| Electronic structure is one of the most important physical properties in condensed matter physics that affects the physical properties of materials,and its variation under different ways of modulation has been always a hot topic.Since the Dutch scientist Kamerlingh Onnes first reported "superconductivity" at extremely low temperature in1911,the electronic structure of superconducting materials has become the most challenging question for condensed matter physicists.Unlike cuprate superconductors which exhibit single band structure,iron-based superconductors exhibit more complex multi-band electronic structure.Therefore,the modulation of electronic structure in iron-based superconductors has remained an important topic in high-temperature superconductivity research until now.On the other hand,topological insulators with exotic band structures are also a significant system for studying electronic structure.The limit of resistance of topological insulators to disorder and the changes of electronic structure under different disorders are of great value for investigating the origin of topological properties.In order to investigate these key issues in iron-based superconductors and topological insulators,this paper investigates the intrinsic properties of the electronic structure of the materials by inducing various modulation methods such as uniaxial strain,electronic doping,and in-situ annealing with angle-resolved photoelectron spectroscopy(ARPES),which can directly probe electronic band structures.The variation in electronic structure of iron-based superconductors and topological insulators is probed.First,a typical iron-based superconductor material Ba Fe2As2 was selected to explore the relationship between nematic order and band structures in iron-based superconductors.A continuously tunable uniaxial strain was applied to the Ba Fe2As2 sample by an in-house designed uniaxial strain device which is compatible with various experimental instruments.The results of X-ray diffraction(XRD)at the low temperature show that the sample remains in the detwined state even under the minimum strain during the tuning process,indicating that the ARPES results are derived from single domain samples.Usually the splitting of two electron bands at the M point in iron-based superconductors can reveal the existence of the nematic order.However,our experiments found that the splitting at MY point is inconsistent with the changes in nematic order under considerable uniaxial strains.Specifically,with the increasing strains,we observed that the resistivity exhibits greater anisotropy;while the energy distance δE of these two electron bands at MY point gradually decreases.It clearly suggests that the splitting of these two electron bands is not induced by the nematic order.This indicates that when applying uniaxial strains on iron-based superconductors,the change of the lattice constant will also lead to a large change in the energy band.Our experimental results reveal the change trend of the energy band after complete detwinning,which provides a preliminary idea for further understanding the change of the electronic band structure of iron-based superconductors under considerable uniaxial strains and understanding the nematic order.Secondly,the effects of surface potassium evaporation on electronic structure and superconducting transition temperature of undoped Fe Se,Co-doped Fe Se and Cudoped Fe Se single crystals are investigated in this paper.For undoped Fe Se samples,we electron doped them by surface potassium evaporation.The electron doping concentration was saturated at 8.8%.Although the Fermi surface of Fe Se did not undergo topological changes during this process,we observed signs of the superconducting gap opening at 17 K,in agreement with previous experimental results,which is higher than 8 K in the reports for Fe Se and comparable to the highest superconducting transition temperature(14.5 K)that can be achieved by doping Fe Se single crystal with Te element substitution.Therefore,our experiment shows that for undoped Fe Se single crystals,the electron doping introduced by surface potassium evaporation can significantly increase their superconducting transition temperature.At the same time,in order to study the effect of this method on the element substituted electron doped samples,we also carried out potassium evaporation studies on the nonsuperconducting magnetically electron doped Fe0.98Co0.02 Se and non-magnetically electron doped Fe0.98Cu0.02 Se,and found that the Fermi surfaces of both samples also did not undergo topological transitions when they reached saturation electron doping concentrations(10.2% and 12%,respectively).Unlike undoped Fe Se,neither sample of Fe0.98Co0.02 Se and Fe0.98Cu0.02 Se showed a superconducting transition at 17 K,indicating that their transition temperature is lower than that of the aforementioned Fe Se samples after potassium evaporation or there is no superconducting transition.Due to the similar properties exhibited by the non-superconducting magnetically electron doped Fe0.98Co0.02 Se and non-magnetically electron doped Fe0.98Cu0.02 Se,we preliminarily excluded the influence of magnetism.The ARPES results indicate that potassium evaporation affects the in-plane lattice of Fe0.98Co0.02 Se and Fe0.98Cu0.02 Se,and both sample surfaces stretch along the in-plane direction after evaporation of potassium.At this point,the lattice perpendicular to the in-plane direction contracts,resulting in a further decrease in the spacing between Fe Se layers in Fe0.98Co0.02 Se and Fe0.98Cu0.02 Se.Therefore,we conclude that the absence of superconductivity at this point may be related to a further reduction in the Fe Se interlayer spacing.Therefore,we believe that the reason why there is no sign of superconducting transition in highly electron-doped Fe0.98Co0.02 Se and Fe0.98Cu0.02 Se is that the enhancement of superconductivity brought by electron doping has not completely overcome the inhibition of superconductivity by Fe Se interlayer spacing reduction brought by element doping.This indicates that the enhancement of superconductivity in iron-based superconductors comes from a variety of factors,and further research is needed on the mechanism regarding the influence on the superconducting transition temperature.Finally,the evolution of electronic structure during the crystallisation of amorphous Bi2Te3 is also investigated in this paper.We successfully synthesised amorphous Bi2Te3 at liquid nitrogen temperature and observed the structural relaxation of amorphous Bi2Te3 with time by RHEED,demonstrating that the samples remain pure Bi2Te3 amorphous states.Subsequently,by increasing the annealing power,samples were obtained for different states of Bi2Te3 throughout the crystallisation process,i.e.,"amorphous state-amorphous nanocrystalline mixed state-nanocrystalline state-nanocrystalline polycrystalline state".The electronic structure of the amorphous Bi2Te3 are directly observed for the first time by in-situ ARPES measurements,indicating that Bi2Te3 are in a topological trivial state until crystallisation to the polycrystalline state.On the other hand,the transition from the polycrystalline to single crystalline state of Bi2Te3 has been achieved by successive annealing,and the critical state during the topological transition is observed by in-situ ARPES. |
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