Quasiparticle Spectroscopic Studies of the Pairing State and Competing Orders in Hole- and Electron-Doped Cuprate Superconductors

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High-temperature superconducting cuprates are doped Mott insulators with strong electronic correlation. Depending on the doping level (d) and the sign of the dopant (hole- or electron-doping) in the CuO(2) planes, the electronic and structural anisotropies, and disorder, different competing orders can emerge in the ground state of these cuprates. These competing orders give rise to various non-universal phenomena that have masked better understanding for the pairing mechanism of cuprate superconductivity. The primary objective of this project is to unravel the physical origin of various non-universal phenomena and to identify truly ubiquitous characteristics among all cuprates. The technical approach will involve systematic experimental studies of hole- and electron-doped cuprates of different doping levels (d) and with controlled substitutions of quantum impurities, using a new homemade variable-temperature high-field-compatible scanning tunneling microscope (STM) and other auxiliary characterization tools in the PI's group. In addition, numerical computations of the quasiparticle local density of states (LDOS) for different model Hamiltonians will be performed. The proposed research is of significant educational value because of the range of experimental skills and the depth of theoretical knowledge required of the participants to accomplish the tasks. Successful completion of the research can scientifically advance current understanding of the pairing mechanism of cuprate superconductivity and of the competing orders in other strongly correlated electronic systems, whereas technologically the STM instrumentation expertise can benefit research on novel nano-scale materials and devices.The physical cause for the occurrence of high-temperature superconductivity (HTS) in the cuprate superconductors remains a mystery despite substantial research progress since the discovery of HTS in 1986. The primary reason for the lack of complete theoretical understanding is due to the variety of low-temperature physical phases that can exist in different families of cuprate superconductors upon simple "tuning" of the chemical properties of these materials. This research project intends to unveil the mystery of HTS by investigating how different physical phases occur in the cuprates, and to identify the truly ubiquitous characteristics among all cuprates that are responsible for HTS. The experimental approach involves systematic studies of different cuprates using a homemade variable-temperature high-field-compatible scanning tunneling microscope (STM) and other auxiliary characterization tools in the PI's group at Caltech. The STM measurements can provide important information about the cuprates with atomic-scale spatial resolution. In addition to the experiments, computer modeling will be performed for comparison with experimental data. The proposed research is of significant educational value because of the range of experimental skills and the depth of theoretical knowledge required of the participants to accomplish the tasks. Successful completion of this project can advance the theoretical understanding and commercial applications of cuprate superconductors. The STM instrumentation developed for this project can further benefit the characterization of novel nano-scale structures and devices in the emerging field of nano-science & technology.