Georg l Lefkidis

Abstract: In this paper, taking one-magnetic-center NiO and two-magnetic-center nanostructures composed of Co and Ni as examples, we review our recent works in this field. To theoretically achieve ultrafast spin flip and transfer in magnetic nanostructures, an ultrafast spin dynamics mechanism, namely Lambda process, was proposed. To properly simulate the laser-matter interaction, the accurate intragap d-states were predicted firstly through quantum chemistry calculations; then the spin localizations on magnetic atoms were analyzed considering external magnetic field and spin-orbit coupling; at last, laser pulse term was turned on to study the time history of the spin switching scenario via the Lambda process. It was shown that spin flip and transfer can be achieved in a subpicosecond regime with linearly polarized light. In order to further realize checking and monitoring of the spin manipulations in magnetic molecules, a CO molecule was attached to one magnetic center to serve as an infrared marker. The calculated spin-state-dependent C-O stretching frequencies indicate that spin manipulations can be indirectly monitored through infrared spectrum experiments.

Abstract: We present a fully ab initio calculation, the synthesis and the characterization of the homodinuclear [Ni2 II(L-N4Me2)- (emb)] complex, which can act as a prototypic, realistic substance for ultrafast laser-induced spin dynamics. The new compound, which has been synthesized and characterized, consists of two magnetic centers with different spin properties and different local symmetries (distorted octahedral versus distorted square-planar) and exhibits strong spin localization. We calculate the vibrational and electronic spectra of the compound and predict a local spin-flip scenario. The very existence and the properties of the compound represent an important step toward ultrafast experimental spin dynamics in ligand-stabilized multicenter compounds and paves the path toward laser-induced magnetic logic on a single molecule.

Abstract: Different from thermally and magnetically induced demagnetization, the laser-induced demagnetization relies on the laser photon field. However, what has been unknown is how the spin moment reduction correlates with the number of photons. Here, our first-principles calculation in ferromagnetic nickel and cobalt clusters shows that the number of photons is not the sole decisive factor for the magnetization change, contrary to earlier belief. For the same number of photons absorbed, the shorter the laser pulse, the larger the induced spin moment reduction. Besides a simple decrease in the magnetic moment, a short pulse also excites a strong coherent spin oscillation, which disappears when using a longer pulse. The longest pulse duration where we observed this oscillation is about 20 fs. Future experiments can directly test our prediction. We show that for our generic ultrafast spinswitching K-process on metallic nanoclusters the electronic correlations constitute a key ingredient, which allows for spin and charge separation. By selectively removing correlational channels, we gradually inhibit the magnetic switching. The dynamics proceeds far from any electronic thermal equilibrium and thus no temperature can be attributed to the system or any subsystem.

Abstract: The photoabsorption spectra of Pt-2 and Pt-4 clusters on the Cu(001) surface are computed using two different theoretical methods: the symmetry-adapted cluster expansion configuration interaction from quantum chemistry and a recently developed linear response approach to treat electron-hole correlations in the presence of an external electromagnetic field. Comparing the energetically low-lying optical excitations, we find very good agreement between the two methods. For different orientations of the adsorbed clusters with respect to the surface, we find that the most intense optical peaks occur when the polarization of the applied laser pulse is parallel to the surface.(C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Abstract: Absorption spectra of closed-shell Na-2, Na-3(+), Na-4, Na-5(+), Na-6, Na-7(+), and Na-8 clusters are calculated using a complex Bethe-Salpeter equation derived using a conserving linear response method. In the framework of a quasiparticle approach, we determine electron-hole correlations in the presence of an external field. The calculated results are in excellent agreement with experimental spectra, and some possible cluster geometries that occur in experiments are analyzed. The position and the broadening of the resonances in the spectra arise from a consistent treatment of the scattering and dephasing contributions in the linear response calculation. Comparison between the experimental and the theoretical results yields information about the cluster geometry, which is not accessible experimentally. (C) 2010 American Institute of Physics. [doi:10.1063/1.3494093]

Abstract: We present a fully ab initio theory of ultrafast spin switching in nanostructures using optical control theory and including spin-orbit coupling thus realizing Lambda processes. These processes are investigated using high-level quantum chemistry in structures with one, two, and three magnetic centers, where the spin localization and transferability are discussed with respect to their geometry. In particular we study metallic chains with two and three magnetic centers interconnected with Na atoms. We discuss the prerequisites for such scenarios for all structures.

