arXiv subjects

Hermann Stoll

Publications and source records attributed to Hermann Stoll.

At least 37 records · Page 2Linked to original sources

arXiv2011

Observation of non-linear magnetic vortex gyration by X-ray microscopy and micromagnetic simulations

Magnetic vortex gyration in a 500 nm wide, 50 nm thick Permalloy square platelet was investigated using time-resolved scanning transmission x-ray microscopy and micromagnetic simulations. The response of the vortex core on an in-plane oscillating magnetic field was studied as a function of the excitation frequency and amplitude. Non-linear behavior was observed in the form of a redshift of the gyrotropic resonance. I.e., when the excitation amplitude was increased, the resonance frequency was found to decrease. The nonlinearity is in agreement with extensive micromagnetic simulations. This work complements previous reports on thinner nanostructures where a frequency blueshift was observed.

cond-mat.mtrl-sci↗

arXiv2011

Magnetic Vortex Core Reversal by Excitation of Spin Waves

Micron-sized magnetic platelets in the flux closed vortex state are characterized by an in-plane curling magnetization and a nanometer-sized perpendicularly magnetized vortex core. Having the simplest non-trivial configuration, these objects are of general interest to micromagnetics and may offer new routes for spintronics applications. Essential progress in the understanding of nonlinear vortex dynamics was achieved when low-field core toggling by excitation of the gyrotropic eigenmode at sub-GHz frequencies was established. At frequencies more than an order of magnitude higher vortex state structures possess spin wave eigenmodes arising from the magneto-static interaction. Here we demonstrate experimentally that the unidirectional vortex core reversal process also occurs when such azimuthal modes are excited. These results are confirmed by micromagnetic simulations which clearly show the selection rules for this novel reversal mechanism. Our analysis reveals that for spin wave excitation the concept of a critical velocity as the switching condition has to be modified.

cond-mat.other↗

arXiv2009

Vortex core switching by coherent excitation with single in-plane magnetic field pulses

The bistability of the core magnetization of nano-scaled magnets with a magnetic vortex configuration has great potential for data storage applications. To exploit this, reliable switching between the two possible states is needed. Time resolved x-ray microscopy was used to study the response of the vortex core to excitation pulses at sub-ns timescales and image the vortex core switching. A reliable switching process by coherent excitation with leading and trailing edges of in-plane magnetic field pulses was found and compared with micromagnetic simulations.

cond-mat.other↗

arXiv2008

Time-Resolved X-ray Microscopy of Spin-Torque-Induced Magnetic Vortex Gyration

Time-resolved X-ray microscopy is used to image the influence of alternating high-density currents on the magnetization dynamics of ferromagnetic vortices. Spin-torque induced vortex gyration is observed in micrometer-sized permalloy squares. The phases of the gyration in structures with different chirality are compared to an analytical model and micromagnetic simulations, considering both alternating spinpolarized currents and the current's Oersted field. In our case the driving force due to spin-transfer torque is about 70% of the total excitation while the remainder originates from the current's Oersted field. This finding has implications to magnetic storage devices using spin-torque driven magnetization switching and domain-wall motion.

cond-mat.other↗

arXiv2005

A simplified method for the computation of correlation effects on the band structure of semiconductors

We present a simplified computational scheme in order to calculate the effects of electron correlations on the energy bands of diamond and silicon. By adopting a quasiparticle picture we compute first the relaxation and polarization effects around an electron set into a conduction band Wannier orbital. This is done by allowing the valence orbitals to relax within a self-consistent field (SCF) calculation. The diagonal matrix element of the Hamiltonian leads to a shift of the center of gravity of the conduction band while the off-diagonal matrix elements result in a small reduction of the conduction-electron band width. This calculation is supplemented by the computation of the loss of ground state correlations due to the blocked Wannier orbital into which the added electron has been placed. The same procedure applies to the removal of an electron, i.e., to the valence bands. But the latter have been calculated previously in some detail and previous results are used in order to estimate the energy gap in the two materials. The numerical data reported here shows that the methods works, in principle, but that also some extension of the scheme is necessary to obtain fully satisfactory results.

