Importanza del processo di knock-out dell'ossigeno nella variazione delle proprietà elettriche del Bi-2212 indotte da radiazione di sincrotrone

Bi2Sr2CaCu2O8+δ (Bi-2212) represents one of the most extensively studied superconducting oxides, because of its high critical temperature Tc of about 80 K. The oxygen nonstoichiometry, which is given by the presence of interstitial oxygen atoms, has a fundamental role since it affects both the structural and electronic properties of the material, being able to tune its Tc and even to drive it to a nonsuperconducting state. It has been recently shown that irradiation with a high-dose of 17 keV X-rays can affect both structural and electronic properties of Bi-2212 by modifying its oxygen content. Subsequently, fully functional devices consisting of a few Josephson junctions have been obtained by locally turning the material into nonsuperconducting with a 17.5 keV X-rays synchrotron beam. (Truccato et al. Nano Lett. 16, 1669−1674 (2016)). Indications have emerged that probably only interstitial O ions have been removed from the crystals and this feature makes this technique interesting as a possible direct-writing patterning method that could be applied to other oxide materials as well. Therefore, in this context, an understanding of the mechanism of oxygen depletion caused by the synchrotron radiation is needed. The aim of my thesis work has been to evaluate the contribution to the depletion of the nonstoichiometric oxygen atoms given by the elastic ¿knock-on¿ scattering of electrons photogenerated in the sample by the X-rays photons. This process is described by the McKinley-Feshbach cross section and can bring to an irreversible displacement of the atom if the photoelectron kinetic energy exceeds some threshold energy. I have carried out the evaluation of the spatial and energy distribution of the photogenerated electrons flux in the sample by means of the Monte Carlo radiation-transport code MCNP6TM by simulating the sample geometry and the properties and position of the X-ray nanobeams. It has turned out that for a 17 keV nanobeam the photoelectron fluence (Φe) decreases by a factor of 10 within about 150 nm from the center of the beam, which indicates that the process is well localized and is a good candidate to be exploited for patterning processes with nanometric resolution. The fraction of displaced interstitial oxygen atoms (fad) is the parameter that quantifies the damage and can be obtained by dividing the density of displaced oxygens (Nad) by the density of targets (Noi). Nad is obtained, for every position in the sample, convoluting the McKinley-Feshbach cross section with the photoelectron fluence. The result is a spatial map of the damage in the sample from which two indications arise. Firstly, the tightness of the irradiation mesh is extremely important to increase the homogeneity of the damage, and secondly the contribution of near irradiation spots significantly increases the maximum value of fad. Since the resistivity of the material can be related to its oxygen content, electrical measurements before and after irradiation are used to find an experimental estimate of fad that can be compared with the spatial average of the results obtained from the simulations. This comparison has allowed to conclude that either the threshold energy for the knock-on process is intermediate between the value of 0.073 eV (that is the binding energy of the interstitial oxygen atoms) and 0,93 eV (activation energy for oxygen diffusion), or the knock-on process alone cannot explain the change of properties caused by X-rays irradiation.