207 research outputs found
Single-hole dynamics in the half-filled two-dimensional Kondo-Hubbard model
We consider the Kondo lattice model in two dimensions at half filling. In
addition to the fermionic hopping integral and the superexchange coupling
the role of a Coulomb repulsion in the conduction band is investigated.
We find the model to display a magnetic order-disorder transition in the U-J
plane with a critical value of J_c which is decreasing as a function of U. The
single particle spectral function A(k,w) is computed across this transition.
For all values of J > 0, and apart from shadow features present in the ordered
state, A(k,w) remains insensitive to the magnetic phase transition with the
first low-energy hole states residing at momenta k = (\pm \pi, \pm \pi). As J
-> 0 the model maps onto the Hubbard Hamiltonian. Only in this limit, the
low-energy spectral weight at k = (\pm \pi, \pm \pi) vanishes with first
electron removal-states emerging at wave vectors on the magnetic Brillouin zone
boundary. Thus, we conclude that (i) the local screening of impurity spins
determines the low energy behavior of the spectral function and (ii) one cannot
deform continuously the spectral function of the Mott-Hubbard insulator at J=0
to that of the Kondo insulator at J > J_c. Our results are based on both, T=0
Quantum Monte-Carlo simulations and a bond-operator mean-field theory.Comment: 8 pages, 7 figures. Submitted to PR
Strong-Coupling Expansions for Multiparticle Excitations: Continuum and Bound States
We present a new linked cluster expansion for calculating properties of
multiparticle excitation spectra to high orders. We use it to obtain the
two-particle spectra for systems of coupled spin-half dimers. We find that even
for weakly coupled dimers the spectrum is very rich, consisting of many bound
states. The number of bound states depends on both geometry of coupling and
frustration. Many of the bound states can only be seen by going to sufficiently
high orders in the perturbation theory, showing the extended character of the
pair-attraction.Comment: 4 pages, 5 figure
Single-hole dynamics in dimerized and frustrated spin-chains
We present a unified account for the coupled single-hole- and spin-dynamics
in the spin-gap phase of dimerized and frustrated spin-chains and two-leg spin
ladders. Based on the strong dimer-limit of a one-dimensional
--model a diagrammatic approach is presented which employs a
mapping of the spin-Hamiltonian onto a pseudo-fermion bond-boson model. Results
for the single-hole spectrum are detailed. A finite quasi-particle weight is
observed and studied for a variety of system parameters. A comparison with
existing exact diagonalization data is performed and good agreement is found.Comment: 10 pages, 12 figure
Particle Content of the Nonlinear Sigma Model with Theta-Term: a Lattice Model Investigation
Using new as well as known results on dimerized quantum spin chains with
frustration, we are able to infer some properties on the low-energy spectrum of
the O(3) Nonlinear Sigma Model with a topological theta-term. In particular,
for sufficiently strong coupling, we find a range of values of theta where a
singlet bound state is stable under the triplet continuum. On the basis of
these results, we propose a new renormalization group flow diagram for the
Nonlinear Sigma Model with theta-term.Comment: 10 pages, 5 figures .eps, iopart format, submitted to JSTA
Discovering Spatio-Temporal Patterns in Precision Agriculture Based on Triclustering
Agriculture has undergone some very important changes over
the last few decades. The emergence and evolution of precision agri culture has allowed to move from the uniform site management to the
site-specific management, with both economic and environmental advan tages. However, to be implemented effectively, site-specific management
requires within-field spatial variability to be well-known and character ized. In this paper, an algorithm that delineates within-field management
zones in a maize plantation is introduced. The algorithm, based on tri clustering, mines clusters from temporal remote sensing data. Data from
maize crops in Alentejo, Portugal, have been used to assess the suit ability of applying triclustering to discover patterns over time, that may
eventually help farmers to improve their harvests.Ministerio de Economía y Competitividad TIN2017-88209-C2Fundaçao para a Ciéncia e a Tecnologia (FCT) UIDB/04561/202
Observation of two-magnon bound states in the two-leg ladders of (Ca,La)14Cu24O41
Phonon-assisted 2-magnon absorption is studied at T=4 K in the spin-1/2
two-leg ladders of Ca_14-x La_x Cu_24 O_41 (x=5 and 4) for polarization of the
electrical field parallel to the legs and the rungs, respectively. Two peaks at
about 2140 and 2800 1/cm reflect van-Hove singularities in the density of
states of the strongly dispersing 2-magnon singlet bound state, and a broad
peak at about 4000 1/cm is identified with the 2-magnon continuum. Two
different theoretical approaches (Jordan-Wigner fermions and perturbation
theory) describe the data very well for J_parallel = 1050 - 1100 1/cm and
J_parallel / J_perp = 1 - 1.1. A striking similarity of the high-energy
continuum absorption of the ladders and of the undoped high T_c cuprates is
observed.Comment: 4 pages, 3 figures, Revte
Hole Dynamics in the Orthogonal-Dimer Spin System
The dynamics of a doped hole in the orthogonal-dimer spin system is
investigated systematically in one, two and three dimensions. By combining the
bond-operator method with the self-consistent
Born approximation, we argue that a dispersive quasi-particle state in the
dimer phase is well defined even for quasi-two-dimensional systems. On the
other hand, a doped hole in the plaquette-singlet phase hardly itinerates,
forming an almost localized mode. We further clarify that although the
quasi-particle weight in the dimer phase is decreased in the presence of the
interchain coupling, it is not suppressed but even enhanced upon the
introduction of the interlayer coupling.Comment: 8 pages, 10 figure
Performance of ab initio and density functional methods for conformational equilibria of CnH2n+2 alkane isomers (n=2-8)
Conformational energies of n-butane, n-pentane, and n-hexane have been
calculated at the CCSD(T) level and at or near the basis set limit.
