1,782 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
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
Electronic-structure-based investigation of magnetism in the Fe8 molecular magnet
We have performed density-functional-based electronic structure calculations on a single Fe8 molecular nanomagnet. Our calculated total moments and local moments are in excellent agreement with experiment. By including spin–orbit coupling we determine the easy, medium, and hard axes and find the ordering of the principle axes also agrees with experiment. From our calculated anisotropy Hamiltonian, we calculate the oscillations in the tunnel splittings and compare to the experimental results
Classical Stern-Gerlach profiles of Mn5 and Mn6 clusters
Mn5 and Mn6 clusters have recently been found to exhibit Stern-Gerlach profiles marked by a central peak that broadens with the increasing field gradient. The profiles neither exhibit a reminiscence of space quantization as observed through a splitting of beams for the case of free atoms, nor a net deflection characteristic of superparamagnetic relaxations observed in other transition metal clusters. It is proposed that this new behavior results from a weak coupling of localized atomic moments. ab initio electronic structure studies are carried out to show that a Mn5cluster has isomers with spin magnetic moments of 3μB, 13μB, and 23μBwhile a Mn6 cluster has isomers with moments of 2μB, 8μB, 16μB, and 26μB, respectively. The isomers can be obtained by sequential turning of the local atomic moments starting from the ferromagnetic state and can be seen in the negative ion photoelectron spectra of the anions. The weak coupling of the atomic moments, however, leads to unconventional spin dynamics that result in classical broadening of the Stern-Gerlach profiles and lower apparent magnetic moments. The theoretical results illustrate how a combination of the negative ion photodetachment spectroscopy and Stern-Gerlach profiles can provide information on the net spin moment, interatomic spin coupling, and spin dynamics
The vibrational stability and electronic structure of B80 fullerene-like cage
We investigate the vibrational stability and the electronic structure of the
proposed icosahedral fullerene-like cage structure of B80 [Szwacki, Sadrzadeh,
and Yakobson, Phys. Rev. Lett. {\bf 98}, 166804 (2007)] by an all electron
density functional theory using polarized Gaussian basis functions containing
41 basis functions per atom. The vibrational analysis of B indicates
that the icosahedral structure is vibrationally unstable with 7 imaginary
frequencies. The equilibrium structure has symmetry and a {\em smaller}
gap of 0.96 eV between the highest occupied and lowest unoccupied molecular
orbital energy levels compared to the icosahedral structure. The static dipole
polarizability of B cage is 149 \AAA and the first ionization energy is
6.4 eV. The B cage has rather large electron affinity of 3 eV making it
useful candidate as electron acceptor if it is synthesized. The infra-red and
Raman spectra of the highly symmetric structure are characterized by a few
absorption peaks.Comment: RevTex, 4 figure
Hydrogen adsorption and magnetic behavior of Fen and Con clusters: Controlling the magnetic moment and anisotropy one atom at a time
Theoretical studies to investigate the effect of H absorption on the magnetic moment of small Fen and Con clusters have been carried out using gradient corrected density-functional approach. Our studies on clusters containing up to four transition metal and 2 H atoms show that the successive addition of H atoms can lead to monotonic or oscillatory change from the free cluster magnetic moment. A detailed analysis of the density of electronic states shows that the variations in the magnetic moment can be related to the location of the lowest unoccupied molecular orbital in the parent cluster. It is shown that the addition of hydrogen can substantially change the magnetic anisotropy. In particular Co3H2is shown to exhibit magnetic anisotropy that is higher than any of the known anisotropies in the molecular nanomagnets
Theoretical calculations of magnetic order and anisotropy energies in molecular magnets
We present theoretical electronic structure calculations on the nature of electronic states and the magnetic coupling in the Mn12O12 free cluster and the Mn12O12(RCOO)16(H2O)4 molecular magnetic crystal. The calculations have been performed with the all-electron full-potential NRLMOL code. We find that the free Mn12O12cluster relaxes to an antiferromagneticcluster with no net moment. However, when coordinated by sixteen HCOO ligands and four H2O groups, as it is in the molecular crystal, we find that the ferrimagnetic ordering and geometrical and magnetic structure observed in the experiments is restored. Local Mn moments for the free and ligandated molecular magnets are presented and compared to experiment. We identify the occupied and unoccupied electronic states that are most responsible for the formation of the large anisotropy barrier and use a recently developed full-space and full-potential method for calculating the spin–orbit coupling interaction and anisotropy energies. Our calculated second-order anisotropy energy is in excellent agreement with experiment
Magnetic moment and anisotropy in FenCom clusters
Electronic structure calculations of FenCom (n+m=5 and 13) are used to examine the effects of alloying on the magnetic moment and magnetic anisotropies. Our density-functional studies show that many mixed clusters have moments comparable to or higher than the pure clusters. The mixed clusters, however, have very low anisotropies and could be ideal as soft magnetic materials. It is shown that shape, composition, and compositional ordering must be considered for optimization of anisotropy energies
Theory for transport through a single magnetic molecule: Endohedral N@C60
We consider transport through a single N@C60 molecule, weakly coupled to
metallic leads. Employing a density-matrix formalism we derive rate equations
for the occupation probabilities of many-particle states of the molecule. We
calculate the current-voltage characteristics and the differential conductance
for N@C60 in a break junction. Our results reveal Coulomb-blockade behavior as
well as a fine structure of the Coulomb-blockade peaks due to the exchange
coupling of the C60 spin to the spin of the encapsulated nitrogen atom.Comment: 5 pages, 4 figures, v2: version as publishe
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