Manifestations of spin and charge fluctuations in spectra of the Hubbard model

The influence of long-range spin and charge fluctuations on spectra of the two-dimensional fermionic Hubbard model is considered using the strong coupling diagram technique. Infinite sequences of diagrams containing ladder inserts, which describe the interaction of electrons with these fluctuations, are summed, and obtained equations are self-consistently solved for the range of Hubbard repulsions $4t\leq U\leq 8t$ and temperatures $0.3t\lesssim T\lesssim t$ with $t$ the intersite hopping constant. It was found that a metal-insulator transition curve goes from larger $U$ and $T$ to smaller values of these parameters. The temperature decrease causes the transition to the long-range antiferromagnetic order. It is responsible for the splitting out of a narrow band from a Hubbard subband with doping for $U=8t$ and low $T$. This segregated band is located near the Fermi level and forms a pseudogap here.