Complementary Visualization Of The Pseudogap States In Bi_2Sr_2CaCu_2O_{8+[Delta]}: Si-Stm Study In Real And Momentum Space

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The high-temperature superconductivity has remained one of the most puzzling phenomena in contemporary condensed matter physics since its discovery in 1986. Despite enormous quantity of experimental results and theoretical proposals over 25 years, the pairing glue as well as the nature of the normal states are still not clearly identified. The enigmatic pseudogap phase, characterized by the loss of spectral weight above the superconducting transition temperature (Tc ), is believed to be a key to understanding the superconductivity. This dissertation primarily investigates the underdoped BSCCO, a double layered cuprate perovskite which exhibits pseudogap over wide range. Spectroscopic-Imaging Scanning Tunneling Microscopy (SI-STM) is a unique probe in that it allows simultaneous access to complimentary spaces. For SI-STM can visualize the electronic structure of a physical system with atomic resolution and register in real space, with its Fourier-transform revealing the dispersion of quasiparticle excitations in momentum space. Three discoveries made with SI-STM are presented here. First, the real-space electronic nematicity observed within CuO2 unit cell demonstrates that the C4 rotational symmetry of the underlying crystal lattice is broken into the C2 symmetry in the electronic structure. Inequivalence of the orthogonal crystallographic axes narrows down the possible models for pseudogap in favor of stripe order. Momentum space provides two other discoveries. Ratio of the differential conductance data measured at opposite bias voltage, which amplifies the particle-hole symmetric dispersion of the Bogoliubov quasiparticle, shows that the pseudogap state retains the tri-partite electronic excitation structure present below Tc . The signature of the Cooper pairs without long-range phase coherence renders the pseudogap as a pre-cursor to the superconducting state. In contrast, the anti-nodal charge-ordering dominant in the raw data but washed out in the ratio map is found to weakly disperse in a continuous but asymmetric manner. The peculiarity of the dispersion distinct in the strongly underdoped regime of cuprate family with high maximum Tc is discussed in the context of the low-energy psdueogap in the last chapter.
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high temperature superconductivity; pseudogap; STM
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Davis, James C.
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Shen, Kyle M.
Kim, Eun-Ah
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Ph. D., Physics
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Doctor of Philosophy
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Government Document
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dissertation or thesis
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