Quantum thermal field fluctuation induced corrections to the interaction between two ground-state atoms

We generalize the formalism proposed by Dalibard, Dupont-Roc, and Cohen-Tannoudji [the DDC formalism] in the fourth order for two atoms in interaction with scalar fields in vacuum to a thermal bath at finite temperature $T$, and then calculate the interatomic interaction energy of two ground-state atoms separately in terms of the contributions of thermal fluctuations and the radiation reaction of the atoms and analyze in detail the thermal corrections to the van der Waals and Casimir-Polder interactions. We discover a particular region, i.e., $\sqrt[4]{\lambda^3\beta}\ll L\ll \lambda$ with $L$, $\beta$ and $\lambda$ denoting the interatomic separation, the wavelength of thermal photons and the transition wavelength of the atoms respectively, where the thermal corrections remarkably render the van der Waals force, which is usually attractive, repulsive, leading to an interesting crossover phenomenon of the interatomic interaction from attractive to repulsive as the temperature increases. We also find that the thermal corrections cause significant changes to the Casimir-Polder force when the temperature is sufficiently high, resulting in an attractive force proportional to $TL^{-3}$ in the $\lambda\ll\beta\ll L$ region, and a force which can be either attractive or repulsive and even vanishing in the $\beta\ll\lambda\ll L$ region depending on the interatomic separation.