2,298 research outputs found
Reduced Density Matrix Functional for Many-Electron Systems
Reduced density matrix functional theory for the case of solids is presented
and a new exchange correlation functional based on a fractional power of the
density matrix is introduced. We show that compared to other functionals, this
produces more accurate results for both finite systems. Moreover, it captures
the correct band gap behavior for conventional semiconductors as well as
strongly correlated Mott insulators, where a gap is obtained in absence of any
magnetic ordering.Comment: 4 figs and 1 tabl
A complete description of the magnetic ground state in spinel vanadates
Capturing the non-collinear magnetic ground state of the spinel vanadates
AVO (A= Mn, Fe and Co) remains an outstanding challenge for
state-of-the-art ab-initio methods. We demonstrate that both the non-collinear
spin texture, as well as the magnitude of local moments, are captured by a
single value of the on-site Hubbard of 2.7~eV in conjunction with the local
spin density approximation (LSDA+), provided the source term (i.e., magnetic
monopole term) is removed from the exchange-correlation magnetic field . We further demonstrate that the magnetic monopole structure in is highly sensitive to the value of , to the extent that the
interplay between on-site localization and local moment magnitude is
qualitatively different depending on whether the source term is removed or not.
This suggests that in treating strongly correlated magnetic materials within
the LSDA+ formalism, subtraction of the unphysical magnetic monopole term
from the exchange-correlation magnetic field is essential to correctly treat
the magnetic ground state.Comment: 4 pages, 3 figure
Ionization potentials and electron affinities from reduced density matrix functional theory
In the recent work of S. Sharma \emph{et al.}, (arxiv.org: arxiv:0912.1118),
a single-electron spectrum associated with the natural orbitals was defined as
the derivative of the total energy with respect to the occupation numbers at
half filling for the orbital of interest. This idea reproduces the bands of
various periodic systems using the appropriate functional quite accurately. In
the present work we apply this approximation to the calculation of the
ionization potentials and electron affinities of molecular systems using
various functionals within the reduced density-matrix functional theory. We
demonstrate that this approximation is very successful in general and in
particular for certain functionals it performs better than the direct
determination of the ionization potentials and electron affinities through the
calculation of positive and negative ions respectively. The reason for this is
identified to be the inaccuracy that arises from different handling of the
open- and closed-shell systems
Ab-initio real-time magnon dynamics in ferromagnetic and ferrimagnetic systems
Magnonics - an emerging field of physics - is based on the collective excitations of ordered spins called spin waves. These low-energy excitations carry pure spin currents, paving the way for future technological devices working at low energies and on ultrafast timescales. The traditional ab-initio approach to predict these spin-wave energies is based on linear-response time-dependent density functional theory (LR-TDDFT) in the momentum and frequency regime. Herein, the simulation of magnon dynamics using real-time time-dependent density functional theory is demonstrated, thus extending the domain of ab-initio magnonic studies. Unlike LR-TDDFT, this enables us to observe atom-resolved dynamics of individual magnon modes and, using a supercell approach, the dynamics of several magnon modes can be observed simultaneously. The energies of these magnon modes are concurrent with those found using LR-TDDFT. Next, the complex dynamics of the superposition of magnon modes is studied, before finally studying the element-resolved modes in multisublattice magnetic systems
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Ab-Initio Real-Time Magnon Dynamics in Ferromagnetic and Ferrimagnetic Systems
Magnonicsâan emerging field of physicsâis based on the collective excitations of ordered spins called spin waves. These low-energy excitations carry pure spin currents, paving the way for future technological devices working at low energies and on ultrafast timescales. The traditional ab-initio approach to predict these spin-wave energies is based on linear-response time-dependent density functional theory (LR-TDDFT) in the momentum and frequency regime. Herein, the simulation of magnon dynamics using real-time time-dependent density functional theory is demonstrated, thus extending the domain of ab-initio magnonic studies. Unlike LR-TDDFT, this enables us to observe atom-resolved dynamics of individual magnon modes and, using a supercell approach, the dynamics of several magnon modes can be observed simultaneously. The energies of these magnon modes are concurrent with those found using LR-TDDFT. Next, the complex dynamics of the superposition of magnon modes is studied, before finally studying the element-resolved modes in multisublattice magnetic systems
Spin-density fluctuations and the fluctuation-dissipation theorem in 3d ferromagnetic metals
Spatial and time scales of spin density fluctuations (SDF) were analyzed in
3d ferromagnets using ab initio linear response calculations of complete
wavevector and energy dependence of the dynamic spin susceptibility tensor. We
demonstrate that SDF are spread continuously over the entire Brillouin zone and
while majority of them reside within the 3d bandwidth, a significant amount
comes from much higher energies. A validity of the adiabatic approximation in
spin dynamics is discussed. The SDF spectrum is shown to have two main
constituents: a minor low-energy spin wave contribution and a much larger
high-energy component from more localized excitations. Using the
fluctuation-dissipation theorem (FDT), the on-site spin correlator (SC) and the
related effective fluctuating moment were properly evaluated and their
universal dependence on the 3d band population is further discussed
Structural Transition Kinetics and Activated Behavior in the Superconducting Vortex Lattice
Using small-angle neutron scattering, we investigated the behavior of a
metastable vortex lattice state in MgB2 as it is driven towards equilibrium by
an AC magnetic field. This shows an activated behavior, where the AC field
amplitude and cycle count are equivalent to, respectively, an effective
"temperature" and "time". The activation barrier increases as the metastable
state is suppressed, corresponding to an aging of the vortex lattice.
Furthermore, we find a cross-over from a partial to a complete suppression of
metastable domains depending on the AC field amplitude, which may empirically
be described by a single free parameter. This represents a novel kind of
collective vortex behavior, most likely governed by the nucleation and growth
of equilibrium vortex lattice domains.Comment: 5 pages plus 3 pages of supplemental materia
Thermal equation of state of cubic boron nitride: Implications for a high-temperature pressure scale
The equation of state of cubic boron nitride (cBN) has been determined to a maximum temperature of 3300 K at a simultaneous static pressure of up to more than 70 GPa. Ab initio calculations to 80 GPa and 2000 K have also been performed. Our experimental data can be reconciled with theoretical results and with the known thermal expansion at 1 bar if we assume a small increase in pressure during heating relative to that measured at ambient temperature. The present data combined with the Raman measurements we presented earlier form the basis of a high-temperature pressure scale that is good to at least 3300 K
Ultrafast demagnetization in bulk versus thin films: An ab initio study
We report on {\it ab-initio} simulations of the quantum dynamics of
electronic charge and spin when subjected to intense laser pulses. By
performing separate calculations for a Ni thin film and bulk Ni, we conclude
that surface effects have a dramatic influence on amplifying the laser induced
demagnetization. We show that the reason for this amplification is due to
increased spin-currents on the surface of the thin film. This enhancement is a
direct consequence of the broken symmetry originating from the surface
formation. We find that the underlying physics of demagnetization, during the
early femtoseconds, for both bulk and thin film is dominated by spin-flips
induced by spin-orbit coupling. After the first fs this changes in
that the dominant cause of demagnetization is the flow of spin-currents, which
leads to stronger demagnetization in the film compared to that of the bulk.Comment: 12 pages, 3 figure
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