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Introduction to the Ising Model

The Ising Model is one of the pillars of statistical mechanics. Each site can have two values (red/white, 1/0, +/-, ...), and neighboring sites have an energetic preference to be the same value. As a system of +/- spins, it is a model for magnetism: like iron, there is a temperature (the Neel point) above which the magnetization "melts" away. Run at high temperatures (say above 3) to see the melted state: run at low temperatures (below 2) to see the magnetized state.

Thought of as sites either occupied or vacant (1/0) on a lattice, it is a model for the liquid-gas transition: dense regions of occupied "liquid" are surrounded by dilute regions of mostly "gas". Our simulation isn't ideal for visualizing this, because we allow atoms to be created and destroyed: one of the possible projects involves writing a version which only allows sites to flip in pairs, moving atoms from occupied to empty sites. Try running at 2.4 for a while (to generate a "clumped up" state), and then drop to 2.0 or so. For a limited time, you will likely observe fairly well defined red regions and white regions: think of a red fluid with a few red vapor atoms in the white regions. As you run for longer, you'll see either the vapor or the fluid win: the last drop will evaporate, or the last bubble will collapse. Of course, real liquid/vapor transitions are in three dimensions: another project will be to write a version of the program which runs a three-dimensional Ising model.

This is an example of Badri Krishnamachari's Ising-type lattice gas to study surface evolution in Barbara Cooper's group.


Other Ising Model Presets

Description of the Model.
Phase Diagram
Magnetization M(T)
Nucleation
Domain Coarsening
Questions for Further Research

Links

The Ising Model
New Tools in Physics: Course Description
Cornell Undergraduate Physics Teaching Lab.
LASSP Home Page, and Entertaining Science Done There,

Last modified: September 11, 1995

James P. Sethna, sethna@lassp.cornell.edu.

Statistical Mechanics: Entropy, Order Parameters, and Complexity, now available at Oxford University Press (USA, Europe).