Silvina Bengio

High-resolution photoemission spectroscopy combined with density functional theory is used to elucidate how chemical bonding governs the electronic structure of the semimetallic topological candidate β-PtBi₂. On the low-temperature-cleaved (111) surface, the Bi4f core level remains at the binding energy of elemental Bi, whereas Pt4f exhibits an anomalous positive shift of approximately 0.5 eV relative to elemental Pt. Reproducing the experimental valence-band spectrum also requires shifting the calculated density of states by about 0.3 eV toward higher binding energy. These observations indicate that the measured binding-energy shifts reflect not only charge redistribution but also inefficient photohole screening associated with covalent-like Pt5d–Bi6p hybridization. Surface-sensitive Bi5d measurements further suggest enhanced surface metallicity without evidence of a chemically distinct surface phase. In contrast, room-temperature cleavage and sputtering/annealing treatments generate additional Bi and Pt core-level components, indicating a non-stoichiometric PtBi₂−x surface phase. The associated core-level shifts and valence-band evolution indicate that Bi vacancies induce Pt5d–Bi6p rehybridization toward a more metallic bonding regime. These findings establish β-PtBi₂ as a model system for understanding how chemical bonding, surface stoichiometry, charge redistribution, and photohole screening collectively determine the photoemission response of semimetallic intermetallic compounds.

Ferromagnetic thin films of La_2/3Ca_1/3MnO₃ manganite were grown by pulsed laser deposition, under different oxygen atmospheres, on silicon substrates. We performed structural, magnetic, spectroscopic, and electrical characterization of the films. Resistive switching between high and low resistance states was obtained upon pulsing with opposite polarities voltages. The I-V curves exhibit sharp transitions between these states. The RS properties are strongly dependant on the films oxygen stochiometry and on the compliance current used for producing the high to low transition. ON/OFF ratios as high as 1000 were obtained for optimal RS conditions. Obtained results are discussed within the framework of mobile oxygen vacancies.