Preparation And Investigation Of The High-T_c Cuprate Thin Films

As one of the core materials in condensed matter physics,high temperature cuprate superconductors and their pairing mechanism have attracted tremendous attention in the last thirty years.All cuprates exhibit a complex phase diagram and numerous electronic states such as the pseudogap and charge density waves.After the thirty-year extensive study,it is generally accepted that the pairing symmetry is in line with the d-wave scenario.In order to probe directly the intrinsic properties of the superconducting layers and to unveil its relationship with various charge ordered states,in this dissertation we have employed ozone-assisted molecular beam epitaxy?O-MBE?and scanning tunneling microscopy?STM?for preparation and in-situ investigation of the superconducting layers in cuprates.The main results are summarized as follows.The CuO₂ plane is the superconducting layer of cuprate.Given the layered crystal structure of cuprate,it is usually sandwiched between the heavily doped charge reservoir layers,and thus difficult to access by most experimental techniques.To overcome this difficulty,we have carried out the growth of high-quality single-crystalline CuO₂ films on Bi₂Sr₂Ca Cu₂O_8+d substrates,from the bottom up.In situ STM measurements directly on the CuO₂ films have revealed the separation of the superconducting gap and the pseudogap,which speaks for their irrelevance.Most importantly,the superconducting gap is U-shaped and its responses to the impurities are in line with the conventional s-wave BCS superconductors.These results strongly suggest that the cuprates may not be d-wave superconductors but rather conventional s-wave ones induced by modulation doping.Under this context,the previously observed complex electronic states are more likely the characteristics of the charge reservoir layers.By controlling the interfacial charge transfer,we have further proved the validity of the modulation doping scenario.These results directly from the superconducting planes offer more convincing information for understanding the high-T_c superconductivity mechanism.La_2-xSr_xCuO₄ is a prototypical cuprate in terms of its relatively simple crystal structure as well as its abundant and robust electronic states,which are ideal for investigating the relationship between superconductivity and various charge ordered states.However,it is very difficult to cleave the material so that few high quality atomic-scale study can be found.We have systematically investigated the O-MBE growth of La_2-xSr_xCuO₄,and have obtained high-quality thin films on both the Sr Ti O₃and Sr La Al O₄ substrates.In-situ STM characterization has revealed for the first time the long-sought charge orders in La_2-xSr_xCuO₄ thin films,which show dramatic dependence on the doping.Based on these results,we have established the doping dependent electronic phase diagram for La_2-xSr_xCuO₄,and have determined the relative spatial location as well as the co-existence/transformation between them.Comparing the different behaviors of the films on the Sr Ti O₃ and Sr La Al O₄ substrates,we have established the competing relation between the superconductivity and charge orders.Our results help understand the secret of charge orders in cuprates.Finally,we have also employed argon ion bombardment and annealing?IBA?technique to investigate the superconductivity of Bi₂Sr₂Ca Cu₂O_8+d.By improving the IBA technique,we are able to realize layer-by-layer etching of the constituent planes of Bi₂Sr₂Ca Cu₂O_8+d.As a result,separation of the superconducting gap and the pseudogap is observed.We have further revealed that the CuO₂ layer induces superconductivity in the neighboring charge reservoir layers via proximity effects,which intertwines with the non-superconducting electronic states from the charge reservoirs.These experiments provide new information to understand previous experimental data and thus raise the high T_c superconductivity research to a new level of sophistication.