| The charge, spin, orbital, and lattice degrees of freedom interact and compete with each other in solids, which determine their diverse physical properties and novel ground states. The progress of condensed matter physics are motivated by emerging materials, newer states of matter, developing theories and technology evolutions. High-TC superconductivity and quantum spin liquid (QSL) are two novel quantum states of condensed matter systems. To understand how the microscopic electronic structures in these systems determine their macroscopic physical properties, we have carried out comprehensive angle-resolved photoemission studies. In this thesis, we investigate the electronic structures of NaFe0.9825Co0.0175As and EtMe3Sb[Pd(dmit)2]2single crystals, which are in novel superconducting state and QSL state, respectively. The main results and conclusion are as follows:1. Novel superconducting ground state in iron-based materials. In chapter3, we have conducted high-resolution angle-resolved photoemission spectroscopy (ARPES) on a unique iron-based superconductor, NaFe0.9825Co0.017sAs, which lies in the spin density wave (SDW) and superconductivity (SC) coexisting regime in the phase diagram. We have investigated the detailed electronic structure of NaFe0.9825Co0.0175As in the3-dimensional Brillouin zone and revealed the signature for the intrinsic microscopic coexistence of SDW and SC orders in momentum space for the first time. Moreover, the superconducting gap distribution is found to be nodeless on all Fermi surface sheets. It is isotropic on the hole pocket around zone center, but it is highly anisotropic on the electron pockets around the zone corner. Compared with previous experimental results and theoretical calculations, we argue that this unique superconducting gap distribution is a distinct consequence of the coexisting SDW order. Our results provide solid experimental foundations for further studies on the pairing symmetry in the theory of iron-based superconductors and help explore the role of magnetism in superconductivity mechanism.2. The electronic structure of organic quantum spin liquid material. In chapter4, we presented the experimental electronic structure of an organic QSL material, EtMe3Sb[Pd(dmit)2]2, for the first time, which naturally explains many phenomena observed in this compound before. We found that the band structure in EtMe3Sb[Pd(dmit)2]2show negligible dispersion, indicating localized electronic structure there. These findings suggest that the hopping integral t and on-site Coulomb interaction U are negligible, contradicting previous theoretical calculations. Moreover, we have carried out the ARPES studies on the sliding Pd(dmit)2compounds, Et2Me2P[Pd(dmit)2]2and EtMe3P[Pd(dmit)2]2, with antiferromagnetic and valence bond solid ground states, respectively. From the fitting analysis on their ARPES spectra with broad polaronic line-shapes, our results indicate that strong electron-phonon/magnon interactions have to be included in theoretical calculation when describing these systems. Our results suggest that a new model, Holstein-Hubbard model, with strong electron-boson coupling, instead of the Hubbard model, is more appropriate for a realistic modeling of the organic QSL’s and related compounds.In the last chapter of this thesis, I introduce the set-up of laser-based ARPES system and some important results based on this system. Laser-based ARPES exhibits higher energy resolution, higher momentum resolution, higher kz resolution and more enhanced bulk sensitivity than traditional ARPES, which are demonstrated from our results. This superior performance is helpful for us to discover the intrinsic electronic structures that often buried in the surface sensitive ARPES information.
|
PDF Full Text Request