Casimir-Lifshitz force with graphene: theory versus experiment, role of spatial non-locality and of losses

We calculate the Casimir-Lifshitz force (CLF) between a metallic sphere and a graphene-coated SiO$_2$ plane and compare our finding with the experiment and theory by M. Liu \emph{et al.}, PRL {\bf 126}, 206802 (2021), where a non-local and lossless model for the graphene conductivity (GC) has been used and shown to be compatible with the experimental results. Recently, that conductivity model has been shown to be not correctly regularized [arXiv:2403.02279], to predict nonphysical results in the non-local regime, and being correct only in its local limit, where its expression is identical to the local Kubo conductivity model (once also losses are introduced). To compare the experimental results with the correctly regularized Kubo theory and to clarify the effective role played by the graphene non-locality and losses in that experiment, we calculated the CLF using three different models for the GC: the correct general non-local Kubo model, the local Kubo model, and the non-regularized and lossless model used by M. Liu \emph{et al.}. For the parameters of the experiment, the predictions for the Casimir-Lifshitz force using the three models are practically identical, implying that, for the experiment, both non-local and lossy effects in the GC are negligible. This explains why the GC model used in M. Liu \emph{et al.} provides results in agreement with the experiment. We find that the experiment cannot distinguish between the Drude and Plasma prescriptions. Our findings are relevant for present and future comparisons with experimental measurement of the Casimir-Lifshitz force involving graphene structures. Indeed, we show that an extremely simple local Kubo model, explicitly depending on Dirac mass, chemical potential, losses and temperature, is largely enough for a totally comprehensive comparison with typical experimental configurations.
Comment: 17 pages, 15 figures