Low Temperature Scanning Tunneling Spectroscopy On Bi₂Sr₂CuO_6+? And ZrSiSe

In addition to characterization of surface morphology,scanning tunneling microscope can also measure the local density of state.Scanning tunneling spectroscopy and its Fourier transform have extended the detection range of scanning tunneling microscope from surface,crystal lattice and impurities to the electronic structure of materials.We can use this technique to study energy bands,Fermi surface,ordered states and so on.Cuprate high-temperature superconductors have been discovered for nearly 30 years,and abundant phases have been found in this material,including Mott insulating phase,superconducting phase,pseudogap and so on.However,the physical mechanism is still controversial,and there are still much filed worth further research.As a new type of material predicted and discovered in recent years,topological nodal-line semimetals is a research hotspot in the field of condensed matter,whose energy bands have nontrivial topological properties protected by symmetry.Both of these materials have rich and interesting electronic structures for study.Based on the technique of scanning tunneling spectroscopy,the electronic properties of cuprate superconductor(Bi,Pb)2(Sr,La)2CuO6+k? and topological nodal-line semimetal ZrSiSe are measured.The main contents are as follows:(1)Study of electronic effect of oxygen defects in(Bi,Pb)2(Sr,La)2CuO6+?.We apply-scanning tunneling microscope to study the O dopants in an optimally doped and an overdoped(Bi,Pb)2(Sr,La)2CuO6+? sample.The spatially dependent differential conductance spectra are collected,accompanying the topographic measurements over a clean area.The conductance maps at characteristic voltages are used to obtain the spatial distributions of the three types of O dopants.For the interstitial O atoms on the SrO layers,which dominate the number of O dopants,we discuss its correlation with the Pb dopants in overdoped sample.The influence of the O dopants on electronic properties is subsequently investigated.The hole carrier density is estimated from the O dopant concentration and compared with the Luttinger count of the measured Fermi surface(FS).Finally,in a statistical analysis on the spatial distributions of O dopants and PG magnitudes,we show that the interstitial O atoms on the SrO layers generally suppress the nearby PG magnitude.(2)Study the charge order and topological defects in overdoped(Bi,Pb)2Sr2CuO6+?.We apply scanning tunneling spectroscopy to explore local properties of the charge order.The ordering wavevector is nondispersive with energy,which can be confirmed and determined.By extracting its order-parameter field,we identify dislocations in the stripe structure of the electronic modulation,which correspond to topological defects.Through differential conductance maps overa series of reduced energies,the development of differentresponse of the charge order is observed and a spatial evolution of topological defects is detected.The intensity of charge-order-induced modulation increases with energy and reaches its maximum when approaching the pseudogap energy.In this evolution,the topological defects decrease in density and migrate in space.Furthermore,we observe appearance and disappearance of closely spaced pairs of defects as energy changes.(3)Study the electronic nematicity in overdoped(Bi,Pb)2Sr2CuO6+?.We confirm a weak nematicity in this material.In our overdoped(Bi,Pb)2Sr2CuO6+?,the two opposite orders coexist when a nonzero collective order is developed around PG.By studying the real-space distribution of the order parameter,we observe the evolution of the nematicity with energy.Nanoscale domains are aggregated in real space for each of two opposite nematicities,which are induced by local fluctuations.With the increase of the reduced energy,the domain size of one nematicity increases,and its order strength gradually dominates although both orders are enhanced.The collective nematic order is established by the combination of these two effects.(4)Study of topological semimetal ZrSiSe.We apply scanning tunneling microscopy to study this new material ZrSiSe.Our study resolves a bias polarity selective topography.Combining the scanning tunneling spectroscopy and theoretical calculation,we can identify the lattice structure of the ZrSe bilayer at a subatomic resolution.In the measurement of Fourier transform-scanning tunneling spectroscopy,we find the pattern of quasiparticle interference is related to the type of defect.We find a special impurity with strong signal of quasiparticle interference of both bulk and surface bands,which can be used to extract the electronic topology of ZrSiSe.By analyzing the pattern of quasiparticle interference with the assistance of the constant energy contour model,we observe the nodal-line state?300 meV above the Fermi level.An extra surface-state Dirac point is determined?400 meV below the Fermi level.Our results are well consistent with the first principle calculation.