Non-monotonic behavior of the Binder parameter in discrete spin systems

We study a non-monotonic behavior of the Binder parameter, which appears in discrete spin systems. We show that the Binder parameters of the Potts model are non-monotonic for q = 3 and 4, while they are monotonic for the Ising case (q = 2). Using the Fortuin-Kasteleyn graph representation, we find that the improved estimator of the Binder parameter consists of two terms with values only in the high- and low-temperature regions. The non-monotonic behavior is found to originate from the low-temperature term. With the appropriately defined order parameter, we can reduce the influence of the low-temperature term and, as a result, the non-monotonic behavior can also be reduced. We propose new definitions for the order parameter, which reduces or eliminates the non-monotonic behavior of the Binder parameter in a system for which the improved estimator of the Binder parameter is unknown.