cond-mat.mtrl-sci↗

arXiv2004

Ab-Initio Calculation of the Metal-Insulator Transition in Lithium rings

We study how the Mott metal-insulator transition (MIT) is affected when we have to deal with electrons with different angular momentum quantum numbers. For that purpose we apply ab-initio quantum-chemical methods to lithium rings in order to investigate the analogue of a MIT. By changing the interatomic distance we analyse the character of the many-body wavefunction and discuss the importance of the $s-p$ orbital quasi-degeneracy within the metallic regime. The charge gap (ionization potential minus electron affinity) shows a minimum and the static electric dipole polarizability has a pronounced maximum at a lattice constant where the character of the wavefunction changes from significant $p$ to essentially $s$-type. In addition, we examine rings with bond alternation in order to answer the question under which conditions a Peierls distortion occurs.

cond-mat.str-el↗

arXiv2004

The convergence of the ab-initio many-body expansion for the cohesive energy of solid mercury

A many-body expansion for mercury clusters of the form E = \sum_{i<j}Δε_{ij} + \sum_{i<j<k}Δε_{ijk} + ... \quad, does not converge smoothly with increasing cluster size towards the solid state. Even for smaller cluster sizes (up to n=6), where van der Waals forces still dominate, one observes bad convergence behaviour. For solid mercury the convergence of the many-body expansion can dramatically be improved by an incremental procedure within an embedded cluster approach. Here one adds the coupled cluster many-body electron correlation contributions of the embedded cluster to the bulk HF energy. In this way we obtain a cohesive energy (not corrected for zero-point vibration) of 0.79 eV in perfect agreement with the experimental value.

cond-mat.mtrl-sci↗

arXiv1999

Wavefunction-based correlated ab initio calculations on crystalline solids

We present a wavefunction-based approach to correlated ab initio calculations on crystalline insulators of infinite extent. It uses the representation of the occupied and the unoccupied (virtual) single-particle states of the infinite solid in terms of Wannier functions. Electron correlation effects are evaluated by considering virtual excitations from a small region in and around the reference cell, keeping the electrons of the rest of the infinite crystal frozen at the Hartree-Fock level. The method is applied to study the ground state properties of the LiH crystal, and is shown to yield rapidly convergent results.

cond-mat↗

arXiv1998

A Wannier-function-based ab initio Hartree-Fock study of polyethylene

In the present letter, we report the extension of our Wannier-function-based ab initio Hartree-Fock approach---meant originally for three-dimensional crystalline insulators---to deal with quasi-one-dimensional periodic systems such as polymers. The system studied is all-transoid polyethylene, and results on optimized lattice parameters, cohesive energy and the band structure utilizing 6-31G** basis sets are presented. Our results are also shown to be in excellent agreement with those obtained with traditional Bloch-orbital-based approaches.

cond-mat↗

arXiv1998

A Wannier function based ab initio Hartree-Fock approach extended to polymers: applications to the LiH chain and trans-polyacetylene

A recently proposed ab initio Hartree-Fock approach aimed at directly obtaining the Wannier functions of a crystalline insulator is applied to polymers. The systems considered are the LiH chain and trans-polyacetylene. In addition to being the first application of our approach to one-dimensional systems, this work also demonstrates its applicability to covalent systems. Both minimal as well as extended basis sets were employed in the present study and excellent agreement was obtained with the Bloch orbital based approaches. Cohesive energies, optimized lattice parameters and the band structure are presented. Localization characteristics of the Wannier functions are also discussed.

cond-mat↗

arXiv1998

Closed-shell interaction in silver and gold chlorides

Hartree-Fock and coupled-cluster calculations have been performed for cubic AgCl and for AuCl having a cubic or the observed structure with space group I4_1/amd. Cohesive energies and lattice constants are in excellent agreement with experiment for AgCl; for AuCl we find good agreement, and the experimental structure is correctly predicted to be lower in energy than the cubic one. Electron-correlation effects on lattice constants are very large, of up to 0.8 Åfor cubic AuCl. We especially discuss the strength of the closed-shell interactions, and for the first time a quantitative analysis of the so-called "aurophilic" Au(I)-Au(I) interaction is presented in solids.

cond-mat.mtrl-sci↗

arXiv1998

Ground-state properties of rutile: electron-correlation effects

Electron-correlation effects on cohesive energy, lattice constant and bulk compressibility of rutile are calculated using an ab-initio scheme. A competition between the two groups of partially covalent Ti-O bonds is the reason that the correlation energy does not change linearly with deviations from the equilibrium geometry, but is dominated by quadratic terms instead. As a consequence, the Hartree-Fock lattice constants are close to the experimental ones, while the compressibility is strongly renormalized by electronic correlations.

cond-mat.mtrl-sci↗

arXiv1998

A Hartree-Fock ab initio band-structure calculation employing Wannier-type orbitals

