Film Thickness Dependence of the Critical Temperature of Superconductivity of LSCO Films: A Bipolaron Model Approach

The superconductivity of (001) oriented La1.85Sr0.15CuO4 cuprate films with the different film thicknesses (d(f)) is considered within the bipolaronic mechanism of high T-c c superconductivity. It is supposed that high-T-c superconductivity of the cuprates is due to Bose-Einstein condensation of an ideal gas of the intersite bipolarons, and therefore, the critical temperature of superconductivity (T-c) is associated with the Bose-Einstein condensation temperature (T-BEC) of the intersite bipolarons. Taking the extended Holstein-Hubbard model as a basis for the cuprate films, a relation between T-BEC of the ideal gas of the intersite bipolarons of La1.85Sr0.15CuO4 film and film thickness is established. It is shown that the calculated values of T-BEC of the ideal gas of the intersite bipolarons of La1.85Sr0.15CuO4 films grown on LaSrAlO4 and SrTiO3 substrates correlate (and at certain values of thickness satisfactorily agree) with the experimental values of T-c of the above systems. This article is the first theoretical work, according to our knowledge, that shows the fundamental possibility of studying the thickness dependence of the high-T-c superconductivity of the cuprate films within the framework of the bipolaronic Bose-Einstein condensation scenario.