Random energy model


In the statistical physics of disordered systems, the random energy model is a toy model of a system with quenched disorder, such as a spin glass, having a first-order phase transition. It concerns the statistics of a collection of spins so that the number of possible states for the system is. The energies of such states are independent and identically distributed Gaussian random variables with zero mean and a variance of. Many properties of this model can be computed exactly. Its simplicity makes this model suitable for pedagogical introduction of concepts like quenched disorder and replica symmetry.

Comparison with other disordered systems

The -spin Infinite Range Model, in which all -spin sets interact with a random, independent, identically distributed interaction constant, becomes the Random-Energy Model in a suitably defined limit.
More precisely, if the Hamiltonian of the model is defined by
where the sum runs over all distinct sets of indices, and, for each such set,, is an independent Gaussian variable of mean 0 and variance, the Random-Energy model is recovered in the limit.

Derivation of thermodynamical quantities

As its name suggests, in the REM each microscopic state has an independent distribution of energy. For a particular realization of the disorder, where refers to the individual spin configurations described by the state and is the energy associated with it. The final extensive variables like the free energy need to be averaged over all realizations of the disorder, just as in the case of the Edwards Anderson model. Averaging over all possible realizations, we find that the probability that a given configuration of the disordered system has an energy equal to is given by
where denotes the average over all realizations of the disorder. Moreover, the joint probability distribution of the energy values of two different microscopic configurations of the spins, and factorizes:
It can be seen that the probability of a given spin configuration only depends on the energy of that state and not on the individual spin configuration.
The entropy of the REM is given by
for. However this expression only holds if the entropy per spin, is finite, i.e., when Since, this corresponds to. For, the system remains "frozen" in a small number of configurations of energy and the entropy per spin vanishes in the thermodynamic limit.