploration of new super-conducting materials or families continuously stimulates the development of the super-conductivity physics,and provides the new platform for superconductivity theroy and experiments.Recently,more and more materials are discovered to show superconduc-tivity under high pressures.So,as an important method to find new superconductors,high pressure has been increasingly emphasized by scientists.While,high pressure not only provides the way of exploration of new superconductors,but also serves as a new method to study the mechanism of superconditivty.This thesis is based on the high pressure measurement technique to investigate the propertites of some novel metals and try to induce the superconductivity by high pressure.We also study the supercon-ducting propertites of semi-metal bismuth under high pressures.In Chapter one,we give a brief introduction to the history and development of superconductivity.We review some important superconducting families and disscuss the current status of exploration of new superconducting materials.In Chapter two,we briefly introduce the basic method for material synthesis,in-cluding solid-state reaction,flux method and chemical vapour transport.Chemical composition analysis,x-ray diffraction experiments,magnetism and resisivity measu-ments are also presented.In particular,magnetism and resisivity measuments under high pressures are introduced.In Chapter three,we report the study of doping and high pressure effect on MAX phase compound Cr2GaN.It belongs to P63/mmc space group.By chemical doping,we partially substituted gallium element with germanium element in this system.By our study,the solubility limit of germanium element is about 25%in Cr2Ga1-xGexN.The lattice parameter of a-axis becomes larger,while c-axis becomes smaller with ger-manium doping.In Cr2GaN,there is a phase transition at 170 K as revealed by both resistivity and magnetic susceptibility measurements at ambient pressure,this transi-tion may correspond to a possible spin-density wave ordering transition.The electrical transport and magnetic behaviour show very similar properties as that of the parent compounds of iron-based superconductors.By doping germanium to this system,the possible SDW transition was successfully suppressed.While with the application of a hydrostatic pressure up to 1.1 GPa,the transition temperature does not show a clear change.No superconductivity was observed in either Ge-doped sample or under a hydrostatic pressure.In chapter four,we report the discovery of supconductivity induced by high pres-sure measurements in Weyl semimetal TaP.Weyl semimetal defines a kind of material with three dimensional Dirac cones which appear in pair due to the breaking of spatial inversion or time reversal symmetry.Superconductivity is the state of quantum con-densation of paired electrons.Turning a Weyl semimetal into superconducting state is very important in having some unprecedented discoveries.In this chapter,by doing resistive measurements on a recently recognized Weyl semimetal TaP under pressure up to about 100 GPa,we observe superconductivity at about 70 GPa.The supercon-ductivity retains when the pressure is released.The systematic evolutions of resistivity and magnetoresistance with pressure are well interpreted by the relative energy shift between the chemical potential and paired Weyl points.Calculations based on the den-sity functional theory and high pressure x-ray diffraction measurements also illustrate the structure transition at about 70 GPa,the phase at higher pressure also has Weyl semi-metal property.Our discovery of superconductivity in TaP by pressure will stim-ulate further study on superconductivity in Weyl semimetals.In chapter five,we study the basic superconducting prepertites of semi-metal bis-muth under high pressures.The temperature dependent resistivity shows a semimetal-lic behavior at ambient and low pressures(below about 1.6 GPa).This is followed by an upturn of resistivity in low temperature region when the pressure is increased,which is explained as a semiconductor behavior.This feature gradually gets enhanced up to a pressure of about 2.52 GPa.Then a non-monotonic temperature dependent re-sistivity appears upon further increasing pressure,which is accompanied by a strong suppression to the low temperature resistivity upturn.Simultaneously,a superconduct-ing transition occurs at about 3.92 K under a pressure of about 2.63 GPa.With further increasing pressure,a second superconducting transition emerges at about 7 K under about 2.8 GPa.For these two superconducting states,the superconductivity induced magnetic screening volume is quite large.As the pressure is further increased to 8.1 GPa,we observe the third superconducting transition at about 8.2 K.The upper critical field(Hc2)for the phase with Tc = 3.92 K is extremely low.Based on the Werthamer-Helfand-Hohenberg(WHH)theory,the estimated value of Hc2 for this phase is about 0.103 T.While the upper critical field for the phase with Tc = 7 K is very high with a value of about 4.56 T.Finally,we present a pressure dependent phase diagram of Bi single crystals.Our results reveal the interesting and rich physics in bismuth single crystals under high pressure.A summary of this thesis is presented in the end.