Ferromagnetism in the Hubbard model on the square lattice: Improved instability criterion for the Nagaoka state

Abstract

The instability of the fully polarized ferromagnetic state (Nagaoka state) with respect to single spin flips is re-examined for the Hubbard model on the square lattice with a large family of variational wave functions which include correlation effects of the majority spins in the vicinity of the flipped spin. We find a critical hole density of ${\delta }_{cr}=0.251$ for $U=\infty$ and a critical coupling of $U_{cr}=77.7t$. Both values improve previous variational results considerably.

10 pages + 1 figure, submitted as uuencoded compressed ps-file, to be published in Annalen der Physik