Model Hamiltonian Calculations Research Articles

Theoretical investigation of carotenoid ultraviolet spectra

AbstractWe have determined the π band‐gaps of the main carotenoids present in poly(methyl)methacrylate/buriti blends, namely, trans‐β‐carotene, 13‐cis‐β‐carotene, 9‐cis‐β‐carotene, phytofluene, and zeaxanthin. Semiempirical, model Hamiltonian, and density functional calculations were carried out to study these structures. The geometries were fully optimized using AM1, PM3, and B3LYP/6‐31G(d,p) methods. The TD‐DFT and ZINDO/S methods were applied for the calculation of the electronic absorption spectra of the optimized B3LYP geometries. The calculated spectra using the polarizable continuum model for the solvent effects were compared with the available experimental. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009

Coexistence of superconductivity and antiferromagnetism in CeRhIn5; model Hamiltonian and ab-initio calculations

The finite temperature phase diagram of CeRhIn5 as a function of pressure and magnetic field has three main highlights: a) the competitive coexistence of metallic antiferromagnetism and superconductivity, b) the abrupt disappearance of antiferromagnetism when the Neel and superconducting temperatures become equal at a critical pressure Pc and c) the reentrance of the antiferromagnetic phase in a range of pressures larger than Pc when a magnetic field is applied. Based on first-principles band structure calculations, we propose a quasi-two-dimensional model of interacting electrons, which reproduces, at the mean-field level, the central features of the phase diagram.

Renormalization of quasiparticle hopping integrals by spin interactions in layered copper oxides

Holes doped within the square $\mathrm{Cu}{\mathrm{O}}_{2}$ network specific to the cuprate superconducting materials have oxygen $2p$ character. We investigate the basic properties of such oxygen holes by wave-function-based quantum chemical calculations on large embedded clusters. We find that a $2p$ hole induces ferromagnetic correlations among the nearest-neighbor $\mathrm{Cu}\phantom{\rule{0.2em}{0ex}}3d$ spins. When moving through the antiferromagnetic background, the hole must bring along this spin-polarization cloud at nearby Cu sites, which gives rise to a substantial reduction of the effective hopping parameters. Such interactions can explain the relatively low values inferred for the effective hoppings by fitting the angle-resolved photoemission data. The effect of the background antiferromagnetic couplings of renormalizing the effective nearest-neighbor hopping is also confirmed by density-matrix renormalization-group model-Hamiltonian calculations for chains and ladders of $\mathrm{Cu}{\mathrm{O}}_{4}$ plaquettes.

Linking model Hamiltonians to ab initio and semiempirical methods in descriptions of impurities in conjugated polymers

AbstractSemiempirical and ab initio electronic structure and model Hamiltonian calculations of the undoped and doped polyacetylene (PA) chain are presented. As a general model Hamiltonian we use the Su–Schrieffer–Heeger (SSH) model combined with the Pariser–Parr–Pople (PPP) model with site‐type impurities. More precisely, band gap and charge distributions of a single conducting PA chain bonding with several atomic and molecular radicals as impurities were investigated. From a comparison between the electronic and SSH‐PPP calculation it was possible to determine what impurity actually corresponds to the charge distribution obtained with model calculations. Therefore, it is shown within the present results what impurities are being represented in model calculations. This correspondence is fundamental in determining the behavior of molecular circuits and molecular devices. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005

Electronic structure of millerite NiS

We investigate the electronic structure of Nickel sulphide (NiS) in the millerite phase using electron spectroscopic measurements and band structure as well as model Hamiltonian calculations. While band structure calculations are found to be relatively more successful in describing the experimental valence band spectrum of this highly conducting phase compared to the hexagonal phase of NiS, cluster calculations including electron correlation effects are found to be necessary for the description of certain features in the experimental spectra, indicating importance of correlation effects even in a highly metallic system. The electronic parameter strengths obtained from these calculations confirm that the millerite NiS is a highly covalent pd-metal. The comparative study of hexagonal and millerite forms of NiS, provides the information concerning the evolution of the spectral function in a pd-metal as a function of covalency.

Density functional calculations for III-V diluted ferromagnetic semiconductors: A Review

In this paper we review the latest achievements of density functional theory in understanding the physics of diluted magnetic semiconductors. We focus on transition-metal-doped III–V semiconductors, which show spontaneous ferromagnetic order at relatively high temperature and good structural compatibility with existing III–V devices. We show that density functional theory is a very powerful tool for (i) studying the effects of local doping defects and disorder on the magnetic properties of these materials, (ii) predicting properties of new materials, and (iii) providing parameters, often not accessible from experiments, for use in model Hamiltonian calculations. Such studies are facilitated by recent advances in numerical implementations of density functional theory, which make the study of systems with a very large number of atoms possible.

