Two-dimensional Hubbard model with strong electron repulsion

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Abstract
The two-dimensional Hubbard model is studied in the limit u/t≫1. It is shown that almost all phase transitions are first-order transitions with a metastable-state lifetime depending discontinuously on the external parameters. The phase transition from the ferromagnetic to the ferrimagnetic state in a strong magnetic field is an exception: It is a second-order transition in a model with a half-filled electron band and it is a weakly-first-order transition in a model with a nearly-half-filled electron band. In the latter case the superconductive state is formed.