2,235 research outputs found
First-principles molecular-dynamics simulations for neutral p-chloranil and its radical anion
APS copyright is acknowledged url = {http://link.aps.org/doi/10.1103/PhysRevB.53.12112}International audienceThe neutral p -chloranil ~ 2,3,5,6-tetrachloro- p -benzoquinone ! and its radical anion have been extensively studied using the Car-Parrinello projector augmented wave method, which is an all-electron electronic structure method for first-principles molecular dynamics based on the local density approximation of density functional theory. Frequencies and eigenmodes are derived by fitting a system of harmonic oscillators to the molecular- dynamics trajectories. The dependence of the bond lengths and vibrational frequencies on the molecular ionicity is discussed, and the electron affinity, Coulomb repulsion, and the spin-splitting parameter of p -chloranil are also derived
Surface resonance of the (2×1) reconstructed lanthanum hexaboride (001)-cleavage plane : a combined STM and DFT study
We performed a combined study of the (001)-cleavage plane of lanthanum hexaboride (LaB6) using scanning tunneling microscopy and density-functional theory (DFT). Experimentally, we found a (2×1) reconstructed surface on a local scale. The reconstruction is only short-range ordered and tends to order perpendicularly to step edges. At larger distances from surface steps, the reconstruction evolves to a labyrinthlike pattern. These findings are supported by low-energy electron diffraction experiments. Slab calculations within the framework of DFT show that the atomic structure consists of parallel lanthanum chains on top of boron octahedra. Scanning tunneling spectroscopy shows a prominent spectral feature at −0.6eV. Using DFT, we identify this structure as a surface resonance of the (2×1) reconstructed LaB6 (100) surface which is dominated by boron dangling bond states and lanthanum d states
Ultrafast spin-nematic and ferroelectric phase transitions induced by femto-second light pulses
Optically-induced phase transitions of the manganite have been simulated using a model Hamiltonian, that
captures the dynamics of strongly correlated charge, orbital, lattice, and spin
degrees of freedom. Its parameters have been extracted from first-principles
calculations. Beyond a critical intensity of a femto-second light pulse, the
material undergoes ultra-fast and non-thermal magnetic phase transition from a
non-collinear to collinear antiferromagnetic phases. The light-pulse excites
selectively either a spin-nematic or a ferroelectric phase depending on the
light-polarization. The behavior can be traced to an optically induced
ferromagnetic coupling between Mn-trimers, i.e. polarons which are delocalized
over three Mn-sites. The polarization guides the polymerization of the
polaronic crystal into distinct patterns of ferromagnetic chains determining
the target phase.Comment: 6 pages, 4 figure
Polar phonons and intrinsic dielectric response of the ferromagnetic insulating spinel CdCrS from first principles
We have studied the dielectric properties of the ferromagnetic spinel
CdCrS from first principles. Zone-center phonons and Born effective
charges were calculated by frozen-phonon and Berry phase techniques within
LSDA+U. We find that all infrared-active phonons are quite stable within the
cubic space group. The calculated static dielectric constant agrees well with
previous measurements. These results suggest that the recently observed
anomalous dielectric behavior in CdCrS is not due to the softening of a
polar mode. We suggest further experiments to clarify this point
Bayesian Error Estimation in Density Functional Theory
We present a practical scheme for performing error estimates for Density
Functional Theory calculations. The approach which is based on ideas from
Bayesian statistics involves creating an ensemble of exchange-correlation
functionals by comparing with an experimental database of binding energies for
molecules and solids. Fluctuations within the ensemble can then be used to
estimate errors relative to experiment on calculated quantities like binding
energies, bond lengths, and vibrational frequencies. It is demonstrated that
the error bars on energy differences may vary by orders of magnitude for
different systems in good agreement with existing experience.Comment: 5 pages, 3 figure
A mixed ultrasoft/normconserved pseudopotential scheme
A variant of the Vanderbilt ultrasoft pseudopotential scheme, where the
normconservation is released for only one or a few angular channels, is
presented. Within this scheme some difficulties of the truly ultrasoft
pseudopotentials are overcome without sacrificing the pseudopotential softness.
