Vanadium dioxide undergoes a first order metal-insulator transition at 340 K. In this work, we develop and carry out state of the art linear scaling DFT calculations refined with non-local dynamical mean-field theory. We identify a complex mechanism, a Peierls-assisted orbital selection Mott instability, which is responsible for the insulating M1 phase, and furthermore survives a moderate degree of disorder.

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 electrons [1][2][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 method 4 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 simplified model Hamiltonians 7-10 substantiated this picture and showed that the actual copper oxides are not far from this metal-insulator boundary.Here we go beyond model studies by incorporating realistic crystal structure and the interplay with magnetism. We study both the electron-and hole-doped cuprates. We find that the parent compound NdCuO 4 of the electron-doped cuprate lies on the metallic side of the metal-insulator boundary. NdCuO 4 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 Nd 2−x Ce x CuO 4 (NCCO), and we compare it with La 2−x Sr x CuO 4 (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).Department of Physics, Rutgers University, Piscataway, New Jersey 0...

We use first-principles calculations to extract two essential microscopic parameters, the charge-transfer energy and the inter-cell oxygen-oxygen hopping, which correlate with the maximum superconducting transition temperature Tc,max across the cuprates. We explore the superconducting state in the three-band model of the copper-oxygen planes using cluster Dynamical Mean-Field Theory. We find that the variation in the charge-transfer energy largely accounts for the empirical trend in Tc,max, resolving a long-standing contradiction with theoretical calculations. perconductors with the charge-transfer energy Abstract -Supplementary material providing the details of the extraction of the materials parameters and the numerical method used to solve the 3-band Hubbard model.Appendix: Table of parameters. -We summarize in Table 1 the parameters extracted via downfolding for the three-band model and discuss the details of the downfolding procedure.

We use the Local Density Approximation in combination with the Dynamical Mean Field Theory to carry out a comparative investigation of a typical electron doped and a typical hole doped copper oxide, NCCO and LSCO respectively. The parent compounds of both materials are strongly correlated electron systems in the vicinity of the metal to charge transfer insulator transition. In NCCO the magnetic long range order is essential to open a charge transfer gap, while Mott physics is responsible for the gap in LSCO. We highlights the role of the apical oxygens in determining the strength of the correlations and obtaining overall good agreement between theory and several experimentally determined quantities. Results for optical conductivity, polarized X-ray absorption and angle resolved photoemission are presented and compared with experiments.

We study the thermal properties of the classical antiferromagnetic Heisenberg model with both nearest (J1) and next-nearest (J2) exchange couplings on the square lattice by extensive Monte Carlo simulations. We show that, for J2/J1>1/2, thermal fluctuations give rise to an effective Z2 symmetry leading to a finite-temperature phase transition. We provide strong numerical evidence that this transition is in the 2D Ising universality class, and that T(c)-->0 with an infinite slope when J2/J1-->1/2.