Angle-resolved photoemission study of unconventional cuprate superconductors
Abstract: Understanding unconventional superconductivity remains one of the most important unsolved problems in physics. A particularly noteworthy case is the copper-based high-temperature superconductor, which stands out due to its remarkably high transition temperature and relatively simple structure. These exceptional properties not only make the study of cuprates valuable for potential practical applications but also provide a prominent platform for deepened understanding of many-particle physics. In the realm of quantum materials, angle-resolved photoemission spectroscopy (ARPES) has emerged as an indispensable tool for examining the intricate electronic structure in momentum space. This methodology directly probes the single-particle spectral function and can uncover the underlying microscopic interactions. In recent years, technological advancements have enabled the development and implementation of time-resolved ARPES (tr-ARPES). tr-ARPES allows access to non-equilibrium transient states and provides valuable insights into the correlated dynamic properties. This thesis work is divided into two main parts. The first part focuses on the development of a high-resolution, high harmonic generation (HHG)-based tr-ARPES setup. The second part involves ARPES investigations of hole- and electron-doped cuprate superconductors.The aim of developing the tr-ARPES setup was to have a light source with specific characteristics, including a narrow bandwidth, a wide range of photon energies (covering the entire first Brillouin zone), good temporal resolution (near the transform limit), and a high repetition rate (to mitigate the space charge effect). To meet these requirements, the chosen technical approach is the HHG method, driven by an unusually long laser pulse (~460 fs) and short wavelength (343 nm) from a frequency tripled Yb fiber laser. The selection of photon energy is achieved through switchable multilayer bandpass mirrors and thin film filters to prevent temporal broadening. As a result, we achieve an energy resolution of ΔE = 9, 14, 18, 111 meV for photon energies of hν = 10.8, 18.1, 25.3, 32.5 eV. For the pump pulse, a tunable range from 0.65 μm to 9 μm is provided by a two-stage optical-parametric amplifier. Further developments on the instrumental side is the exploration of using a spherical grating to select the HHG harmonics. This is realized by designing a very low-density grating so that the temporal broadening can be minimized. The spherical grating has been numerically calculated, fabricated, and experimentally characterized. This monochromator solution was compared with the mirror+filter configuration and has shown much higher efficiency (3.3 times higher for 10.8 eV and 12.9 times higher for 18.1 eV) with insignificant temporal broadening (6.8% increase for 18.1 eV). This experimental development provides a compact and efficient layout for ultrafast pulse extraction.In the cuprate section, a thorough investigation was conducted on the optimally doped n-type cuprate (NdCeCuO) using static ARPES. The much-refined experimental conditions have enabled us to obtain unprecedented signal-to-noise ratio and detailed observations in this material. The results demonstrate two distinct sectors of states: the reconstructed main band, which exhibited a gap due to the antiferromagnetic (AF) interactions, and a remaining dispersion observed within this AF pseudogap. This in-gap dispersion forms a 'gossamer' Fermi surface that plays a crucial role in the electron pairing. Additionally, a replica band corresponding to the AF folding feature was observed, displaying a consistent energy difference (approximately 60 meV) in both momentum and temperature dependence, possibly suggesting a connection between the AF order and phonon coupling. Furthermore, the hole-doped cuprate (Bi-2212) was studied using the recently developed tr-ARPES system. Leveraging the use of time-of-flight detection, node-antinode information could be gathered from a single measurement. The dynamics of the in-gap states at the antinode exhibited disparities with the near-node region, potentially reflecting phenomena associated with the pseudogap.
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