Post-CCSD(T) contribution were considered and found to be unimportant. The data
thus obtained were used to assess the performance of a variety of density
functional methods. Double-hybrid functionals like B2GP-PLYP and B2K-PLYP,
especially with a small Grimme-type empirical dispersion correction, are
capable of rendering conformational energies of CCSD(T) quality. These were
then used as a `secondary standard' for a larger sample of alkanes, including
isopentane and the branched hexanes as well as key isomers of heptane and
octane. Popular DFT functionals like B3LYP, B3PW91, BLYP, PBE, and PBE0 tend to
overestimate conformer energies without dispersion correction, while the M06
family severely underestimates GG interaction energies. Grimme-type dispersion
corrections for these overcorrect and lead to qualitatively wrong conformer
orderings. All of these functionals also exhibit deficiencies in the conformer
geometries, particularly the backbone torsion angles. The PW6B95 and, to a
lesser extent, BMK functionals are relatively free of these deficiencies.
Performance of these methods is further investigated to derive conformer
ensemble corrections to the enthalpy function, , and the Gibbs
energy function, , of these alkanes. While
is only moderately sensitive to the level of theory, exhibits more pronounced sensitivity. Once again, double hybrids
acquit themselves very well.Comment: J. Phys. Chem. A, revised [Walter Thiel festschrift
Ab initio van der Waals interactions in simulations of water alter structure from mainly tetrahedral to high-density-like
The structure of liquid water at ambient conditions is studied in ab initio
molecular dynamics simulations using van der Waals (vdW) density-functional
theory, i.e. using the new exchange-correlation functionals optPBE-vdW and
vdW-DF2. Inclusion of the more isotropic vdW interactions counteracts highly
directional hydrogen-bonds, which are enhanced by standard functionals. This
brings about a softening of the microscopic structure of water, as seen from
the broadening of angular distribution functions and, in particular, from the
much lower and broader first peak in the oxygen-oxygen pair-correlation
function (PCF), indicating loss of structure in the outer solvation shells. In
combination with softer non-local correlation terms, as in the new
parameterization of vdW-DF, inclusion of vdW interactions is shown to shift the
balance of resulting structures from open tetrahedral to more close-packed. The
resulting O-O PCF shows some resemblance with experiment for high-density water
(A. K. Soper and M. A. Ricci, Phys. Rev. Lett., 84:2881, 2000), but not
directly with experiment for ambient water. However, an O-O PCF consisting of a
linear combination of 70% from vdW-DF2 and 30% from experiment on low-density
liquid water reproduces near-quantitatively the experimental O-O PCF for
ambient water, indicating consistency with a two-liquid model with fluctuations
between high- and low-density regions
Unraveling the performance of dispersion-corrected functionals for the accurate description of weakly bound natural polyphenols
Long-range non-covalent interactions play a key role in the chemistry of natural polyphenols. We have previously proposed a description of supramolecular polyphenol complexes by the B3P86 density functional coupled with some corrections for dispersion. We couple here the B3P86 functional with the D3 correction for dispersion, assessing systematically the accuracy of the new B3P86-D3 model using for that the well-known S66, HB23, NCCE31, and S12L datasets for non-covalent interactions. Furthermore, the association energies of these complexes were carefully compared to those obtained by other dispersion-corrected functionals, such as B(3)LYP-D3, BP86-D3 or B3P86-NL. Finally, this set of models were also applied to a database composed of seven non-covalent polyphenol complexes of the most interest.FDM acknowledges financial support from the Swedish Research Council (Grant No. 621-2014-4646) and SNIC (Swedish National Infrastructure for Computing) for providing computer resources. The work in Limoges (IB and PT) is supported by the “Conseil Régional du Limousin”. PT gratefully acknowledges the support by the Operational Program Research and Development Fund (project CZ.1.05/2.1.00/03.0058 of the Ministry of Education, Youth and Sports of the Czech Republic). IB gratefully acknowledges financial support from “Association Djerbienne en France”
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