1,545 research outputs found
Vibrational signatures for low-energy intermediate-sized Si clusters
We report low-energy locally stable structures for the clusters Si20 and Si21. The structures were obtained by performing geometry optimizations within the local density approximation. Our calculated binding energies for these clusters are larger than any previously reported for this size regime. To aid in the experimental identification of the structures, we have computed the full vibrational spectra of the clusters, along with the Raman and IR activities of the various modes using a recently developed first-principles technique. These represent, to our knowledge, the first calculations of Raman and IR spectra for Si clusters of this size
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
Electric control of a single-molecule magnet in a single-electron transistor
Using first-principles methods we study theoretically the properties of an
individual single-molecule magnet (SMM) attached to metallic leads
in a single-electron transistor geometry. We show that the conductive leads do
not affect the spin ordering and magnetic anisotropy of the neutral SMM. On the
other hand, the leads have a strong effect on the anisotropy of the charged
states of the molecule, which are probed in Coulomb blockade transport.
Furthermore, we demonstrate that an external electric potential, modeling a
gate electrode, can be used to manipulate the magnetic properties of the
system. For a charged molecule, by localizing the extra charge with the gate
voltage closer to the magnetic core, the anisotropy magnitude and spin ordering
converges to the values found for the isolated SMM. We compare these
findings with the results of recent quantum transport experiments in
three-terminal devices
Non-collinear first-principles studies of the spin-electric coupling in frustrated triangular molecular magnets
Frustrated triangular molecular magnets (MMs) with anti-ferromagnetic ground
states (GS) are an important class of magnetic systems with potential
applications in quantum information processing. The two-fold degenerate GS of
these molecules, characterized by spin chirality, can be utilized to encode
qubits for quantum computing. Furthermore, because of the lack of inversion
symmetry in these molecules, an electric field couples directly states of
opposite chirality, allowing a very efficient and fast control of the qubits.
In this work we present a theoretical method to calculate the spin-electric
coupling for triangular MMs with effective {\it local} spins larger than
1/2, which is amenable to a first-principles implementation based on density
functional theory (DFT). In contrast to MMs where the net magnetization at the
magnetic atoms is ( is the Bohr magneton), the
DFT treatment of frustrated triangular MMs with larger local magnetizations
requires a fully non-collinear approach, which we have implemented in the
NRLMOL DFT code. As an example, we have used these methods to evaluate the
spin-electric coupling for a spin triangular MM,
where this effect has been observed experimentally for the first time quite
recently. Our theoretical and computational methods will help elucidate and
further guide ongoing experimental work in the field of quantum molecular
spintronics.Comment: 9 pages, 6 figure
Jurassic earthquake sequence recorded by multiple generations of sand blows, Zion National Park, Utah
Earthquakes along convergent plate boundaries commonly occur in sequences that are complete within 1 yr, and may include 8–10 events strong enough to generate sand blows. Dune crossbeds within the Jurassic Navajo Sandstone of Utah (western United States) enclose intact and truncated sand blows, and the intrusive structures that fed them. We mapped the distribution of more than 800 soft-sediment dikes and pipes at two small sites. All water-escape structures intersect a single paleo-surface, and are limited to the upper portion of the underlying set of cross-strata and the lower portion of the overlying set. A small portion of one set of crossbeds that represents ~1 yr of dune migration encloses eight generations of eruptive events. We interpret these superimposed depositional and deformational structures as the record of a single shock-aftershock earthquake sequence. The completeness and temporal detail of this paleoseismic record are unique, and were made possible when sand blows repeatedly erupted onto lee slopes of migrating dunes. Similar records should be sought in modern dunefields with shallow water tables
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
- …