【作者】 张波；

【导师】 孙喆；

【作者基本信息】 中国科学技术大学 ， 核科学与技术， 2020， 博士

【摘要】 自三维拓扑绝缘体被实验发现之后,拓扑半金属材料备受科研领域的青睐。拓扑半金属是费米能级附近受晶格对称性保护由体态电子形成能带简并点的一类具有金属态的物质。不同于在动量空间中形成离散简并点的狄拉克半金属(DSM)和外尔半金属(WSM),拓扑节点线半金属(TNLSM)的能带简并点可以形成更具复杂性的一维构型,例如节点线、节点环、节点链等,同时在实验中发现了巨磁阻、超导等新颖的物理现象,使得其在凝聚态物理领域的研究热度一直高居不下。材料的物理和化学性质往往由体系内部的电子行为所决定,因此对于材料电子结构信息的探索至关重要,而角分辨光电子能谱(ARPES)是唯一可以直接探测材料三维电子结构的实验技术。在这篇论文中,我们主要通过ARPES技术实现对层状拓扑半金属电子结构的测量,取得了如下的研究成果:1.利用ARPES和第一性原理计算,我们系统研究了拓扑节点线半金属ZrSnTe和ZrSiTe的电子结构。通过进行大范围的动量空间和光子能量依赖测量,识别了来自体态和表面态的电子结构。同时沿着M-A和X-R方向上Dirac节点线的发现无疑证实了 ZrSnTe和ZrSiTe是拓扑Dirac节点线半金属。除了在WHM系列中普遍存在的floating band外,我们在ZrSiTe中还发现了具有不同能带趋势的鼓膜状表面态和无间隙的表面态。通过表面钾原子沉积实验,沿X-R方向发现了位于费米能级的狄拉克线节点,这与我们的第一性原理的计算结果完全一致。重要的是,这些体态和表面态在表面钾沉积时表现出不同的能带响应。这种差异不仅反映在整个能带结构的非刚性位移上,而且还反映在表面态色散的明显改变。我们的研究结果表明,具有非平庸电子结构的WHM材料可以通过钾原子沉积或表面修饰等方式实现对能带结构的调制,甚至可以产生新的拓扑输运行为。2.结合ARPES技术和第一性原理计算,对ZrSiX(X=Se,Te)的电子能带结构进行了比较。由于Te的原子半径大于Se,使得ZrSiTe具有更强的二维性,沿着高对称方向上的Dirac节点线电子色散明显弱于ZrSiSe。此外,X原子从Se到Te的变化,导致SOC强度的增强,使得表面态和体带产生明显的能带劈裂。尽管晶格应力和SOC并没有解除Dirac节点线的简并度,但它们可以共同改变Dirac点的相对位置。这为我们通过调整狄拉克点的位置实现新的输运行为提供了一种可行的方法。值得注意的是,先前的理论计算表明,ZrSiSe可能是平庸的拓扑半金属,而ZrSiTe被证明具有非平庸的拓扑行为,这强烈表明ZrSiSe和ZrSiTe之间存在拓扑相变。因此,对ZrSiX(X=Se,Te)及其相关化合物的进一步研究对拓扑相变的探索具有重要的指导意义。3.利用能够直观精确探测晶体电子结构的APRES技术和其他表征手段研究Nb3SiTe6单晶的电子结构。我们的实验结果揭示了在S-R路径上节点线的缺失,同时给出了沿着S-R方向费米能级处hourglass狄拉克色散稳定存在的实质性证据。不仅如此,实验结果还发现在Ky=π平面形成能够在自旋轨道耦合存在的情况下稳定存在的节点面,这与ZrSiS是截然不同的。更值得注意的是,在垂直于解理面的kz方向(沿着U-R方向)观测到节点线的能带结构,表明Nb3SiTe6是一个多种拓扑行为共存的新型拓扑半金属。4.研究Se替代和Cr插层对于ZrTe2电子结构的影响。对于Se替代ZrTe2的体系ZrTe2(1-x)Se2x,在r点处的价带由于自旋轨道相互作用导致能带发生劈裂,并且随着Se替代的增加导致劈裂幅度逐渐减弱。同时,我们还发现劈裂后最上层价带的有效质量随着Se含量的增加呈现出增大的趋势。不仅如此,Se替代导致ZrTe2(1-x)Se2x间接能隙的打开,使得体系发生从拓扑半金属到半导体的转变。对于Cr0.4ZrTe2单晶,我们通过ARPES系统研究超导体Cr0.4ZrTe2的电子结构。通过与母体化合物ZrTe2的比较,我们发现Cr的插入导致能带结构发生了明显的变化。Cr元素的插层并没有带来简单的电荷掺杂效应,而是引起了能带结构的剧烈变化,从无能隙半金属型转变为具有间接带隙半金属型,布里渊区中心附近的价带移动到费米能级以下。最终,在Cr0.4ZrTe2中,M点周围的电子占据了电子输运性质的主导地位。此外,M点周围的额外电子态在较大的能量尺度上表现出强烈的温度依赖行为,这表明与极化子的存在密切相关。更多还原

