Investigation Of The Two-dimensional Topological Quantum Material By Angle-resolved Photoemission Spectroscopy

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=?plane 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 ? 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.