Doping Regulation And ARPES Study Of Copper Oxide Superconductors

One of the hot topics in condensed matter physics is cuprate high-temperature superconductors,which have more complex phase diagrams and superconductivity mechanisms than conventional superconductors.This is partly because the doping effect plays an extremely important role in cuprate superconductors,where small changes in the doping level can significantly alter their physical properties;on the other hand,different parent types can cause the electron-hole doping asymmetry,which is not conducive to a unified superconductivity mechanism in copper oxide superconductors.In order to better explore their superconducting mechanism,the evolution of Mott insulators with doping concentration needs to be studied over a large doping range.For the typical cuprate superconductor Bi₂Sr₂CaCu₂O_8+δ（Bi-2212）,conventional doping regulation techniques have significant limitations in terms of the regulation range.To this end,we have developed an ozone/vacuum annealing method using oxide molecular beam epitaxy（OMBE）technology that enables continuously doping regulation of the entire superconducting phase region.In addition,we have also developed the Ca H₂ annealing method,which allows for the first time the continuously doping of Bi-2212 from the optimally hole-doped region to the electron-doped superconducting phase region.The development of these two doping regulation techniques provides an excellent platform for us to study fundamental issues such as pseudogap,electron-hole doping symmetry,and superconductivity mechanisms in a very large doping range using angle-resolved photoemission spectroscopy（ARPES）.The following are the main results of our study:1.Using ozone/vacuum annealing techniques,we have achieved a wide range of hole-doped regulation from 0.07 to 0.27 in the Bi-2212 system,and the Fermi surface gradually evolves from a large Fermi surface to a Fermi arc as the doping decreases.The energy gap is anisotropic in momentum space,with the largest energy gapΔ_AN,measured at the antinode and increasing with decreasing doping concentration.The energy gap satisfies the d-wave symmetry well in the extremely overdoping region,but as the Fermi arc appears,Δ_AN gradually deviates from the original d-wave symmetry because the pseudogap opens from the antinode and extends towards the node.The characteristic temperature corresponding to the superconducting gapΔ₀fitted with the d-wave symmetry function is the onset temperature_onset of the Nernst effect,verifying that electron pairing and phase coherence do not occur simultaneously in cuprate superconductors,and that the pairing strength is significantly higher than that of BCS superconductors,which belongs to the category of strong correlation.2.Using the CaH₂ annealing technique,we can easily regulate the optimally hole-doped Bi-2212 samples into the vicinity of the parent with better data quality than conventional surface adsorption techniques.With the annealing time increases,the resistance of the sample increases and then decreases and is accompanied by a change in Hall coefficient,which means that we have continuously doped Bi-2212 to the electron-doped region and we have also measured the electron-doped superconducting phase at dilution refrigerator temperature.We measured the hole-and electron-doped Bi-2212 by ARPES and found that the evolution of the energy bands and the constant binding energy contour is continuous,which is different from the electron-hole doping asymmetry exhibited in the conventionaland^′structures,and we speculate that this asymmetry due to the differences of the crystal structure.