Quantum Phase Transition And Properties Of Several Semimetals Under High Pressure

Emergence indicates the exotic phenomena or properties that caused by the collective behavior in many-body systems,which is common in condensed matter,especially in the strongly correlated electron system.In modern physics,more emergent phenomena were observed,including high-temperature superconductor,Mott transition,topological insulator etc.,and this gives rise to the broader concept of quantum material,for which the traditional theories in condensed matter physics cannot be applied anymore due to the limitation of simple-electron state.We can tune their properties by controlling the quantum effect,from which the quantum transitions are induced and support essential information for understanding the underlying interactions between elements.Recently,the topological semimetal has attracted intense attention due to the exotic electronic band structure.The linear dispersion Dirac cones exist near the Fermi level and form the special Dirac node,Weyl pair or nodal-line in the momentum space,giving rise to some novel properties.The pressure can be used as a useful and "clear" tool to easily change the band topology by introducing volume collapse or symmetry breaking.By adopting high-pressure measurements of transport property,Raman spectroscopy,x-ray diffraction and the first principle calculations,the topological phase transitions and superconductivity have been observed in Fermi liquid 1T-TiTe2,nodal-line semimetal ZrSiSe and the extreme magnetoresistance(XMR)materials of LaSb.The main contents are follows:In Chapter Ⅰ,the theoretical and experimental progress of quantum material are briefly reviewed,mainly on the topological quantum material and superconductivity.The general picture on pressure effect are addressed as well.In Chapter Ⅱ,the high pressure instrument,diagnostic and the relevant method which were used in this thesis are introduced,including the diamond unveil cell(DAC),the transport measurement platform,Raman spectroscopy,x-ray diffraction,the single crystal growth methods of chemical vapor transport(CVT)and mental flux method.In Chapter Ⅲ,the pressure induced topological phase transitions in 1T-TiTe2 are reported.The semimetal 1T-TiTe2 single crystal was grown by chemical vapor transport method,and the high-pressure Raman spectroscopy and resistivity measurement were performed.According to the changes of the pressure coefficients of Raman shift and resistivity,three singularities were found at 1.7 GPa,3 GPa,and 8 GPa,respectively,leading to four phases of Ⅰ,Ⅱ,Ⅲ,Ⅳ.Through the VASP package,the orbital projection band structure were calculated under high pressure.The orbital hybridization show enhancement and consequently some bands inversion appear at some pressure points.Band inversion can be the signal of the topology.Here,because of the inversion symmetry in 1T-TiTe2 system,parity analysis can be utilized to easily calculate the Z2 topological invariant.By comparing the experimental and calculated results,we found that phase Ⅱ and phase Ⅲ correspond to a strong topological state and a weak topological state,respectively,while phase Ⅳ is another structural phase.In a word,Thus,under high pressure,the rich phase transitions were discovered in 1T-TiTe2 with the path of nontopology-strong topology-weak topology-structural phase transition.The system seems provide an ideal platform to study the topology and possible applications.In Chapter Ⅳ,two electronic topological phase transitions(ETT)and one isostructural phase transition were observed in nodal-line semimetal of ZrSiSe under high pressure.ZrSiSe single-crystals were prepared by chemical vapor transport.And the high pressure transport properties,Raman spectroscopy,and synchrotron x-ray diffraction(HP-XRD)were measured.ZrSiSe undergoes two ETTs and one isostrucrural phase transition at 3.6 GPa,6.8 GPa,11 GPa,respectively.At 11 GPa,the HP-XRD results indicate one minimum value of the axial ratio c/a.This is related to the appearance of the maximum value of the Se-Zr-Se bond angle,which is induced by the change of the atomic occupation under high pressure.Meanwhile,the evolutions of the band structure and Fermi suface with pressure were calculatd by the first-principles calculations.Under lower pressure,the present results reveal that the electric band structures and the Fermi surfaces are significantly modulated by pressure.The hole pocket along the Γ-M path escaped from the Fermi level with increasing pressure and the crossing between electrons pocket and Fermi level appears near Γ at higher pressure.The movement of the electron and hole pockets cause significant changes in the Fermi surfaces shape and further introduces two ETTs at 3.6 GPa,6,8 GPa,respectivily.Therefore,a series of discontinuous changes in Raman modes and resistance were observed corresponding to the phase transitions.This work open a new avenue for understanding ZrSiSe and motivates the further exploration of novel electronic states in the WHM family with high pressure.In Chapter Ⅴ,we investigated the temperature-pressure diagram of LaSb.The electrical transport measurements conducted in this work reveal the superconductivity rooted in the high-pressure PT phase of LaSb.The Tc-P phase diagram was obtained,and,corresponding to the transition from the B1 to PT phase,an abrupt drop appears both in residual resistance and MR between 10.8 and 13.7 GPa,where a typical superconducting dome starts to form.Based on the previous electronic band calculations,the superconductivity is attributed to the significant increase of N(EF)due to structural instability.Conclusively,one mixed phase containing B1 and PT phases is proposed in the range 10.8-13.7 GPa,leading to the initial increase of Tc,after which a typical linear decrease of Tc is observed.In the B1 phase,possible signals of the topological phase transition near 5.5 GPa are also found,but further confirmation is needed.In light of the common structural phase transition occurring in LnX,more superconducting compounds and related physics can be expected in the entire LaX family.It should be noted that the superconductivity identified in the B1 phase of LaBi was not observed in LaSb,and the distinct transport properties reported here indicate promising prospects for further investigation of the LaX family,in particular the elements’ effects and the high-pressure behaviors.In Chapter Ⅵ,we give the summary and a general outlook based on the works we discussed above.