Imaging the changes in electronic structure across phase transitions in the strongly correlated materials V₂O₃ and URu₂Si₂

In systems with strongly correlated electrons, different competing ground states can lead to exotic states of matter and new phase transitions. This thesis explores experimentally, using angle-resolved photoelectron spectroscopy (ARPES), how the electronic structure changes across two such paradigmatic transitions : the Mott metal-to-insulator transition as a function of temperature in V2O3 and the quantum phase transi- tion as a function of doping in URu2Si2.
In bulk V2O3, a first-order Metal-to-Insulator Transition occurs at a transition temperature of TMIT 160 K, as evidenced by a sharp rise in resistivity of more than 6 orders of magnitude and the opening of a gap of about 750 meV. The thesis presents a comprehensive study of the electronic structure using angle-resolved photoemission spectroscopy (ARPES), across the temperature-induced MIT in V2O3 films, whose crystal integrity, contrary to single crystals, is not affected by the transition.
The results show dispersive quasi-particle and Mott-Hubbard bands in the metallic state, and the observation of unique spectral signatures, different to those of a conventional Fermi-surface instability, accompanying the MIT : the opening of a gap at the Fermi level associated to a decrease in spectral weight of the quasiparticle band, without any change in its effective mass, while the Mott- Hubbard-band remains unaffected by the MIT. The spectral weight is transferred from the conduction band to a non-dispersive band of different orbital character, as the latter moves down in binding energy. Furthermore a clear thermal hysteresis in the gap and the quasi-particle’s spectral weight is ob- served, which is interpreted in terms of the evolution of ratios of metallic/insulating microscopic domains, and that precisely capture the hysteresis in resistivity measured in the same samples.
URu2Si2 undergoes a phase transition at 17.5 K, whose order parameter is still unknown to researchers over three decades after its first observation in specific heat measurements. The transition temperature can be lowered by phosphorous doping up to its suppression at a quantum-critical point. Stronger doping then gives rise to an an- tiferromagnetic ground state. The doping-induced transitions are of interest on a fundamental level : A continuous phase transition at absolute zero im- plies quantum criticality, where the transition into the new ground state cannot be driven by thermal excitations like in conventional first order phase transitions, and requires instead zero-point fluctu- ations due to Heisenberg uncertainty.
This thesis presents angle-resolved photoemission spectroscopy measurements of the electronic structure of URu2Si2 across the transitions from the low temperature hidden order (HO) to the paramagnetic and then into the antiferromagnetic phase driven by Phosphorous doping. It is found that the changes in electronic structure around the Gamma high symmetry point, that have been previously reported across a purely temperature- driven Hidden Order transition, can be reproduced by the doping, independently of temperature. Furthermore, at the X-point, strong changes in the dispersion of a hole-like surface state and suppression of its hybridization with the U5f-quasiparticle band in the paramagnetic state are observed with the doping, in contrast to the temperature-driven transition, indicating that such hybridization is a unique feature of the hidden order phase.