Transport Properties of Transition Metal Oxide Thin Films and Interfaces under Light Irradiation

Complex oxide materials are special in their ability to show highly diverse electronic properties and go through many different phase transitions, including ferromagnetic, ferroelectric, metal-to-insulator and superconducting. Furthermore, the similarity of crystal structures makes it possible to stack in epitaxial heterostructures layers possessing different functional properties and interacting with each other through electric or magnetic fields, charge transfer, exchange bias, strain, etc, opening the way to the design of almost unlimited multifunctional device concepts. In this context, this thesis is devoted to the transport characterization of transition metal oxides (TMOs) under the external perturbation of a photon field, with special attention dedicated to two quite outstanding cases, that are: i) Pr1-xCaxMnO3 (PCMO), a narrow band manganite showing an insulating behaviour across it's whole phase diagram, and ii) polar-non polar oxide interfaces, where the formation of a high mobility 2-dimensional electron gas (2DEG) in SrTiO3 (STO) is triggered by the deposition of a polar overlayer. Both systems are correlated systems, since their electronic properties are dominated by the narrow d-bands of the transition metals, Mn and Ti. STO is insulating, but doped STO has d carriers. The system is for this reason often defined as a 2-dimensional electron liquid (2DEL). In particular, there is a strong current interest arising on the photoconductance properties of both kinds of systems addressed in this thesis. In the first case it is established that by shedding light we do not simply dope the system with photocarriers within a rigid band picture, but we undisclose a hidden state which is quasi-degenerate in energy with the insulating ground state and has a different distribution of its density of states in k space. The second case might well be envisaged as a simple photodoping of STO, but, as we will show in this thesis work, there are some quite intriguing aspects to consider. The effects seen in interfaces is not seen in pure STO, and †" furthermore †" persistent photoconductivity (PC) with very long decay times (on the time scale of months) and at photon energies far below the STO gap is found. Also in view of the recently suggested phase separation, we can not exclude that something more complex than photodoping is taking place. In a more general framework, the present investigation may be considered as an attempt to add a tile to the wide mosaic of the physics of inhomogeneous states in TMO.