【作者】 胡健；

【导师】 吴国庆； 王秋良；

【作者基本信息】 扬州大学 ， 工程硕士（专业学位）， 2020， 硕士

【摘要】 拓扑半金属是具有奇异物理特性和重要科技应用的一种新颖拓扑量子材料。拓扑半金属在其费米能级附近,导带和价带间有重叠的部分而形成有限个点或者节线圈。根据其能带结构及能级简并特征,拓扑半金属主要可分为狄拉克半金属(Dirac semimetal)、外尔半金属(Weyl semimetal)和节线半金属(Nodal-line semimetal)。这种拓扑能带结构由于受到时间反演对称性和空间反演对称性的保护,因而拥有很强的稳定性。高压强(high pressure)是一种环境参数,可以缩短材料内原子间距离,增加相邻电子间的轨道重叠,可引起材料内晶体结构发生变化,导致超导、磁性或电性的有序排列等各种可能的相变,以及破坏拓扑半金属材料的拓扑对称性保护等,从而高压强可以导致新的物理现象或者全新的量子态。核磁共振(nuclear magnetic resonance,NMR)是指处于外磁场中的材料内的原子核系统受到相应频率的电磁波的辐射时,在其自旋磁能级之间发生的共振跃迁现象(当电磁波辐射的能量hv,恰好等于原子核自旋相邻两磁能级间的能量差ΔE时,处于低能态的核自旋吸收电磁辐射能跃迁到高能态)。核磁共振由于其可深入材料内部而不破坏样品,并具有迅速、准确、分辨率高等优点,而广泛用于分析材料的性质及用来研究材料性质变化的物理学机制。本文利用高压强和核磁共振技术对拓扑节线半金属材料ZrGeSe的基本物理学性能进行了深入的研究。本文的工作主要有三个部分。第一部分,关于高压实验平台及其电性测量系统的设计及构建。文中介绍了本人搭建的高压强实验平台及自主设计并加工制成的高压强电性测量探杆设备,叙述了自主组建的电阻测量的电子电路系统,并对高压强的获得即金刚石对顶压砧(DAC)的使用,及给ZrGeSe施加高压强的实验过程做出了系统性的阐述。第二部分,关于高压强下ZrGeSe的电学性质的研究。通过测量出的ZrGeSe的电阻-温度曲线,结果发现随着对样品施加的压强的增加,样品的电阻明显降低,即材料的导电性能明显增强。通过对理论模型的分析发现,随着压强的增加,ZrGeSe导电的散射机制发生了变化。当施加的压强达到～50GPa(1 GPa=10000大气压),虽然材料的导电性能得到明显增强,但并没有观察到超导现象或结构相变的发生。进一步的高压强实验测量发现,在高压强环境下(24.55 GPa)加入磁场(≤16 T)后,该材料并没有像常压环境下那样出现磁场导致的金属-绝缘体交叉(metal-insulatorcrossover)现象,而是表现出很强的金属性质,即高压强有效抑制了强磁场下能隙的出现。这也说明了在高压强下ZrGeSe的拓扑能带结构的稳定性得到增强。第三部分,关于ZrGeSe的核磁共振实验研究。通过对77Se-核的核磁共振光谱和奈特频移(Knightshift)的测量,结果发现在不同磁场环境条件下,随着温度的降低,在金属-绝缘体交叉温度TMI附近,77Se-核的核磁共振光谱并没有出现分裂,但谱线宽度(the full width at half maximum,FWHM)明显增加,即77Se-核周围的电子的自旋产生的磁场的静态分布发生了变化,表明77Se-核周围磁场大小的分布的不均匀性(field inhomogeneity)得到显著增加,也就是材料内的电子在金属-绝缘体交叉温度TMI处出现了由磁场导致的自旋排列的有序(spinorder)。这种由磁场导致的自旋排列有序,符合物理学上的条纹有序(stripe ordering)现象。为了进一步观测金属-绝缘体交叉温度TMI处出现的由磁场导致的自旋有序的发生,我们进行了 77S-核的自旋晶格弛豫时间T1的测量。通过测量,结果发现在金属-绝缘体交叉温度TMI以上,1/T1T与温度T之间不满足科林伽(Korringa)关系,而是存在一定的电子自旋反铁磁相关和自旋涨落。并且,1/T1T在金属-绝缘体交叉温度TMI处,随温度的降低而出现突然的减少,即电子自旋的磁化率χs(T)在金属-绝缘体交叉温度处发生了减少的突变,从而进一步揭示了金属-绝缘体交叉温度时电子自旋排列发生了有序的变化和有自旋能隙的出现。因此,结合77Se-核的核磁共振光谱、奈特频移和77Se-核的自旋晶格弛豫时间T1的实验数据,我们发现由磁场导致的自旋排列的有序变化,是导致拓扑节线半金属ZrGeSe在强磁场条件下出现金属-绝缘体交叉现象的根本原因。因而,核磁共振实验,从原子尺度上展示了金属-绝缘体交叉温度附近,77Se-核周围电子自旋的静、动态性质,从而有效揭示出了拓扑节线半金属ZrGeSe在强磁场条件下出现的金属-绝缘体交叉现象的物理学机制。更多还原

【Abstract】 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=ΔE,where ΔE 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.更多还原