High Pressure And NMR Investigation On Nodal-line Topological Semimetal ZrGeSe

Topological semimetal is a novel type of topological quantum material which can show singularity in physical property and has important technological applications.Near the Fermi energy of a topological semimetal,there are superimposed parts between valance band and conduction band that form finite points or a nodal line.Based on its energy band structure and the character of its energy degeneracy level,topological semimetal can be mainly divided by Dirac semimetal,Weyl semimetal and Nodal-line semimetal.Its energy band has strong stability,since its band structure has protections by time reversal symmetry and inversion symmetry.High pressure is an environmental parameter,which can shorten the distance between atoms in a material,increase an orbital superposition,cause crystal structure changes,lead to various phase transitions,such as superconductivity,magnetic and/or electronic order,and destroy topological symmetries.Thus high pressure can induce new physical phenomenon or new quantum states.Nuclear magnetic resonance(NMR)is a phenomenon where resonance transition occurs between neighboring energy levels of nuclear spins,when the nuclear system in a material receives radiations from electromagnetic waves in certain frequency(the nuclear spins at a lower energy level receive radiations of electromagnetic waves and make a transition to a higher energy level,when the radiation energy hv=?E,where ?E is the energy difference between the neighboring energy levels).Because NMR happens inside a material and does not damage the sample material,and has advantages in measurement accuracy and resolution,it has been widely used in analyzing materials'property and studying the mechanism of the property changes.This thesis is a report for the investigation of the basic physical properties of the topological semimetal ZrGeSe,using the techniques of high pressure and NMR.There are three major aspects in this work.First,it is about the design and construction of the high pressure experimental platform and the electronic property measurement system designed by ourselves.In the text,the self-designed and constructed high pressure experimental platform and the self-made probe for the high pressure electronic property measurements are introduced.The self-designed electronic circuit system for the measurements of the high pressure electronic properties is also described.There is a detailed description for the use of the diamond anvil cells(DAC)and for the process of the high pressure applications as well.Second,it is about the study of the electronic properties of ZrGeSe under high pressure.With the increase of high pressure,we found that the sample resistance decreases apparently,i.e.,the conductivity of it increased significantly.With the analysis of a theoretical model,we found that the scattering mechanism of its conductivity is changed by the application of high pressure.When high pressure reaches?50 GPa(1 GPa=10000 atm),even though its electrical conductivity is improved,there is still no appearance of superconductivity or structure phase transition.By further high pressure measurements,we found that with the application of magnetic field(up to 16 T)under high pressure at 24.55 GPa it does not show the phenomenon of metal-insulator crossover,which is unlike the case under normal pressure environment.Instead,it shows a strong metallic phase,indicating that energy gap is fully restricted by high pressure.This also means that the stability of the energy structure of ZrGeSe in enhanced under high pressure.Third,it is about the nuclear magnetic resonance(NMR)investigation.We found that there is no occurrence of the breaking of the 77Se-NMR spectrum lines of ZrGeSe in the temperature regime near where the metal-insulator cross-over appears,through the measurements of the 77Se-NMR spectrum and Knight shift under various applied magnetic field.However,the 77Se-NMR linewidth(FWHM)broadens significantly,i.e.,the distribution of the static local magnetic field surrounding the 77Se-nucleaus broadens,indicating that the local field inhomogeneity around the 77Se-nucleaus is increased,i.e.,there is an occurrence of the electron spin order at the metal-insulator cross-over temperature TMI.This magnetic field induced electron spin order matches the so-called stripe order in physics.For further observation of the magnetic field induced electron spin order at the metal-insulator crossover temperature TMI,we performed the measurements of the 77Se-NMR spin-lattice time T1?We found that the spin-lattice time,plotted as 1/T1T versus T,does not satisfy the Korringa law in the temperature regime above TMI,while it shows antiferromagnetic spin correlations and electron spin fluctuations.Below TMI,1/T1T decreases suddenly,which indicates that there is an immediate decrease of the electron spin susceptibility,further revealing the electron spin order and a spin energy gap opening correspondingly at the metal-insulator crossover temperature TMI?Thus,considering all the data of the 77Se-NMR spectrum.Knight shift and spin-lattice relaxation time of ZrGeSe,we conclude that the magnetic field induced spin order is the mechanism of the occurrence of the metal-insulator crossover in ZrGeSe under the application of magnetic field.Therefore,our NMR experiment exhibited the electron spin dynamics of the topological nodal-line semimetal ZrGeSe at atomic scale,and the mechanism for the occurrence of the magnetic field induced metal-insulator crossover in ZrGeSe is revealed.