Critical Casimir forces for ${\cal O}(n)$ systems with long-range interaction in the spherical limit
We present exact results on the behavior of the thermodynamic Casimir force and the excess free energy in the framework of the $d$-dimensional spherical model with a power law long-range interaction decaying at large distances $r$ as $r^{-d-σ}$, where $σ T_c$ and $T T_c$ it decays as $L^{-d-σ}$, where $L$ is the thickness of the film. We consider both the case of a finite system that has no phase transition of its own, when $d-1<σ$, as well as the case with $d-1>σ$, when one observes a dimensional crossover from $d$ to a $d-1$ dimensional critical behavior. The behavior of the force along the phase coexistence line for a magnetic field H=0 and $T 1$ and a decreasing one for $σ<1$. For any $d$ and $σ$ the minimum of the force at $T=T_c$ is always achieved at some $H\ne 0$.