First-principles study of ground, excitonic, and hole states in Sr₂CuO₃

First-principles, periodic, spin-unrestricted Hartree-Fock (UHF) calculations predict the ground state of Immm Sr2CuO3 to be a highly ionic (d(9)) insulator with strong coupling of antiferromagnetically aligned d(x2-y2) spins within the corner-sharing CuO4 units. The distribution of ground-state valence-band states is in good agreement with He I and He II photoemission spectra with O(p) states occupying the upper part of the band and Cu(d) states the lower part. The conduction-band lower edge consists largely of Cu(d) states leading to essentially a charge-transfer classification. Compared with the experimental values, the local Cu spin moment is grossly overestimated and the (Ising model) coupling constant grossly underestimated, due, it is suggested, to zero-point fluctuations. Local d-->d excitons obtained directly from variationally minimized excited states are found between similar to1.2 and similar to2.2 eV, of which the lowest-energy state, d(xy)-->d(x2-y2), is predicted to occur within the optical gap of similar to1.5 eV. Hole states are found to be strongly polaronic, of essentially O(p) character and confined (at low temperature) to non-corner-sharing oxygen sites. Taking account of electron correlation, the lowest-energy hole state is predicted to be locally singlet with an unpaired O p, electron aligned antiferromagnetically to (d(x2-y2))(1) in the form of a modified (two-center) Zhang-Rice singlet. Antiferromagnetic alignment is predicted to persist (at zero Kelvin) even for hole densities of 50%.