Critical behavior of hard squares in strong confinement

We examine the phase behavior of a quasi-one-dimensional system of hard squares with side-length $\sigma$, where the particles are confined between two parallel walls and only nearest neighbor interactions occur. As in our previous work (PRE, 94, 050603 (2016)), the transfer operator method is used, but here we impose a restricted orientation and position approximation to yield an analytic description of the physical properties. This allows us to study the parallel fluid-like to zigzag solid-like structural transition, where the compressibility and heat capacity peaks sharpen and get higher as $H \rightarrow H_c=2\sqrt{2}-1\approx 1.8284$ and $p \rightarrow p_c= \infty$. Here $H$ is the width of the channel measured in $\sigma$ units and $p$ is the pressure. We have found that this structural change becomes critical at the $(p_c,H_c)$ point. The obtained critical exponents belong to the universality class of the one-dimensional Ising model. We believe this behavior holds for the unrestricted orientational and positional case.