Substrate and thickness influence on YBa$$_2$$Cu$$_3$$O$$_7-\delta $$ thin films grown by PLD deposition

We present a study of high (T_mathrm{c}) superconductor YBa(_2)Cu(_3)O(_7-delta ) (YBCO) thin films of different thickness, grown on two different substrates SrLaAlO(_4) (SLAO) and SrTiO(_3) (STO). The samples were deposited using the Pulsed Laser Deposition (PLD) technique. Structural properties were analyzed with X-ray diffraction, AFM imaging, and Raman spectroscopy. The samples grown on SLAO exhibit an epitaxial strain for thickness less than 60 nm but no strain is achieved through thickness reduction in the STO case. For both types of substrates, no changes in oxygen content are observed through Raman spectroscopy. For the 60 nm YBCO film on SLAO substrate, certain frequency mode shifts indicate the presence of strain in the sample, consistent with the level of strain detected by X-ray diffraction. No similar detection was found for growth of the sample on STO, no relevant change in the critical temperature is observed. However, when samples are grown on SLAO, a small decrease in the critical temperature with the number of deposited unit cells is perceived. Models based on size and strain effects, related to quantum charge fluctuations, are introduced to explain the decrease of critical temperature with decreasing thickness for YBCO films on SLAO substrates. It is found that when an in-plane compressive strain appears it dominates over the size effect, producing a sharp drop in the critical temperature, otherwise the size effect can be observed.