Measurement of London penetration depth from holographic images of superconducting vortices: The influence of specimen thickness

High-resolution holographic data of superconducting vortices are presented and analyzed in order to extract a measurement of the London penetration depth by fitting the reconstructed phase across the core using an analytical one-dimensional London model. The resulting value of $50\pm 5\mathrm{nm}$ for the London penetration depth is obtained, which is about two times larger than the commonly accepted value of 30 nm used in previous simulations. It is shown that this discrepancy can be removed by taking into account the influence on the phase shift of the specimen thickness and of the associated broadening of the field lines near the surface. These results highlight the importance of the assumed model in order to extract from the analysis of experimental data reliable quantitative estimates of critical parameters such as the London penetration depth.