An ab initio Wannier-function-based approach to electronic ground-state calculations for crystalline solids is outlined. In the framework of the linear combination of atomic orbitals method the infinite character of the solid is rigorously taken into account. The Hartree-Fock ground-state energy, cohesive energy, lattice constant and bulk modulus are calculated in a fully ab initio manner as it is demonstrated for sodium chloride, NaCl, using basis sets close to the Hartree-Fock limit. It is demonstrated that the Hartree-Fock band-structure can easily be recovered with the current approach and agrees with the one obtained from a more conventional Bloch-orbital-based calculation. It is argued that the advantage of the present approach lies in its capability to include electron correlation effects for crystalline insulators by means of well-established quantum chemical procedures.

cond-mat↗

arXiv1998

Towards a quantum-chemical description of crystalline insulators: A Wannier-function-based Hartree-Fock study of Li2O and Na2O

A recently proposed approach for performing electronic-structure calculations on crystalline insulators in terms of localized orthogonal orbitals is applied to the oxides of lithium and sodium, Li2O and Na2O. Cohesive energies, lattice constants and bulk moduli of the aforementioned systems are determined at the Hartree-Fock level, and the corresponding values are shown to be in excellent agreement with the values obtained by a traditional Bloch-orbital-based Hartree-Fock approach. The present Wannier-function-based approach is expected to be advantageous in the treatment of electron-correlation effects in an infinite solid by conventional quantum-chemical methods.

cond-mat↗

arXiv1998

Ground state properties of heavy alkali halides

We extend previous work on alkali halides by calculations for the heavy-atom species RbF, RbCl, LiBr, NaBr, KBr, RbBr, LiI, NaI, KI, and RbI. Relativistic effects are included by means of energy-consistent pseudopotentials, correlations are treated at the coupled-cluster level. A striking deficiency of the Hartree-Fock approach are lattice constants deviating by up to 7.5 % from experimental values which is reduced to a maximum error of 2.4 % by taking into account electron correlation. Besides, we provide ab-initio data for in-crystal polarizabilities and van der Waals coefficients.

cond-mat.mtrl-sci↗

arXiv1997

Obtaining Wannier Functions of a Crystalline Insulator within a Hartree-Fock approach: Applications to LiF and LiCl

An ab initio Hartree-Fock approach aimed at directly obtaining the localized orthogonal orbitals (Wannier functions) of a crystalline insulator is described in detail. The method is used to perform all-electron calculations on the ground states of crystalline lithium fluoride and lithium chloride, without the use of any pseudo or model potentials. Quantities such as total energy, x-ray structure factors and Compton profiles obtained using the localized Hartree-Fock orbitals are shown to be in excellent agreement with the corresponding quantities calculated using the conventional Bloch-orbital based Hartree-Fock approach. Localization characteristics of these orbitals are also discussed in detail.

cond-mat↗

arXiv1997

Cohesive properties of alkali halides

We calculate cohesive properties of LiF, NaF, KF, LiCl, NaCl, and KCl with ab-initio quantum chemical methods. The coupled-cluster approach is used to correct the Hartree-Fock crystal results for correlations and to systematically improve cohesive energies, lattice constants and bulk moduli. After inclusion of correlations, we recover 95-98 % of the total cohesive energies. The lattice constants deviate from experiment by at most 1.1 %, bulk moduli by at most 8 %. We also find good agreement for spectroscopic properties of the corresponding diatomic molecules.

cond-mat.mtrl-sci↗

arXiv1997

Correlated ab-initio calculations for ground-state properties of II-VI semiconductors

Correlated ab-initio ground-state calculations, using relativistic energy-consistent pseudopotentials, are performed for six II-VI semiconductors. Valence ($ns,np$) correlations are evaluated using the coupled cluster approach with single and double excitations. An incremental scheme is applied based on correlation contributions of localized bond orbitals and of pairs and triples of such bonds. In view of the high polarity of the bonds in II-VI compounds, we examine both, ionic and covalent embedding schemes for the calculation of individual bond increments. Also, a partitioning of the correlation energy according to local ionic increments is tested. Core-valence ($nsp,(n-1)d$) correlation effects are taken into account via a core-polarization potential. Combining the results at the correlated level with corresponding Hartree-Fock data we recover about 94% of the experimental cohesive energies; lattice constants are accurate to \sim 1%; bulk moduli are on average 10% too large compared with experiment.

cond-mat.mtrl-sci↗