Ultrafast charge and structural dynamics in rare-earth perovskites

Materials with strong correla on between charges, spins and la ce o en exhibit complex phase diagrams, hin ng at an underlying compe on amongst different groundstates with comparable energy scales. Such correlated systems are thus very sensi ve to external parameters such as temperature, pressure or magne c fields, enabling efficient control of the material proper es. Intense short laser pulses can also be used to engineer transient states, which may or may not exist in thermal equilibrium, and even induce phase transforma ons, such as mel ng of magne c order, insulator-to-metal transi on or transient superconducvity. This thesis aims at understanding the pathways to these highly out-of-equilibrium states and reports on the inves ga ons of two correlatedmaterials exhibi ng photo-induced insulator-to-metal transi on. Photo-induced insulator-to-metal transi ons in the inves gated transi on metal oxides are o en accompanied by the mel ng of associated long-range electronic orders and structural distor ons stabilizing the insula ng ground state. Long range order of atoms or charges is ideally probed by diffrac on-based techniques, taking advantage of the wave nature of par cles like x-ray photons, neutrons or electrons. Their dynamics can be measured using state-of-the-art light sources such as free-electron lasers, which generate ultrashort x-ray pulses of the order of few tens of femtosecond, enabling the extension of well-established sta c diffrac on techniques to the me domain via the pump-probe scheme. With such tools at hands, it has recently become possible to probe charge and la ce dynamics on subpicosecond mescales. A first part of this work is dedicated to the study of the dynamics of chargeand orbitallyordered manganites following op cal excita on across the insula ng gap. These materials aremostly known for displaying colossal magnetoresistance but have also beenwidely invesgated in me-resolved experiments, because of the strong electron-phonon coupling and the ensuing rich structural dynamics. Here, the focus is on three aspects that have seldom been considered so far: the doping dependence of the structural dynamics, the monoclinicto-orthorhombic transi on and the recovery to the groundstate. Comparing these results with studies of the charge order and magne c dynamics, a comprehensive picture of the phase transi on is proposed. Two dis nct regimes are iden fied in the transi on: the charge order and the fast component of the structural dynamics are shown to be con nuous and can be assimilated to a second order phase transi on, while the slower changes of the unit cell, including amonoclinic-to-orthorhombic transi on, and the emergence of ferromagne c correla on display a threshold behavior, a characteris c trait of a first-order transi on. In a second part, an alterna ve excita on mechanism is explored. Instead of exci ng materials with visible or near-infrared light, induc ng direct changes in the electronic system, low energy mid-infrared pulses are used to coherently excite phonon modes to large amplitude in a manganite. Such excita on was shown to also induced a fast insulator-tometal transi on, but the details of the underlying dynamics remained unclear. Resonant x-ray diffrac on is employed to disentangle the charge and structural components of this transi on and their dynamics are compared to that of the electronic excita on discussed in the first part. Contrary to near-infrared excita on, the reduc on of charge order scales highly nonlinearly with fluence. The response of the la ce, however, follows that of the charge order in a similar way, ruling out a nonlinear phonon-phonon coupling as the driving mechanism for the transi on. Density func onal theory calcula ons reveal a strong nonlinear electron-phonon coupling to be the origin of the observed nonlinearity. Finally, a last part is dedicated to the study of the charge order dynamics in a nickelate film. The nickelates have a similar structure than that of the manganites and also display a large conduc vity change at the ordering temperature. The core of the study is a meresolved resonant diffrac on experiment, where the charge order dynamics is examined after photoexcita on of the electronic system. The me-dependent order parameter model that was developed for the manganites is successfully tested on the nickelates, poin ng towards a possible universality of this concept. The temperatureand fluence-dependent transient responses are also probed by a complementary me-resolved reflec vity experiment. Clear signatures of the phase transi on are observed in both the temperature and fluence dependence of the op cal reflec vity.