i) Ghost states are easily avoided without including semicore shells. ii) The
ultrasoft pseudo-charge-augmentation functions can be made more soft. iii) The
number of nonlocal operators is reduced. The scheme will be most useful for
transition metals, and the feasibility and accuracy of the scheme is
demonstrated for the 4d transition metal rhodium.Comment: 4 pages, 2 figure
First-principles study of spontaneous polarization in multiferroic BiFeO
The ground-state structural and electronic properties of ferroelectric
BiFeO are calculated using density functional theory within the local
spin-density approximation and the LSDA+U method. The crystal structure is
computed to be rhombohedral with space group , and the electronic
structure is found to be insulating and antiferromagnetic, both in excellent
agreement with available experiments. A large ferroelectric polarization of
90-100 C/cm is predicted, consistent with the large atomic
displacements in the ferroelectric phase and with recent experimental reports,
but differing by an order of magnitude from early experiments. One possible
explanation is that the latter may have suffered from large leakage currents.
However both past and contemporary measurements are shown to be consistent with
the modern theory of polarization, suggesting that the range of reported
polarizations may instead correspond to distinct switching paths in structural
space. Modern measurements on well-characterized bulk samples are required to
confirm this interpretation.Comment: (9 pages, 5 figures, 5 tables
Appearance of Flat Bands and Edge States in Boron-Carbon-Nitride Nanoribbons
Presence of flat bands and edge states at the Fermi level in graphene
nanoribbons with zigzag edges is one of the most interesting and attracting
properties of nanocarbon materials but it is believed that they are quite
fragile states and disappear when B and N atoms are doped at around the edges.
In this paper, we theoretically investigate electronic and magnetic properties
of boron-carbon-nitride (BCN) nanoribbons with zigzag edges where the outermost
C atoms on the edges are alternately replaced with B and N atoms using the
first principles calculations. We show that BCN nanoribbons have the flat bands
and edge states at the Fermi level in both H_2 rich and poor environments. The
flat bands are similar to those at graphene nanoribbons with zigzag edges, but
the distributions of charge and spin densities are different between them. A
tight binding model and the Hubbard model analysis show that the difference in
the distribution of charge and spin densities is caused by the different site
energies of B and N atoms compared with C atoms.Comment: 5 pages; 3 figure
Neutral-ionic phase transition : a thorough ab-initio study of TTF-CA
The prototype compound for the neutral-ionic phase transition, namely TTF-CA,
is theoretically investigated by first-principles density functional theory
calculations. The study is based on three neutron diffraction structures
collected at 40, 90 and 300 K (Le Cointe et al., Phys. Rev. B 51, 3374 (1995)).
By means of a topological analysis of the total charge densities, we provide a
very precise picture of intra and inter-chain interactions. Moreover, our
calculations reveal that the thermal lattice contraction reduces the indirect
band gap of this organic semi-conductor in the neutral phase, and nearly closes
it in the vicinity of the transition temperature. A possible mechanism of the
neutral-ionic phase transition is discussed. The charge transfer from TTF to CA
is also derived by using three different technics.Comment: 11 pages, 9 figures, 7 table
Surface resonance of the (21) reconstructed lanthanum hexaboride (001)-cleavage plane: a combined STM and DFT study
We performed a combined study of the (001)-cleavage plane of lanthanum
hexaboride (LaB) using scanning tunneling microscopy (STM) and
density functional theory (DFT). Experimentally, we found a (21)
reconstructed surface on a local scale. The reconstruction is only short-range
ordered and tends to order perpendicularly to step edges. At larger distances
from surface steps, the reconstruction evolves to a labyrinth-like pattern.
These findings are supported by low-energy electron diffraction (LEED)
experiments. Slab calculations within the framework of DFT shows that the
atomic structure consists of parallel lanthanum chains on top of boron
octahedra. Scanning tunneling spectroscopy (STS) shows a prominent spectral
feature at -0.6 eV. Using DFT, we identify this structure as a surface
resonance of the (21) reconstructed LaB (100)-surface which
is dominated by boron dangling bond-states and lanthanum d-states.Comment: 10 pages, 16 figure
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