Abstract: A fully ab initio controlled ultrafast magnetooptical switching mechanism in small magnetic clusters is achieved through exploiting spin-orbit-coupling enabled ¤ processes. The idea is that in the magnetic molecules a fast transition between two almost degenerate states with di®erent spins can be triggered by a laser pulse, which leads to an electron excitation from one of the degenerate states to a highly spin-mixed state and a deexcitation to the state of opposite spin. In this paper a CO molecule is attached to one magnetic center of the clusters, which serves as an experimental marker to map the laser-induced spin manipulation to the IR spectrum of CO. The predicted spin-state-dependent CO frequencies can facilitate experimental monitoring of the processes. We show that spin flip in magnetic atoms can be achieved in structurally optimized magnetic clusters in a subpicosecond regime with linearly polarized light.

Abstract: We present an ab initio theory of ultrafast nanologic elements and show that controlled spin manipulation is feasible with the inclusion of spin-orbit coupling in ¤-processes. We show that in branched metallic chains with three magnetic centers both spin flips and spin transfers are possible within a hundred femtoseconds. A static external magnetic field and the magnetic state of one magnetic center serve as input poles (input bits), while the magnetic state of the cluster after a controlled laser pulse can be mapped to the output. Combining laser- induced spin-manipulation scenarios we are able to construct the NAND logic. Thus multicenter magnetic clusters can extend spin dynamics to optically triggered and functionalized magnetic transport on a subpicosecond timescale and nanometer spatial scale.

Abstract: The magneto-optical Kerr effect (MOKE) is a powerful tool for studying changes in the magnetization of ferromagnetic materials. It works by measuring changes in the polarization of reflected light. However, because the conventional theoretical basis for interpreting a MOKE signal assumes measurement with continuous-wave light(1,2), its use for understanding highspeed magnetization dynamics of a material probed with femtosecond optical pulses(3,4) has been controversial(5-10). Here we establish a new paradigm for interpreting time-resolved MOKE measurements, through a first-principles investigation of ferromagnetic nickel. We show that the time-resolved optical and magnetic responses energetically follow their respective optical and magneto-optical susceptibilities. As a result, the one-to-one correspondence between them sensitively depends on the incident photon energy. In nickel, for photon energies below 2 eV the magnetic response is faithfully reflected in the optical response, but above 2 eV they decouple. By constructing a phase-sensitive polarization versus magnetization plot, we find that for short pulses the magnetic signals are delayed by 10 fs with respect to the optical signals. For longer pulses, the delay shortens and the behaviour approaches the continuous-wave response. This finally resolves the long-standing dispute over the interpretation in the time-resolved MOKE measurements and lays a solid foundation for understanding femtomagnetism(3,4).

Abstract: When an ultrafast laser impinges a magnetic material, it excites charge and then, via spin-orbit-coupling, spin. This holds great promise for the future magnetic storage. However, the coupling of the two dynamics is far from clear, which hampers the experimental effort in femtosecond magnetism. Since not every excitation induces the same spin excitation, a clear understanding of the correlation between charge and spin is crucial. In this paper we investigate in a complete first-principles manner the energy dispersion of the spin-moment change in ferromagnetic Ni and the effect of the distance between the magnetic centers upon the spin localization and local-spin-flip times in metallic chains. Thus we establish the missing link between the spin-momentum change and the density-of-states change, and derive rules-of-thumb for localized spin manipulation. (c) 2009 American Institute of Physics. [DOI: 10.1063/1.3058704]