【Abstract】 Since the discovery of topological insulators,layered topological semimetal materials have widely concerned in the field of scientific research.Topological semimetal are a class of materials with metallic states that degenerate band crossing are formed by bulk bands near the Fermi level are protected by lattice symmetry.Unlike DSM and WSM,which form discrete degenerate points in momentum space,the band crossing of TNLSM can form complex one-dimensional configurations,such as nodal lines,nodal rings,node chains.Moreover,many novel physical phenomena such as giant magnetoresistance and superconductivity have been discovered in experiments,which has made it a hot field in current condensed matter physics.The physical and transport properties of materials are often determined by its internal electronic structures,so it is vital to explore the electronic structures.Angle-resolved photoelectron spectroscopy(ARPES)is the only experimental method that can directly detect the three-dimensional band structures of materials.In this dissertation,we mainly use ARPES technology to systematically investigate the electronic structures of layered topological semimetals,and the following results have been achieved:1.Using ARPES and combining DFT calculations,we revealed the band structures of the both ZrSnTe and ZrSiTe.By carrying out broad range momentum space and photon energy-dependent measurements,electronic structures from bulk bands and surface states can be easily identified.The observation of the Dirac line nodes along the X-R and M-A direction unambiguously confirms that the both of ZrSnTe and ZrSiTe are a topological Dirac line-node semimetals.In addition to the floating surface states that are ubiquitous in the WHM series,we found drumhead surface states with different band trendency and gapless surface states in ZrSiTe.With potassium deposition,the Dirac line node around the Fermi level are unveiled along the X-R direction,which is in agreement with our first-principles calculations.Importantly,these bulk bands and surface states shows different responses upon in-situ potassium deposition.This discrepancy is not only reflected in the non-rigid shift of the overall band structures,but also in the apparent change in the dispersion of the surface states.Since quasi-two-dimensional ZrSiTe belongs to the WHM materials possessing similar band structures,our findings indicate that the WHM materials with nontrivial electronic structures is a ideal platform to realize the modulation of band structures and trigger novel quantum phases by applying potassium deposition or surface decoration.2.We present the comparison of the electronic band structures on ZrSiX(X=Se,Te)by combining ARPES technique and first-principles calculations.The atomic radius of Te is larger than Se,resulting in a stronger two-dimensionality of ZrSiTe,so that the dispersion of line node is significantly weaker than that of ZrSiSe.Furthermore,the enhancement of the SOC strength due to the X atom from Se to Te,leading to more obvious band splitting of the surface states and bulk bands.Although the lattice strains and SOC do not lift the degeneracy of the Dirac line node,they collectively change the relative position of the Dirac point.This provides a feasible methods for us to realize many novel transport phenomena by adjusting the position of Dirac point.Remarkably,the theoretical calculations show that ZrSiSe most likely are topologically trivial,while ZrSiTe has been proven to have a nontrivial topology,which strongly suggests the presence of topological phase transition between ZrSiSe and ZrSiTe.Therefore,the further study of ZrSiX(X=Se,Te)and related compounds is of great significant for the exploration of topological phase transition.3.The electronic structures of Nb3SiTe6 was studied by making use of the combination of ARPES and the related characterization experiments.We reveal the lack of nodal line in the S-R path,and give substantial evidence of the stable existence of hourglass Dirac dispersion near the Fermi level along the S-R direction.Moreover,our data also found that nodal surfaces can be formed stably in the ky=πplane when the spin-orbit coupling included.Importantly,the band structures of the Dirac line node was observed along the U-R direction,indicating that Nb3SiTe6 is a new topological semimetal with multiple topological behaviors coexisting.4.The influences of Se substitution and Cr intercalation on the band structures of ZrTe2 were investigated.The valence bands split at Γ due to the stronger spin-orbital interaction of Te,whereas the splitting magnitude decreased with Se substitution in ZrTe2(1-x)Se2x.In addition,we also found that the effective mass of the uppermost valence band after split showed an increasing trend with increasing Se content.The indirect gap in ZrTe2(1-x)Se2xthat opened with Se substitution,resulting in a semimetal-to-semiconductor transition.For Cr0.4ZrTe2,we have studied the electronic structure of Cr-intercalated ZrTe2 superconductor Cr0.4ZrTe2.Comparing with the parent compound ZrTe2,we found that the band structures are significantly changed due to Cr intercalation.The intercalation of Cr element does not bring about a simple charge doping effect,instead it causes a drastic change of band structures,from the semi-metal type of ZrTe2 to the indirect-bandgap type in Cr0.4ZrTe2,with the valence bands around the zone center sinking below the Fermi level.Eventually,the electron pocket around the M point dominates the electronic transport properties in Cr0.4ZrTe2.In addition,the electronic states around the M point shows a strong temperature-dependent behavior over a large energy scale,which suggests the existence of polarons.更多还原