Microscopic theory for insulator to metal transition in cuprates
Journal article, 2012
Hubbard U is equal to the energy of the insulator to metal transition in Mott insulators. Hubbard U is, therefore, also the excitation energy for metal atom to other metal atom transitions and the key concept in photoinduced conductivity spectra of pure and doped cuprates. In this article, the electronic structure of Cu(II) and Cu(III) sites is first discussed. It is shown that Hubbard U depends on the location of the excitation in the CuO2 plane relative to the Cu(III) sites. Far from the Cu(III) sites, the ground-state wave function continues to be of spin-coupled type. Near to a Cu(III) site, it changes character and is mixed with charge components. The Hubbard gap thus depends on locality in the CuO2 plane. Close to a Cu(III) site, it tends to zero and induces local conductivity. Far from a Cu(III) site, it is large but converges to zero as the doping levels are raised. In fact, the Hubbard gap has many features in common with the pseudogap.