Abstract: Using ab initio methods. we theoretically investigate the adsorption of one and two Pt dimers oil Cu(001) Treating the interaction of adsorbates oil surfaces as a local phenomenon, a representation of the substrate by a large cluster of at least 62 Cu atoms allows one to treat the electronic structures of both systems, that is, the adsorbate and the surface, on equal footing. Theoretical results concerning the adsorbate energetics, structure, and density of states are presented We find that the Pt atoms of the adsorbates prefer to locate on top of the hollow sites of the Cu lattice. A symmetry-adapted-cluster expansion configuration-interaction method is used to calculate the excited states and the optical absorption spectra of the systems. The most intense peaks of the absorption spectra are around 1 eV and result from excited states of B-2 symmetry Last but not least, we identify surface-mediated interdimer interactions.

Abstract: We present a fully ab initio ultrafast magnetooptical-switching mechanism in NiO. After obtaining all intragap d-character states of the bulk and the (001) surface with the use of highly correlational quantum chemistry we propagate in time under the influence of a static magnetic field and a laser pulse. We find that switching can be best achieved in a subpicosecond regime with linearly polarized light. The electric-dipole approximation suffices for the surface, however, for the centrosymmetric bulk the presence of an optical phonon is used as a symmetry-lowering mechanism. Lattice (contrary to electronic) temperature is found to enhance the process. (C) 2008 Elsevier B.V. All rights reserved.

Abstract: We present a first-principles scenario where a realistic three-magnetic-center metallic cluster acts as a prototypic magnetic-logic element within the frame of a unified optically induced spin manipulation. We find that the spins of the energetically low-lying triplet states of a Ni3Na2 cluster are always localized at a single magnetic center and that controlled spin flips and transfers are possible within a hundred femtoseconds with suitable static external magnetic field and laser pulses. The magnetic state or the position of the spins and the static magnetic field can be used as input bits while the output bit is the final state of the magnetic centers, thus the gates AND, OR, XOR (CNOT), and NAND can be built.

Abstract: In an ultrafast laser-induced magnetization-dynamics scenario we demonstrate for the first time an exact microscopic spin-switch mechanism. Combining ab initio electronic many-body theory and quantum optics analysis we show in detail how the coherently induced material polarization for every elementary process leads to angular-momentum exchange between the light and the irradiated antiferromagnetic NiO. Thus we answer the long-standing question where the angular momentum goes. The calculation also predicts a dynamic Kerr effect, which provides a signature for monitoring spin dynamics, by simply measuring the transient rotation and ellipticity of the reflected light.

Abstract: We present a fully ab initio ultrafast spin manipulation calculation in two-magnetic-center clusters with CO attached to one of the magnetic centers. CO serves as an experimental marker for certain magnetic states between which spin flip and transfer can be achieved. The predicted spin-state-dependent CO vibrational frequencies indicate that spin manipulation can be readily monitored through the infrared spectrum. The feasibility is demonstrated by two charged clusters [CoMg2Ni-CO](+) and [NiCo-CO](+). Spin transfer between magnetic centers is achieved with a fidelity of 99.8%.

Abstract: In this paper localized optically induced spin dynamics is presented, based on highly correlational ab initio calculations. Two-magnetic-centre metallic chains are chosen as a material on which the total spin is always found to lie on one of the magnetic centres only. Switching is achieved through a Lambda-process driven by a laser pulse whose parameters are optimized with a genetic algorithm. Locally switching the spin on the iron side of a Co-Na-Fe cluster is given as an example of local spin manipulation.

Abstract: Using highly correlational quantum chemistry we compute from first principles the contributions of the electric dipoles and quadupoles, and magnetic dipoles to the nonlinear optics of NiO. The material is modeled as a doubly embedded cluster and all magnetic intragap d-character states are calculated, thus explaining the experimental results.

