 9111 B.Toth
 Failure of saturated ferromagnetism for the Hubbard model with two holes
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Jul 13, 91

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Abstract. We consider the Hubbard model on a finite set of sites with nonpositive
hopping matrix elements and infinitely strong onsite repulsion. Nagaoka's
theorem states that in this model the relative ground state in the sector with
one unoccupied site is maximally ferromagnetic. We show that this phenomenon is
a consequence of a combinatorial coincidence valid in the onehole regime only. In the case of more than one hole there is no reason to expect maximally
ferromagnetic ground states. We prove this claim for the case of two holes for
models defined on a class of graphs which contains all tori that are not too
small.
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