A first-principles density-functional description of the electronic structures of the high-T c cuprates has remained a long-standing problem since their discovery in 1986, with calculations failing to capture either the insulating (magnetic) state of the pristine compound or the transition from the insulating to metallic state with doping. Here, by taking lanthanum cuprate as an exemplar high-T c cuprate, we show that the recently developed non-empirical, strongly constrained and appropriately normed density functional accurately describes both the antiferromagnetic insulating ground state of the pristine compound and the metallic state of the doped system. Our study yields new insight into the low-energy spectra of cuprates and opens up a pathway toward wide-ranging first-principles investigations of electronic structures of cuprates and other correlated materials.

Realistic modeling of competing phases in complex quantum materials has proven extremely challenging. For example, much of the existing density-functional-theory-based first-principles framework fails in the cuprate superconductors. Various many-body approaches involve generic model Hamiltonians and do not account for the couplings between spin, charge, and lattice. Here, by deploying the recently constructed stronglyconstrained-and-appropriately-normed density functional, we show how landscapes of competing stripe and magnetic phases can be addressed on a first-principles basis in YBa2Cu3O6 and YBa2Cu3O7 as archetype cuprate compounds. We invoke no free parameters such as the Hubbard U, which has been the basis of much of the cuprate literature. Lattice degrees of freedom are found to be crucially important in stabilizing the various phases.Competing orders lie at the heart of myriad fascinating properties of complex materials and their evolution with external controls of temperature, pressure, doping and magnetic field. The half-filled parent compounds of the cuprates, for example, are anti-ferromagnetic (AFM) insulators, which become high-temperature superconductors when doped with holes or electrons. This insulator-superconductor transformation is very complex and involves the presence of an intervening pseudogap phase and mechanisms for arresting superconductivity with increasing doping, so that the superconducting phase occupies a characteristic dome-shaped region in the phase diagram. Along these lines, a wide variety of orders and the associated phase diagrams are exhibited by iron-based superconductors, heavy-fermion compounds, organics, iridates, among other correlated materials of current interest.Complex materials, which often involve strong electronic correlations, present a challenge to first-principles approaches. For example, the local-spin-density and the generalized-gradient approximations used commonly within the first-principles density functional theory (DFT) framework, yield a metallic rather than the experimentally observed insulating AFM ground state in undoped cuprates, so that a meaningful treatment of doping-dependent electronic structures of cuprates on this basis becomes impossible. The AFM state can be stabilized by invoking an ad Page 2 of 10 hoc Hubbard U parameter, but that limits the predictive power of the theory. An alternate route that has been pursued is to deploy effective model Hamiltonians and attempt an exact treatment of electron interaction effects using quantum Monte Carlo (QMC) and other techniques. However, in view of their heavy computational cost, such studies have to be limited to fairly small clusters and one-band or at most three-band models in the cuprates, and cannot be material-specific or allow modifications in the Hamiltonian in response to interactions between the charge, spin and lattice degrees of freedom in the system.Recent progress in constructing advanced density-functionals offers a new pathway for addressing at the first-principles level the electr...

Heterocrystals: rotationally oriented stacks of incommensurate 2D materials with tunable and laser-reconfigurable properties.

We show how an accurate first-principles treatment of the antiferromagnetic (AFM) ground state of La2CuO4 can be obtained without invoking any free parameters such as the Hubbard U . The magnitude and orientation of our theoretically predicted magnetic moment of 0.495µB on Cu-sites along the (100) direction are in excellent accord with experimental results. The computed values of the band gap (1.00 eV) and the exchange-coupling (-138 meV) match the corresponding experimental values. We identify interesting band splittings below the Fermi energy, including an appreciable Hund's splitting of 1.25 eV. The magnetic form factor obtained from neutron scattering experiments is also well described by our calculations. Our study opens up a new pathway for first-principles investigations of electronic and atomic structures and phase diagrams of cuprates and other complex materials.arXiv:1808.06283v1 [cond-mat.str-el]