Weakly Nonextensive Thermostatistics and the Ising Model with Long--range Interactions

We introduce a nonextensive entropic measure $S_{\chi }$ that grows like $N^{\chi }$, where $N$ is the size of the system under consideration. This kind of nonextensivity arises in a natural way in some $N$-body systems endowed with long-range interactions described by $r^{-\alpha }$ interparticle potentials. The power law (weakly nonextensive) behavior exhibited by $S_{\chi }$ is intermediate between (1) the linear (extensive) regime characterizing the standard Boltzmann-Gibbs entropy and the (2) the exponential law (strongly nonextensive) behavior associated with the Tsallis generalized $q$-entropies. The functional $S_{\chi }$ is parametrized by the real number $\chi \in [1,2]$ in such a way that the standard logarithmic entropy is recovered when $\chi =1$

. We study the mathematical properties of the new entropy, showing that the basic requirements for a well behaved entropy functional are verified, i.e., $S_{\chi }$ possesses the usual properties of positivity, equiprobability, concavity and irreversibility and verifies Khinchin axioms except the one related to additivity since $S_{\chi }$ is nonextensive. For $1<\chi <2$, the entropy $S_{\chi }$ becomes superadditive in the thermodynamic limit. The present formalism is illustrated by a numerical study of the thermodynamic scaling laws of a ferromagnetic Ising model with long-range interactions.