Evanescent character of the repulsive thermal Casimir force

TL;DR

This paper clarifies the physical origin of the negative thermal correction to the Casimir force, showing that evanescent waves cause repulsion at small distances, while at large distances the force is predominantly attractive.

Contribution

It provides a detailed analysis of the thermal Casimir force's asymptotic behavior, highlighting the role of evanescent waves in the repulsive component at small separations.

Findings

Evanescent waves dominate the repulsive thermal Casimir force at small distances.

At large distances, the Casimir force is mainly attractive due to cancellation of wave contributions.

The study explains the naturalness of the large thermal correction in metal-metal interactions.

Abstract

The physical origin of the negative thermal correction to the Casimir force between metals is clarified. For this purpose the asymptotic behavior of the thermal Casimir force is analyzed at large and small distances in the real frequency representation. Contributions from propagating and evanescent waves are considered separately. At large distances they cancel each other in substantial degree so that only the attractive Lifshitz limit survives. At smaller separations the repulsive evanescent contribution of s-polarization dominates in the case of two metals or a metal and a high-permittivity dielectric. Common origin and order of magnitude of the repulsion in these two cases demonstrate naturalness of the controversial large thermal correction between metals.