Strength of correlations in electron- and hole-doped cuprates

The introduction of holes in a parent compound consisting of copper oxide layers results in high-temperature superconductivity. It is also possible to dope the cuprate parent compound with electrons1–3. The physical properties of these electrondoped materials bear some similarities to but also significant differences from those of their hole-doped counterparts. Here, we use a recently developed first-principles method4 to study the electron-doped cuprates and elucidate the deep physical reasons behind their behaviour being so different from that of the hole-doped materials. The crystal structure of the electrondoped compounds is characterized by a lack of apical oxygens, and we find that it results in a parent compound that is a Slater insulator—a material in which the insulating behaviour is the result of the presence of magnetic long-range order. This is in sharp contrast with the hole-doped materials, which are insulating owing to the strong electronic correlations but not owing to magnetism. The understanding of late-transition-metal oxides begins with seminal work by Zaanen et al.5, which established that the lowestenergy excitations in materials such as the copper oxides are the charge-transfer excitations of an electron from the oxygen anions to the copper cation. If the energy cost of the charge-transfer process is less then the kinetic energy gain resulting from this process, the system is metallic. Otherwise it is insulating. Materials on the metallic side of this metal–insulator boundary can turn into insulators if the band structure is such that Bragg scattering from the zone boundary can open a bandgap, as stressed in ref. 6. On the other hand, in the charge-transfer insulators, the magnetic order that occurs at lower temperatures is a consequence rather than the cause of the insulating behaviour. The metal–insulator boundary is a ‘mean-field’ theoretical boundary separating the itinerant and the localized regimes of the low-energy electronic excitations. Early studies on simplifiedmodel Hamiltonians7–10 substantiated this picture and showed that the actual copper oxides are not far from thismetal–insulator boundary. Here we go beyond model studies by incorporating realistic crystal structure and the interplay with magnetism. We study both the electronand hole-doped cuprates. We find that the parent compound NdCuO4 of the electron-doped cuprate lies on the metallic side of the metal–insulator boundary. NdCuO4 is hence an insulator only as a result of the magnetic long-range order. This is in sharp contrast with the hole-doped cuprates, where the parent compound is a charge-transfer insulator. We study the single-layer electron-doped compound Nd2−xCexCuO4 (NCCO), and we compare it with La2−xSrxCuO4 (LSCO), a single-layer hole-doped material in the related T structure. We use a realistic theoretical approach, the local-density approximation combined with the dynamical mean-field theory (LDA+DMFT; refs 4,11,12).