Microscopic electronic heterogeneity and its effect on the optical spectrum of high-temperature superconductors

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We investigated the effect of microscopic heterogeneity on the optical spectrum of high-Tc copper oxide superconductors using various spectroscopic methods, particularly STM/STS, applied to well-controlled single crystalline samples. The major results of the present research are:(1)Elucidation of the roles played by two distinct quasiparticles (QP), nodal and antinodal quasiparticles, in d-wave superconductors. Nodal QP has longer lifetime/coherence which gives rise to the periodic modulations of the local-electronic-density-of-states (LDOS) due to quantum-mechanical interference. On the other hand, antinodal QP is incoherent, and strongly influenced by lattice disorder, playing a major role in the microscopic heterogeneity in high-Tc cuprates.(2)Among disorder in various lattice sites of cuprates, the disorder in the apical oxygen block next to the CuO2 plane is found to most strongly affects the electronic state of the CuO2 plane. This disorder stabilizes a competing order which shows up as a pseudogap in the various spectroscopies. Using a special analysis on the STM/STS result, we revealed that the competing order/the pseudogap phase is a sort of electromic glass.(3)The antinodal QP is intimately related to the c-axis optical spectrum in which the superconducting response appears as a Josephson plasma mode because cuprates are regarded as a series of Josephson junctions along the c-axis. We demonstrated that magnetic fields applied parallel to the CuO2 planes make the Josephson junctions heterogeneous, and, as a consequence, a new Josephson plasma mode appears in the terahertz frequency region. We also showed that the frequency of the new Josephson plasma mode can be tuned by changing the strength of applied magnetic field.