Laser-based angle-resolved photoemission spectroscopy (ARPES) and studies of topological insulators and heavy fermions

摘要

Angle-resolve photoemission spectroscopy (ARPES) as an experimental method that can directly measure electronic structure has been playing an important role in studies of novel materials, such as high temperature superconductors, topological insulators and many others. In this thesis, we will discuss the development of a tunable vacuum UV Laser ARPES system as well as ARPES studies of topological insulators and heavy fermion materials. The main results are as follows:1. We developed an angle-resolved photoemission spectrometer with tunable vacuum ul- traviolet laser as a photon source. The photon source is based on the fourth harmonic generation of a near IR beam from a Ti:sapphire laser pumped by a CW green laser and tunable between 5.3 eV and 7 eV. The most important part of the set-up is a compact, vacuum enclosed fourth harmonic generator based on potassium beryllium fluoroborate crystals, grown hydrothermally in the US. This source can deliver a photon flux of over 1014 photon/s. We demonstrate that this energy range is sufficient to measure the kz dis- persion in an iron arsenic high temperature superconductor and rare-earth antimonides, which was previously only possible at synchrotron facilities.2. We studied a nontrivial surface state in a pseudobinary Bi2Te2.28Se0.58 topological insu- lator. We demonstrated that, unlike in previously studied binaries, this is an intrinsic topological insulator with the conduction bulk band residing well above the chemical potential. Our data shows that under a good vacuum condition there are no significant aging effects for more than two weeks after cleaving. We also demonstrated that the shift of the Kramers point at low temperature is caused by UV-assisted absorption of atomic hydrogen.3. We systematically studied the electronic structure of quasi-2D heavy fermion material Ce2RhIn8. The lack of significant kz dispersion confirms the quasi two dimensionality of the electronic structure. Fermi surface is quite complicated and consists of several hole and electron pockets. Using comparison with DFT calculation we demonstrated that the data is consistent with a localized picture of f electrons. This provides clues to understanding their unusual transport and thermodynamical properties.