Molecular Beam Epitaxy Growth And Scanning Tunneling Microscopy Study On Tetragonal CuO Thin Films

Cuprates have been regarded as the most promising high-T_c superconductors in the past 30 years.Whether the electron-phonon interaction of BCS theory is correct to explain the high-T_c superconductivity of cuprates is still controversial.The structural commonalities of various cuprates,such as the layered structure,CuO₂ superconducting layer and antiferromagnetic Mott insulator,are good starting points to understand the microscopic mechanism of high-T_c superconductivity of cuprates.Although the charge carriers responsible for the superconductivity are located in the superconducting CuO₂layers,the dopants sit in the non-superconducting layers.Therefore,the research on CuO₂superconducting plane plays an important role in understanding the microscopic mechanism of the high-T_c superconductivity of cuprates.It is well known that copper-based superconductors typically contain at least three elements and their composition are relatively complex.The simplest cuprates is CuO or Cu₂O.Cu₂O has only one cubic phase,while CuO can be monoclinic,tetragonal and cubic.In nature,CuO is stable in monoclinic phase.Tetragonal CuO thin films can be epitaxially grown on Sr Ti O₃ substrate.Studies on cubic CuO are rare.According to previous studies,CuO(001)plane can be regarded as the superposition of two sets of CuO₂ sublattices with weak coupling,and the electronic structure of CuO may be analogous to CuO₂superconducting planes in cuprates.Therefore,it is particularly important to prepare high quality tetragonal CuO thin films and further study their electronic structure and properties.In this thesis,the morphology and electronic structure of(001)plane in tetragonal CuO thin films are systematically studied by the combination of molecular beam epitaxy(MBE)and low temperature scanning tunneling microscope(STM).We successfully grow tetragonal CuO thin films with thickness from 1 ML to 10 ML on Sr Ti O₃ substrate,beyond the previous-reported critical thickness.The structure characterization by STM,XRD,and XPS fully proves the four-fold symmetry of CuO thin films.Using in situ STM,we observe the atomic resolution images on the tetragonal CuO(001)plane for the first time;two sets of CuO₂ sublattices can be distinguished.The phase inversion of the CuO₂sublattices at the domain boundary can be interpreted as two different antiferromagnetic directions of CuO₂.To reduce the whole energy of system,domains with different antiferromagnetic directions will spontaneously coexist.By recording d I/d V spectra,we obtained the value of charge transfer gap in 1 ML tetragonal CuO thin film as 3.67 e V,which is consistent with the previous ARPES and XPS results.The effects of tip-induced band bending on the thick samples are also discussed.Our work may stimulate other studies in tetragonal CuO on electronic structures,magnetic structures,electron-phonon coupling and high-T_c superconductivity.