Physical Properties Of Several Topological And Correlated Electronic Materials Under High Pressure

Topology is a mathematical concept used to describe the overall geometric properties that do not change with continuous deformation.In recent years,people have introduced the concept of topology into semimetal materials,and developed the concept of topological semimetal,i.e,the band structure contains the degeneracy point where the conduction band and the valence band linearly cross.According to the degeneracy of crossing points,topological semimetals can be classified into Dirac semimetals,Weyl semimetals,nodal semimetals or triple degeneracy semimetals.There are many novel quantum effects and phenomena in these materials,such as quantum anomalous Hall effect,possible Majorana zero-energy mode,extreme magnetoresistance effect and negative magnetoresistance effect caused by chirality anomalies,etc.These discoveries broaden people's understanding of condensed matter physics.The novel physical properties of materials also provide a new direction for the development of low energy consumption electronic devices and topological computing devices.The introduction of correlation effects in topological materials will induce new physical properties and new phenomena.Kondo lattice materials have always been an important research content in the field of strong correlation.The charge,spin,orbital,degree of freedom of the lattice in this type of material are strongly coupled to form a variety of quantum states such as large effective mass fermions,magnetic order,unconventional superconducting states,non-Fermi liquids,etc.Strong correlated topological materials maybe found in this system.The ground-state interaction energy scale of strongly correlated electronic materials is equivalent and can be easily affected by the external environment.It also can be tuned by means of magnetic field and pressure.After decades of development,pressure has become a common experimental method in laboratories.In particular,diamond anvil cell equipment develop rapidly has become one of the main tools for high pressure.At present,the high hydrostatic pressure has reached the order of millions of atmosphere,which can effectively tune the crystal structure and topological electronic state of the material.Therefore,we selected several types of topological semimetals and strong correlated electronic materials,and carried out high pressure experiments by using diamond anvil cell.The research content is mainly as follows:1.Pressure effect on Dirac-like semimetal PdSn4This chapter mainly introduces the experimental results of high pressure electric transport and high pressure synchrotron radiation X-ray diffraction of topological semi-metal PdSn4.The high pressure XRD results show that the crystal structure exists stably in ambient pressure range within the pressure range of 0?44.5 GPa,and no structural phase transition was observed.The high pressure resistance conforms to the Fermi liquid behavior in the low-temperature section and does not change with pressure.At 3.2?20.0 GPa,the magnetoresistance and the high magnetic field maintain a linear relationship.We believe that the topological characteristics of PdSn4 are robust to pressure.2.Pressure induced structural modulation in Dirac-like semimetal LaAgSb2 single crystalIn this chapter,we use high pressure methods to study the relationship between the charge density wave and the topological state of LaAgSb2.We systematically studied high pressure electrical transport property,high pressure synchrotron X-ray diffraction,and high pressure Raman experiments of LaAgSb2.High pressure electric transport experiments show that the charge density wave is gradually suppressed under pressure,and disappears around PC?22 GPa.The resistance at 300 K increases abruptly at the Pc.In LaAgSb2,the carrier at ambient pressure is dominated by holes,the Hall coefficient at Pc suddenly changes sign from positive to negative,and the carriers become dominant by electrons.In the range of 1.7?41.6 GPa,there is no obvious change in the structure,but there is a change in the slope of the lattice constant ratio c/a at Pc.The high pressure Raman spectrum shows a new peak at Pc.Combining these experimental results,we believe that LaAgSb2 undergoes an isostructural phase transition at the Pc.3.High pressure effect on colossal magnetoresistance effect in n-type CdCr2Se4 polycrystallineThis chapter mainly introduces the evolution of electrical transport with pressure in p-type and n-type CdCr2Se4 polycrystalline.The high pressure transport measurements show that the resistance of p-type polycrystalline is a semiconducting behavior without colossal magnetoresistance effect.The resistance decreases monotonously under pressure,and shows no signs of metallization up to 27.9 GPa.In n-type CdCr2Se4 polycrystalline,the colossal magnetoresistance effect is suppressed by the pressure and disappears around 9 GPa.The high-pressure magnetization measurements show that the magnetic ground state of n-type CdCr2Se4 polycrystal is gradually suppressed by pressure and disappears at the same pressure point.By analyzing high pressure XRD data,we found that the bond length is decreased below 10 GPa,which causes the reduction of ferromagnetic interaction,resulting in the disappearance of the colossal magnetoresistance effect.At 11 GPa and 15 GPa,the structural phase transitions occur,which is consistent with previous reports.4.High pressure study of topological Kondo lattice semimetal CeSbTeThis chapter mainly introduces the evolution of CeSbTe electrical transport and structure under pressure.At ambient pressure,temperature dependence of resistance is semiconducting behavior.As pressure increases to 10 GPa,the resistance suddenly drops at the low temperature,and around 25 GPa,drop behavior disappears completely and becomes semiconducting behavior again.At higher pressures,the resistance behavior gradually becomes metallized.The series of changes in resistance under pressure are closely related to the lattice structure.Through the analysis of the high-pressure XRD data,we preliminarily believe that the separate of bragg peak at about 10 GPa is related to the different compressibility between lattice parameters a and c under pressure.The structural phase change occurs at 25 GPa from tetragonal phase to orthorhombic phase.Therefore,the change in the structure leads to an abnormal behavior in resistance.5.Summary and outlook of the full text...