1,547 research outputs found
The Hamiltonian of the V Spin System from first-principles Density-Functional Calculations
We report first-principles all-electron density-functional based studies of
the electronic structure, magnetic ordering and anisotropy for the V
molecular magnet. From these calculations, we determine a Heisenberg
Hamiltonian with four antiferromagnetic and one {\em ferromagnetic} coupling.
We perform direct diagonalization to determine the temperature dependence of
the susceptibility. This Hamiltonian reproduces the experimentally observed
spin =1/2 ground state and low-lying =3/2 excited state. A small
anisotropy term is necessary to account for the temperature independent part of
the magnetization curve.Comment: 4 pages in RevTeX format + 2 ps-figures, accepted by PRL Feb. 2001
(previous version was an older version of the paper
Photo-excitation of a light-harvesting supra-molecular triad: a Time-Dependent DFT study
We present the first time-dependent density-functional theory (TDDFT)
calculation on a light harvesting triad carotenoid-diaryl-porphyrin-C60.
Besides the numerical challenge that the ab initio study of the electronic
structure of such a large system presents, we show that TDDFT is able to
provide an accurate description of the excited state properties of the system.
In particular we calculate the photo-absorption spectrum of the supra-molecular
assembly, and we provide an interpretation of the photo-excitation mechanism in
terms of the properties of the component moieties. The spectrum is in good
agreement with experimental data, and provides useful insight on the
photo-induced charge transfer mechanism which characterizes the system.Comment: Accepted for publication on JPC, March 09th 200
Electric control of spin states in frustrated triangular molecular magnets
Frustrated triangular molecular magnets are a very important class of
magnetic molecules since the absence of inversion symmetry allows an external
electric field to couple directly with the spin chirality that characterizes
their ground state. The spin-electric coupling in these molecular magnets leads
to an efficient and fast method of manipulating spin states, making them an
exciting candidate for quantum information processing. The efficiency of the
spin-electric coupling depends on the electric dipole coupling between the
chiral ground states of these molecules. In this paper, we report on
first-principles calculations of spin-electric coupling in triangular
magnetic molecule. We have explicitly calculated the spin-induced charge
redistribution within the magnetic centers that is responsible for the
spin-electric coupling. Furthermore, we have generalized the method of
calculating the strength of the spin-electric coupling to calculate any
triangular spin 1/2 molecule with symmetry and have applied it to
calculate the coupling strength in molecular magnets
Effect of local Coulomb interactions on the electronic structure and exchange interactions in Mn12 magnetic molecules
We have studied the effect of local Coulomb interactions on the electronic
structure of the molecular magnet Mn12-acetate within the LDA+U approach. The
account of the on-site repulsion results in a finite energy gap and an integer
value of the molecule's magnetic moment, both quantities being in a good
agreement with the experimental results. The resulting magnetic moments and
charge states of non-equivalent manganese ions agree very well with
experiments. The calculated values of the intramolecular exchange parameters
depend on the molecule's spin configuration, differing by 25-30% between the
ferrimagnetic ground state and the completely ferromagnetic configurations. The
values of the ground-state exchange coupling parameters are in reasonable
agreement with the recent data on the magnetization jumps in megagauss magnetic
fields. Simple estimates show that the obtained exchange parameters can be
applied, at least qualitatively, to the description of the spin excitations in
Mn12-acetate.Comment: RevTeX, LaTeX2e, 4 EPS figure
The Puzzle of Sibling Attachment Non-Concordance: Implications of Categorical versus Continuous Approaches to Attachment
PURPOSE OF THE CURRENT STUDY: To determine whether characterizing the quality of attachment as a continuous measure impacts the extent to which siblings’ attachment relationships are judged concordant. To investigate whether continuous measures of attachment provide additional information regarding the similarity of more specific aspects of siblings attachment relationships
Yield and Forage Value of a Dual-Purpose \u3ci\u3ebmr-12\u3c/i\u3e Sorghum Hybrid
Grain sorghum [Sorghum bicolor (L.) Moench] is an important crop for rainfed production systems with 2.7 million ha grown in the United States in 2013. The brown-midrib (bmr) mutations, especially bmr-12, have resulted in low stover lignin and high fiber digestibility without reducing grain yield in some sorghum lines. However, the effect of the bmr trait on beef cattle (Bos taurus) performance when grazing crop residue is unknown. Our objectives were to validate previous small-plot results reporting no grain yield difference between near-isogenic bmr-12 (BMR) and wild-type control (CON) A Wheatland Ă— R
Tx430 sorghum hybrids in a field-scale experiment and to determine if BMR stover enhances beef production in a grazing experiment. Four replicated paddocks (2.3 ha) were planted in 2006 and 2008 near Mead, NE. Crossbred yearling steers (240 ± 17 kg hd-1) grazed (2.6 steers ha-1) paddocks following grain harvest for 72 d in 2006 and 61 d in 2008. Forage was sampled 4, 30, and 60 d after grazing began. Grain yield of BMR was 6% less (P = 0.01) than CON with no difference in stover neutral detergent fiber (NDF) content, but BMR stover had higher in vitro NDF digestibility (IVNDFD) (31%; P \u3c 0.0001), steer average daily gain (ADG; 0.18 kg hd-1 d-1; P = 0.001), and body weight (BW) gain (29 kg ha-1; P = 0.002), resulting in an estimated increase in net return of $133.84 ha-1 due to BMR. Results suggest that the A Wheatland × R Tx430 bmr-12 hybrid is an effective dual-purpose sorghum crop for both grain and beef production
Variational Hilbert space truncation approach to quantum Heisenberg antiferromagnets on frustrated clusters
We study the spin- Heisenberg antiferromagnet on a series of
finite-size clusters with features inspired by the fullerenes. Frustration due
to the presence of pentagonal rings makes such structures challenging in the
context of quantum Monte-Carlo methods. We use an exact diagonalization
approach combined with a truncation method in which only the most important
basis states of the Hilbert space are retained. We describe an efficient
variational method for finding an optimal truncation of a given size which
minimizes the error in the ground state energy. Ground state energies and
spin-spin correlations are obtained for clusters with up to thirty-two sites
without the need to restrict the symmetry of the structures. The results are
compared to full-space calculations and to unfrustrated structures based on the
honeycomb lattice.Comment: 22 pages and 12 Postscript figure
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