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 AV$_2$O$_4$ (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 $U$ of 2.7~eV in conjunction with the local spin density approximation (LSDA+$U$), provided the source term (i.e., magnetic monopole term) is removed from the exchange-correlation magnetic field ${\bf B}_{XC}$. We further demonstrate that the magnetic monopole structure in ${\bf B}_{XC}$ is highly sensitive to the value of $U$, 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+$U$ 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

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

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 $\sim 40$ 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