Temperature dependence of the spectral weight in undoped Mott insulators

As recent angular-resolved photoemission spectroscopy experiments have shown, even an insulator can exhibit a "remnant" Fermi surface (FS), i.e., a boundary line between mostly occupied and mostly unoccupied states in the Brillouin zone. We derive an analytic expression for the remnant FS valid to order (t/U)(2) for the large-U limit of the single-band Hubbard model at half-filling. The shape of the remnant FS at low temperature (T<<J=4t(2)/U) is found to be determined by local magnetic correlations, and is largely independent of the details of the band FS. However, miraculously, at intermediate temperatures (J<<T<<U), where magnetic correlations are weak, the location of the remnant FS coincides with the band FS.