Covalency-driven unusual metal-insulator transition in nickelates.

Electronic structures of metallic ${\mathrm{LaNiO}}_{3}$ and insulating ${\mathrm{NdNiO}}_{3}$ are investigated using electron spectroscopies as well as band-structure and model-Hamiltonian calculations. It is shown that there is an increase in the transfer integral, ${\mathit{t}}_{\mathit{p}\mathit{d}}$ in ${\mathrm{LaNiO}}_{3}$ compared to ${\mathrm{NdNiO}}_{3}$ though ${\mathit{U}}_{\mathit{d}\mathit{d}}$ and \ensuremath{\Delta} are very similar. A phase diagram obtained with a multiband Hubbard model explains the metal-insulator transition as well as the anomalous resistivity of ${\mathrm{NdNiO}}_{3}$ under pressure while indicating various interesting possibilities near the metal-insulator phase line.

Electronic structure of transition metal compounds: Photoemission experiments and model Hamiltonian calculations

We discuss the case of NiO in detail to exemplify the use of photoemission experiments and model Hamiltonian calculations in order to unravel the electronic structure of transition metal compounds. Then we discuss the transition metal compounds and their metallic/nonmetallic properties in terms of a generalized phase diagram as a function of the Coulomb interaction strength and charge transfer energy. Finally, we discuss the electronic structure of the high- T c cuprate superconductors as deduced from photoemission experiments and model calculations in relation to the above-mentioned phase diagram.

Cluster-model calculation of the electronic structure of CuO: A model material for the high-Tc superconductors.

In this paper we describe the details of several model Hamiltonian cluster calculations, suitable for describing various spectroscopic data of CuO. By treating the d-d Coulomb and exchange interactions within the full atomic multiplet theory and using symmetry-dependent Cu-O hybridizations, we do a detailed comparison to photoelectron spectroscopic data, thereby obtaining reliable values for the parameters of an Anderson-model Hamiltonian. We present a study of the allowable ranges of such parameters and a discussion of the applicability to high-${T}_{c}$ copper compounds. For the latter we investigate the influence of the out-of-plane apex oxygen, which is found to be small for the photoelectron spectrum for known Cu-O distances. From a study of the dependence of the nature of the first ionization state on the apex-O--to--Cu distance as well as on the apex-O 2p state orbital energy, we determine the values for which this state changes from a singlet to a triplet. However, in all cases this state remains ${d}^{9}$L in character. From the parameters obtained for CuO, we derive an O 2p--Cu 3d exchange interaction of 3.4 eV for ${x}^{2}$-${y}^{2}$ symmetry orbitals. In addition we calculate the energies of the optical d-d transitions and find all three of these to be clustered around 1.4\ifmmode\pm\else\textpm\fi{}0.1 eV.

Electronic structure of small gap polymers

The following factors have a large effect on the electronic structures and energy gaps (E g) of conjugated polymers: (1) perturbations due to hetero-atoms and side groups, (2) topology (fused rings, ladder structures, etc.) and (3) geometrical rearrangements. It is shown, how total energy minimization yields reliable enough geometrical parameters to be used in subsequent model Hamiltonian calculations of the electronic structures. Derivatives of polythiophene and polypyrrole, and thiophene-vinylene polymers are studied. Agreements with experimental E g values, where available, are good. A few proposed systems are predicted to have a gap in the vicinity of 1 eV.

Model Hamiltonian calculations from helium interacting with stepped transition metal surfaces

Model Hamiltonian calculations from helium interacting with stepped transition metal surfaces

Model Hamiltonian calculations for helium interacting with stepped transition metal surfaces

A theory is presented to describe adsorption on surfaces with impurities. It is based on embedding a cluster of auxillary basis function into an indented Sommerfeld metal. The cluster of auxiliary functions is chosen in such a way that the boundary conditions for the impurities are satisfied. The method is applied to calculate the interaction of a helium a atom with a stepped Cu(110) surface. The interaction with the d-electrons is included as well. The charge density for the stepped surface exhibits oscillations near the step and a pronounced Smoluchowsky effect. Compared to the flat surface case the helium binding energy is increased right at the step and decreases further out on the low terrace.