Abstract
On the basis of exact diagonalizations a comparative study of optical (two-particle) and single-particle excitations is presented for a 2-D multiorbital Hubbard model. The single-particle excitations display strongly correlated states related to the Zhang-Rice Cu-O singlet construction. These states define the gap (to the upper Hubbard band) at half-filling and become partially occupied by doping holes in our 2 × 2 unit cell system. At a hole concentration δ = 0.25 the imaginary part of the self energy does not vanish close to the Fermi energy, indicating a non-Fermi liquid behavior of the Zhang-Rice states. The optical results show three allowed optical transitions in the low-energy regime: (i) The Drude peak at ω = 0 with spectral weight proportional to z; (ii) a small structure due to transitions within the singlet band complex, which may be related to the experimentally observed midinfrared absorption. Both transitions (i) and (ii) stem from itinerant motion of the Zhang-Rice singlets. (iii) Unbinding of the O hole from the Cu spin in the singlet. This gives a strong absorption peak due to non-bonding oxygen → CuO singlet transitions, again with relative weight ≈ z.
Published Version
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