Development of ultrahigh-resolution photoemission spectrometer and study of quasiparticles in high-temperature superconductors

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In order to clarify the origin of quasiparticles in high-temperature superconductors, we have constructed a new ultrahigh-resolution angle-resolved photoemission (ARPES) spectrometer at Tohoku University. The spectrometer consists of (1) electron-energy analyzer, (2) light source, (3) analysis chamber, and (4) sample preparation chamber. As for the analyzer, we have improved the electric stability of the hemispherical part and the electron lens system, and also performed accurate sensitivity calibration of the CCD camera and the two-dimensional photoelectron detector. As for the light source, we have constructed a double differential pumping system as well as a VUV-filter which can pass ultraviolet-ray from the He discharge lamp but completely blocks residual gasses from the lamp. As for the sample chamber, we have developed a variable-angle manipulator which can rotate the angle with respect to the analyzer at low temperature. To effectively perform the ARPES experiment, we have devel … More oped a sample transfer system around the sample preparation chamber. To improve the vacuum during the measurement and to achieve low temperature, we have constructed multi-wall radiation shields with He-cycling cryostat. By making these efforts, we have finally achieved high performances as follows : (1) energy resolution : 1.3meV, (2) momentum resolution : 0.005Å^<-1>, (3) lowest temperature : 3.7K, and (4) vacuum during measurements : 2x10^<-11>Torr. By using this ultrahigh-resolution spectrometer, we have performed high-resolution ARPES measurements on following subjects.1)Observation of Fermi surface, superconducting gap, and pseudogap in Bi-based high-temperature superconductors.2)Direct observation of Bogoliubov quasiparticles on a triple-layered superconductor Bi2223.3)Elucidation of the origin of kink in the dispersion in hole-doped high-temperature superconductors.4)Direct observation of multiple-superconducting gaps in MgB_2.5)Observation of magnetic isotope effect in energy dispersion of Bi-based high-temperature superconductors.6)Direct observation of nonmonotonic d_x2_<-y2> wave superconducting gap in electron-doped high-temperature superconductors. Less