Dominance of many-body effects over the one-electron mechanism for band structure doping dependence in Nd 2− x Ce x CuO 4 : the LDA+GTB approach (original) (raw)
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Physical Review B, 2002
We use inelastic neutron scattering to study the evolution of the generalized phonon density of states (GDOS) of the n-type high-Tc superconductor Nd2−xCexCuO 4+δ (NCCO), from the halffilled Mott-insulator (x = 0) to the Tc = 24 K superconductor (x = 0.15). Upon doping the CuO2 planes in Nd2CuO 4+δ (NCO) with electrons by Ce substitution, the most significant change in the GDOS is the softening of the highest phonon branches associated with the Cu-O bond stretching and out-of-plane oxygen vibration modes. However, the softening occurs within the first few percent of Ce-doping and is not related to the electron doping induced nonsuperconducting-superconducting transition (NST) at x ≈ 0.12. These results suggest that the electron-lattice coupling in the n-type high-Tc superconductors is different from that in the p-type materials.
Renormalized band structure of CuO2 layers in superconducting compounds: A mean-field approach
Physical Review B, 1990
We have analyzed the band structure of a CuOz plane including two copper orbitals {d 2 2 and X d») and two oxygen orbitals {p", p~) in a tight-binding approximation. A Hubbard interaction between copper d holes has been considered with use of a slave-boson technique, while oxygen and copper holes interact via a nearest-neighbors Coulombic repulsion treated in the Hartree-Fock approximation. The resulting band structure opens the way to a comparison with the experiments and to some theoretical implications on the pairing mechanism of high-T, superconductors. The electronic structure of the high-temperature superconductors has been a controversial issue ever since their discovery by Bednorz and Muller. ' There is a general agreement in the scientific community in assigning a major role to the Cu02 layers in determining the normal and superconducting properties of these materials. At zero doping the Cu02 system has one hole per unit cell residing on Cu sites with d, 2(b,) symmetry. The additionx al holes introduced by doping in the superconducting samples reside on oxygen sites because of the large Hubbard repulsion on Cu sites Ud (of the order of 8-10 eV).
Physical Review Letters, 2007
Realistic electronic-structure calculations for correlated Mott insulators are notoriously hard. Here we present an ab initio multiconfiguration scheme that adequately describes strong correlation effects involving Cu 3d and O 2p electrons in layered cuprates. In particular, the O 2p states giving rise to the Zhang-Rice band are explicitly considered. Renormalization effects due to nonlocal spin interactions are also treated consistently. We show that the dispersion of the lowest band observed in photoemission is reproduced with quantitative accuracy. Additionally, the evolution of the Fermi surface with doping follows directly from our ab initio data. Our results thus open a new avenue for the first-principles investigation of the electronic structure of correlated Mott insulators.
Superconducting phase diagram of electron-doped cuprates Nd2−xCexCuO4
Physica C: Superconductivity, 2003
Detailed superconducting (SC) phase diagram of electron-doped cuprates Nd 2Àx Ce x CuO 4 has been studied by magnetic susceptibility measurements on single crystals with Ce concentration x up to 0.17. On increasing x, fractional superconductivity appears at around x ¼ 0:075. On further doping, bulk superconductivity sets in at around x ¼ 0:135, where the volume fraction of antiferromagnetic phase drastically degrades. SC samples with different onset T c $ 20 and 25 K were prepared by heat treatment in both fractional and bulk SC phases at a fixed x, and there exists no region where the onset T c depends on doping in the studied x-region. The boundary between fractional and bulk SC phases slightly shifts depending on the conditions of heat treatment.
Superconductivity induced by structural reorganization in the electron-doped cuprate <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML">mml:mrowmml:msubmml:miNdmml:mrowmml:mn2mml:mo−mml:mixmml:msubmml:miCe<...
Physical review, 2022
Electron-doped and hole-doped superconducting cuprates exhibit a symmetric phase diagram as a function of doping. This symmetry is however only approximate. Indeed, electron-doped cuprates become superconductors only after a specific annealing process: This annealing affects the oxygen content by only a tiny amount, but has a dramatic impact on the electronic properties of the sample. Here we report the occurrence of superconductivity in oxygen-deficient Nd2-xCexCuO4 thin films grown in an oxygen-free environment, after annealing in pure argon flow. As verified by x-ray diffraction, annealing induces an increase of the interlayer distance between CuO2 planes in the crystal structure. Since this distance is correlated to the concentration of oxygens in apical positions, and since oxygen content cannot substantially increase during annealing, our experiments indicate that the superconducting phase transition has to be ascribed to a migration of oxygen ions to apical positions during annealing. Moreover, as we confirm via first-principles density functional theory calculations, the changes in the structural and transport properties of the films can be theoretically described by a specific redistribution of the existing oxygen ions at apical positions with respect to CuO2 planes, which remodulates the electronic band structure and suppresses the antiferromagnetic order, allowing the emergence of hole superconductivity.
Physical Review Letters, 2008
The mechanism of field-effect doping in the "123" high critical temperature superconductors (HTS) has been investigated by x-ray absorption spectroscopy in the presence of an electric field. We demonstrate that holes are created at the CuO chains of the charge reservoir and that field-effect doping of the CuO2 planes occurs by charge transfer, from the chains to the planes, of a fraction of the overall induced holes. The electronic properties of the charge reservoir and of the dielectric/HTS interface determine the electric field doping of the CuO2 planes.
Possible electronic properties in high-Tc superconducting cuprates from cluster calculations
Chemical Physics Letters, 1996
Electronic structure calculations have been performed on a Ca2Cu404 cluster to model possible local properties of relevance to superconductivity in cuprates. Explicit account of near-degeneracies suggests the pairing potential mixes anisotropic s and d symmetries as a result of going beyond the independent particle description. Pairing results from the screening of cation repulsion across the CuO 2 plane. Pair-breaking energies in the range 0-60 meV are computed dependent on the cation to plane distance. A possible mechanism for superconductivity emerges.
Physical Review B, 1998
Polarization-dependent x-ray-absorption spectroscopy at the Cu L 3 and Ce M 4,5 edges has been performed on epitaxial films of Nd 1.85 Ce 0.15 CuO 4Ϫ␦ vs defect concentration induced by He ϩ ion irradiation. The Cu L 3 edge exhibits an increase in the integral intensity of both the white line in Eʈab (ϳ15%) and the first peak in Eʈc (ϳ20%) after superconductivity suppression. At the same time, partial filling of Ce 4 f orbitals (ϳ16%) takes place, though the shape of Ce M 4,5 edges remains the same as for the formally tetravalent Ce ion. The mechanism of superconductivity suppression in Nd 1.85 Ce 0.15 CuO 4Ϫ␦ under He ϩ ion irradiation is discussed. The Cu L 3 -edge measurements vs Ce doping indicate that the major part (ϳ70%) of the excess electrons fill Cu 3d x 2 Ϫy 2 states in Nd 1.85 Ce 0.15 CuO 4Ϫ␦ . The remaining part extends beyond the CuO 2 superconducting plane forming ''impurity'' states that seem to be localized within a few unit cells.