166 research outputs found
Magnetic ordering in the striped nickelate La5/3Sr1/3NiO4: A band structure point of view
We report on a comprehensive study of the electronic and magnetic structure
of the striped nickelate La5/3Sr1/3NiO4. The investigation is carried out using
band structure calculations based on density functional theory. A magnetic
structure compatible with experiment is obtained from spin-polarized
calculations within the generalized gradient approximation (GGA), whereas
inclusion of a local Coulomb interaction in the LDA+U framework results in a
different ground state. The influence of the various interaction parameters is
discussed in detail.Comment: 5 pages, 4 figures, 2 tables, accepted by Europhys. Let
Microscopic origin of diagonal stripe phases in doped nickelates
We investigate the electron density distribution and the stability of stripe
phases in the realistic two-band model with hopping elements between e_g
orbitals at Ni sites on the square lattice, and compare these results with
those obtained for the doubly degenerate Hubbard model with two equivalent
orbitals and diagonal hopping. For both models we determine the stability
regions of filled and half-filled stripe phases for increasing hole doping
x=2-n in the range of x<0.4, using Hartree-Fock approximation for large
clusters. In the parameter range relevant to the nickelates, we obtain the most
stable diagonal stripe structures with filling of nearly one hole per atom, as
observed experimentally. In contrast, for the doubly degenerate Hubbard model
the most stable stripes are somewhat reminiscent of the cuprates, with
half-filled atoms at the domain wall sites. This difference elucidates the
crucial role of the off-diagonal e_g hopping terms for the stripe formation in
La_2-xSr_xNiO_4. The influence of crystal field is discussed as well.Comment: 15 pages, 12 figure
Slave-boson approach to the metallic stripe phases with large unit cells
Using a rotationally invariant version of the slave-boson approach in spin
space we analyze the stability of stripe phases with large unit cells in the
two-dimensional Hubbard model. This approach allows one to treat strong
electron correlations in the stripe phases relevant in the low doping regime,
and gives results representative of the thermodynamic limit. Thereby we resolve
the longstanding controversy concerning the role played by the kinetic energy
in stripe phases. While the transverse hopping across the domain walls yields
the largest kinetic energy gain in the case of the insulating stripes with one
hole per site, the holes propagating along the domain walls stabilize the
metallic vertical stripes with one hole per two sites, as observed in the
cuprates. We also show that a finite next-nearest neighbor hopping can tip
the energy balance between the filled diagonal and half-filled vertical
stripes, which might explain a change in the spatial orientation of stripes
observed in the high cuprates at the doping .Comment: 16 pages, 14 figure
Automated Reasoning and Presentation Support for Formalizing Mathematics in Mizar
This paper presents a combination of several automated reasoning and proof
presentation tools with the Mizar system for formalization of mathematics. The
combination forms an online service called MizAR, similar to the SystemOnTPTP
service for first-order automated reasoning. The main differences to
SystemOnTPTP are the use of the Mizar language that is oriented towards human
mathematicians (rather than the pure first-order logic used in SystemOnTPTP),
and setting the service in the context of the large Mizar Mathematical Library
of previous theorems,definitions, and proofs (rather than the isolated problems
that are solved in SystemOnTPTP). These differences poses new challenges and
new opportunities for automated reasoning and for proof presentation tools.
This paper describes the overall structure of MizAR, and presents the automated
reasoning systems and proof presentation tools that are combined to make MizAR
a useful mathematical service.Comment: To appear in 10th International Conference on. Artificial
Intelligence and Symbolic Computation AISC 201
Model study of adsorbed metallic quantum dots: Na on Cu(111)
We model electronic properties of the second monolayer Na adatom islands
(quantum dots) on the Cu(111) surface covered homogeneously by the first Na
monolayer. An axially-symmetric three-dimensional jellium model, taking into
account the effects due to the first Na monolayer and the Cu substrate, has
been developed. The electronic structure is solved within the local-density
approximation of the density-functional theory using a real-space multigrid
method. The model enables the study of systems consisting of thousands of
Na-atoms. The results for the local density of states are compared with
differential conductance () spectra and constant current topographs from
Scanning Tunneling Microscopy.Comment: 10 pages, 8 figures. For better quality figures, download
http://www.fyslab.hut.fi/~tto/cylart1.pd
Analysis of Resonant Inelastic X-Ray Scattering in Stripe-Ordered Nickelate
We analyze theoretically the resonant inelastic x-ray scattering (RIXS) at
the Ni K edge in the stripe-ordered state of La_{2-x}Sr_xNiO_4 at x=1/3. In the
calculation of RIXS spectra, the stripe-ordered ground state is described
within the Hartree-Fock approximation by using a realistic tight-binding model
for Ni3d\gamma and O2p_{x, y} orbitals, and the electron correlations in the
electronic excitation processes are taken into account within the random-phase
approximation. The calculated RIXS spectrum shows a tail toward the low-energy
region when the momentum transfer of photons equals the stripe vector Q, being
consistent with a recent experimental result. The origin of this anomalous
momentum dependence of RIXS spectra is discussed microscopically.Comment: 23 pages, 9 figures. Published version in J. Phys. Soc. Jp
Electronic resonance states in metallic nanowires during the breaking process simulated with the ultimate jellium model
We investigate the elongation and breaking process of metallic nanowires
using the ultimate jellium model in self-consistent density-functional
calculations of the electron structure. In this model the positive background
charge deforms to follow the electron density and the energy minimization
determines the shape of the system. However, we restrict the shape of the wires
by assuming rotational invariance about the wire axis. First we study the
stability of infinite wires and show that the quantum mechanical
shell-structure stabilizes the uniform cylindrical geometry at given magic
radii. Next, we focus on finite nanowires supported by leads modeled by
freezing the shape of a uniform wire outside the constriction volume. We
calculate the conductance during the elongation process using the adiabatic
approximation and the WKB transmission formula. We also observe the correlated
oscillations of the elongation force. In different stages of the elongation
process two kinds of electronic structures appear: one with extended states
throughout the wire and one with an atom-cluster like unit in the constriction
and with well localized states. We discuss the origin of these structures.Comment: 11 pages, 8 figure
Stripes in cuprate superconductors: Excitations and dynamic dichotomy
We present a short account of the present experimental situation of stripes
in cuprates followed by a review of our present understanding of their ground
state and excited state properties. Collective modes, the dynamical structure
factor, and the optical conductivity of stripes are computed using the
time-dependent Gutzwiller approximation applied to realistic one band and three
band Hubbard models, and are found to be in excellent agreement with
experiment. On the other hand, experiments like angle-resolved photoemission
and scanning tunneling microscopy show the coexistence of stripes at high
energies with Fermi liquid quasiparticles at low energies. We show that a
phenomenological model going beyond mean-field can reconcile this dynamic
dichotomy.Comment: 20 pages, 14 figures. Review paper for a Special Issue of Physica C
on "Stripes and Electronic Liquid Crystals in Strongly Correlated Systems
Formalization of Definitions and Theorems Related to an Elliptic Curve Over a Finite Prime Field by Using Mizar
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