Bayo Lau

Theory of the Magnetic and Metal-Insulator Transitions in RNiO3 Bulk and Layered Structures

A slave rotor--Hartree-Fock formalism is presented for studying the properties of the p-d model describing perovskite transition metal oxides, and a flexible and efficient numerical formalism is developed for its solution. The methodology is shown to yield, within a unified formulation, the significant aspects of the rare-earth nickelate phase diagram, including the paramagnetic metal state observed for the LaNiO3 and the correct ground-state magnetic order of insulating compounds. It is then used to elucidate ground state changes occurring as morphology is varied from bulk to strained and unstrained thin-film form. For ultrathin films, epitaxial strain and charge transfer to the apical out-of-plane oxygen sites are shown to have significant impact on the phase diagram.

High-spin polaron in lightly doped CuO2 planes.

We derive and investigate numerically a minimal yet detailed spin-polaron model that describes lightly doped CuO2 layers. The low-energy physics of a hole is studied by total-spin-resolved exact diagonalization on clusters of up to 32 CuO2 unit cells, revealing features missed by previous studies. In particular, spin-polaron states with total spin 3/2 are the lowest eigenstates in some regions of the Brillouin zone. In these regions, and also at other points, the quasiparticle weight is identically zero indicating orthogonal states to those represented in the one electron Greens function. This highlights the importance of the proper treatment of spin fluctuations in the many-body background.

Computational approach to a doped antiferromagnet: Correlations between two spin polarons in the lightly doped CuO2 plane

We extend the methods recently introduced [ Phys. Rev. Lett.106, 036401 (2011)] to investigate correlations between two spin-polarons in a quasi-two-dimensional CuO2 layer. The low-energy wave functions for two doped holes introduced in a half filled CuO2 plane with 32 copper and 64 oxygen sites are calculated explicitly using an efficient yet accurate truncation scheme to model the antiferromagnet background. The energetics and wave functions show that the charges form three spin-polarons and the spin is carried by a disturbance around the three spin-polaron core. The low-energy band results from the competition between the kinetic energy and a local attractive potential, which favors dx2-y2 states. Last, we point out features that are expected to be robust for larger systems.

Single-polaron properties of the one-dimensional breathing-mode Hamiltonian

We investigate numerically various properties of the one-dimensional 1D breathing-mode polaron. We use an extension of a variational scheme to compute the energies and wave functions of the two lowest-energy eigenstates for any momentum, as well as a scheme to compute directly the polaron’s Green’s function. We contrast these results with the results for the 1D Holstein polaron. In particular, we find that the crossover from a large to a small polaron is significantly sharper. Unlike for the Holstein model, at moderate and large couplings, the breathing-mode polaron dispersion has nonmonotonic dependence on the polaron momentum k. Neither of these aspects is revealed by a previous study based on the self-consistent Born approximation.

Computational scheme for the spin-1/2 Heisenberg antiferromagnet based on an octapartite description of the square lattice

We introduce a novel parametrization scheme for the Hilbert space of a spin-1/2 Heisenberg antiferromagnet (AFM) based on an octapartite description of the square lattice. Our formulation provides an efficient yet systematic way to model the singlet ground-state wave function within a truncated basis. We demonstrate its effectiveness by using exact diagonalization to study systems with up to 64 spins. At significantly reduced computational cost, we obtain ground-state energies within less than 1% of the lowest values published. The non-iterative nature of this formulation and the fact that spatial symmetries remain unexploited lead to opportunities for extensions such as the incorporation into established iterative methods and the application to interacting models, for example in the study of the propagation of charged fermions in a 2D antiferromagnet.