Casimir-Polder attraction-repulsion crossover criterion

Abstract: The mutual electromagnetic correlations between two spatially separated systems gives rise to Casimir and Casimir-Polder effect. The corresponding forces, which are generally attractive for most vacuum-separated metallic or dielectric geometries, are due to the contribution to the ground-state energy of the coupled system. We investigate here the Casimir-Polder free energy corresponding to interactions of a magnetically and electrically polarizable micro-particle with a magneto-dielectric sheet. Our semi-phenomenological study shows that such an interaction is reversibly tunable in strength and sign.The latter, particularly, is true provided we look for the exotic materials fabricated at scales between the micron and the nanometer. The crossover between attractive and repulsive behavior is found to depend on the polarizability ratio of the micro-particle and the electromagnetic impedance of the magneto-dielectric sheet.