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Abstract

Since the discovery of cuprate superconductors in 1986, the subject of high temperature superconductivity has triggered a lot of scientific activity. Despite enormous efforts, the origin of high-temperature superconductivity is still the subject of intense debate, revived recently by the discovery of Fe-base superconducting pnictides. Angle-resolved photoemission spectroscopy (ARPES) has been a fundamental tool in the study of the electronic band structure of cuprate and Fe-based superconductors and of the connection between the electronic degrees of freedom and the origin of unconventional superconductivity in these systems. This thesis presents an experimental study of pnictide LaRu2P2 and of cuprate La2−xSrxCuO4 (LSCO) by means of ARPES carried out at Surface/Interface spectroscopy (SIS) beamline and at ADRESS soft x-ray beamline of Swiss Light Source (SLS). In the first part of the thesis I present soft x-ray (SX) ARPES measurments of superconducting LaRu2P2 (Tc = 4K) which is isostructural with “122” Fe-pnictides superconductors. The results show that both the Fermi surface topology and the bandwidth renormalization of LaRu2P2 are very different from their high-temperature superconducting Fe-pnictide counterparts. We also demonstrate that, compared to ultraviolet (UV) ARPES, the increased photoelectron mean free path of SX-ARPES is essential in determining of the bulk electronic structure of LaRu2 P2 . In the second part of the thesis I present a comprehensive ARPES study of the electronic structure of La2−xSrxCuO4 in a wide doping range (x), from non-superconducting samples (x = 0.02) to highly doped superconducting ones (x = 0.22). The measurements show that, while the main Fermi surface changes monotonically with adding more charge carriers to the system, two folded bands that resemble a copy of the main band but shifted by (π, 0) or (π, π) have an unusual dependence on doping. Third part of the thesis deals with the ARPES study of the momentum and temperature dependence of the energy gap in the electronic excitation spectra of highly underdoped superconducting (x = 0.08) and non-superconducting (x = 0.02 − 0.03) LSCO. This study reveals that in this compound the low-energy electronic excitation spectra are gapped along the entire underlying Fermi surface at low temperatures and that the gap structure evolves to a dx2−y2 form as increasing temperature or doping, consistent with the vast majority of ARPES studies of cuprates.

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