Casimir force within Ising chain with competing interactions

We derive exact results for the critical Casimir force (CCF) within the one-dimensional Ising model with periodic boundary conditions (PBC's) and long-range equivalent-neighbor ferromagnetic interactions of strength Jₗ/N>0 superimposed on the nearest-neighbor interactions of strength Jₛ which could be either ferromagnetic (Jₛ>0) or antiferromagnetic (Jₛ<0). In the infinite system limit the model, also known as the Nagle-Kardar model, exhibits in the plane (Kₛ=β Jₛ,Kₗ=β Jₗ) a critical line 2 Kₗ=exp(-2 Kₛ), Kₛ>-ln3/4, which ends at a tricritical point (Kₗ=-√3/2, Kₛ=-ln3/4). The critical Casimir amplitudes are: ΔCas⁽ᶜʳ⁾=1/4 at the critical line, and ΔCas⁽ᵗʳ⁾=1/3 at the tricritical point. Quite unexpectedly, with the imposed PBC's the CCF exhibits very unusual behavior as a function of temperature and magnetic field. It is repulsive near the critical line and tricritical point, decaying rapidly with separation from those two singular regimes fast away from them and becoming attractive, displaying in which the maximum amplitude of the attraction exceeds the maximum amplitude of repulsion. This represents a violation of the widely-accepted "boundary condition rule", which holds that the CCF is attractive for equivalent BC's and repulsive for conflicting BC's independently of the actual bulk universality class of the phase transition under investigation.