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--&gt;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)--&gt;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%.</p

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--&gt;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)--&gt;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%.</p

First principles calculations of solute-hole interactions in the high T_c oxyfluoride superconductor Sr₂CuO₂F₂

First principles periodic Hartree-Fock calculations are reported of the electronic and magnetic structures and hole states in the T (La2CuO4) phases of Sr2CuO2F2, Sr2CuO2F2.5 and NaSrCuO2F2. Sr2CuO2F2 is predicted to be a highly ionic antiferromagnetic charge transfer insulator with Cu in a d(9) ((d(x2-y2))(1)) configuration. Holes are predicted to be highly localised spin polarons which destroy the AF ordering at concentrations of 0.5 per formula unit.</p

First principles calculations of solute-hole interactions in the high T_c oxyfluoride superconductor Sr₂CuO₂F₂

First principles periodic Hartree-Fock calculations are reported of the electronic and magnetic structures and hole states in the T (La2CuO4) phases of Sr2CuO2F2, Sr2CuO2F2.5 and NaSrCuO2F2. Sr2CuO2F2 is predicted to be a highly ionic antiferromagnetic charge transfer insulator with Cu in a d(9) ((d(x2-y2))(1)) configuration. Holes are predicted to be highly localised spin polarons which destroy the AF ordering at concentrations of 0.5 per formula unit.</p