Publication
The discovery in 1986 of the high-Tc superconducting cuprates has ushered in a new era of research in condensed matter physics. There is still a great interest for these materials, which stems not only from the lack of a clear theoretical picture, but also from the perspective of practical applications which could have an enormous impact on everyday's life. The complexity of the cuprates has stimulated a critical reassessment of many theoretical concepts and the development of new ideas, and fueled an unprecedented experimental activity to characterize their electronic structure. Besides the obvious occurrence of superconductivity, the two-dimensional character and the proximity to other ordered states, appear as prominent characteristics of these materials. The interplay of these properties is the subject of intense theoretical activity, since it is believed that it could play an important role in the emergence of the unconventional superconducting state. At the same time, it has been realized that similar situations may also arise in other classes of materials, where they lead to complex phase diagrams and to the emergence of new, unusual properties. These materials therefore offer interesting opportunities for related research. This thesis presents an investigation of the electronic structure of selected quasi-2D materials, and of their electronic instabilities – charge-density-wave (CDW), Mott metal-insulator transition, superconductivity – by means of angle resolved photoemission spectroscopy (ARPES). ARPES is a very powerful probe of solids which, thanks to its unique energy and momentum selectivity, provides a clear and direct view of the electronic states and of their interactions. Most of this work is concentrated on the low-energy excitations near the Fermi surface, the quasiparticles. These states play a crucial role in the thermodynamic, magnetic and transport of solids. Together with the shape of the Fermi surface, they determine the possible occurrence of electronic instabilities like charge- or spin-density waves, or again superconductivity (SC). A leitmotiv of this work is the study of how the nature of the quasiparticles, reflected in the ARPES spectral function, and the Fermi surface are influenced by an underlying instability, or by the competition between several instabilities. I have performed high-resolution ARPES experiments at the LSE-ICMP, and at two synchrotron radiation laboratories: the Swiss Light Source (PSI-Villigen) and SOLEIL (Paris). A large part of this work is concerned with the electronic properties of compounds which belong to the class of transition metal dichalcogenides (TMDs). TMDs are layered materials with rather strongly two-dimensional (2D) electronic properties. They often exhibit charge-density-wave (CDW) instabilities, and in selected cases superconductivity, but also peculiar metal-insulator (Mott) transitions. Among the TMDs, the 1T and 2H polytypes of TaS2 are representative of CDW materials whic