Stability of Ferromagnetism in Hubbard models with degenerate single-particle ground states
نویسنده
چکیده
A Hubbard model with a Nd-fold degenerate single-particle ground state has ferromagnetic ground states if the number of electrons is less or equal to Nd . It is shown rigorously that the local stability of ferromagnetism in such a model implies global stability: The model has only ferromagnetic ground states, if there are no single spin-flip ground states. If the number of electrons is equal to Nd , it is well known that the ferromagnetic ground state is unique if and only if the single-particle density matrix is irreducible. We present a simplified proof for this result. PACS-numbers: 75.10.Lp, 71.10.Fd
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