Abstract: Ultrafast demagnetization in ferromagnets has attracted much attention both experimentally and theoretically for over a decade. This includes a lively debate as to whether the observed signal represents a true magnetization process. Here, a first-principles and time-dependent calculation is performed for ferromagnetic nickel under laser illumination to directly compute the magnetization and polarization changes. It is found that while the polarization exhibits both rapid and slow oscillations, the magnetization mainly shows a slow oscillation. The slow component of the polarization generally follows the magnetization evolution with a margin of 2-3 fs deviation. The correlation between the magnetization and polarization tends to be better in the absence of laser radiation.

Abstract: We present a fully ab initio ultrafast magneto-optical switching mechanism in NiO. All intragap d states of the bulk and the (001) surface are obtained with highly correlational quantum chemistry and propagated in time under the influence of a static magnetic field and a laser pulse. We find that demagnetization and switching can be best achieved in a subpicosecond regime with linearly rather than circularly polarized light. Going beyond the electric dipole approximation is mandatory for the bulk and greatly enhances the process even for the surface, where it is not required by symmetry.

Abstract: We present a method for calculating second harmonic generation on a purely ab initio basis in NiO, both for the (001) surface and for the bulk. We go beyond the electric dipole approximation and we incorporate magnetic dipoles and electric quadrupoles in our results. A full detailed symmetry analysis of these contributions is given. Then we calculate phononic contributions to the second order susceptibility tensor for the bulk within the frozen phonon approximation. It is shown that transient lattice distortions can lead to a second harmonic signal, even in centrosymmetric materials like NiO. The second order susceptibility tensor is calculated from first principles, including nonlocalities in two different ways: (i) with phononic contributions on a time averaged way and (ii) with a time resolved single optical phonon at the Gamma point. Furthermore the effects of electronic redistribution prior to the probe pulse are given.

Abstract: In this Letter we develop a new systematic approach to study optical second harmonic generation in NiO, on both the (001) surface and the bulk. NiO is modeled as a doubly embedded cluster on which two highly correlated quantum chemistry methods are applied in order to obtain the wave functions of all the intragap d states and the low lying charge transfer states. The optical gap is calculated and the electric dipole, magnetic dipole, and electric quadrupole contributions to the second order susceptibility tensor are computed for the first time from first principles. Going beyond the electric dipole approximation gives new insight into the experimentally observed spectrum. A method is proposed for monitoring the spin dynamics of the NiO(001) surface.

Abstract: Second harmonic generation is a powerful tool for monitoring ultrafast dynamics and investigating domain structures in transition-metal oxides. A symmetry analysis of the contributions to the second order susceptibility tensor of different antiferromagnetic domains is performed, and the role of the phonons is discussed. (c) 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Abstract: An embedded cluster approach is applied to study the electronic excitations on the NiO(001) surface. Starting from the unrestricted Hartree-Fock level of theory, we calculate the ground-state properties to provide some insight into electronic structure and excitations and we estimate the excitation energies and oscillator strengths using the single excitation configuration-interaction (CIS) technique. We compute the second harmonic generation (SHG) tensor for the NiO(001) surface, using the many-body wave functions and energies resulting from double excitation configuration-interaction (CID) and quadratic configuration-interaction with single and double excitations. and energy contributions from triple excitations [QCISD(T)] calculations. From that. the intensity of the nonlinear optical response as a function of photon energy at different polarizations of the incident and outgoing photons is obtained. Combining optical control theory with the ab initio results and the crystal field theory. the laser induced femtosecond spin dynamics is explored. A scenario for ultrafast all-optical magnetic switching that results from the combination of spin-orbit coupling with appropriately shaped short laser pulses is proposed. The application of the theory to the multiplet states within the gap of NiO(001) is found to predict the possibility of all-optical spin switching within 150 femtoseconds.

Abstract:

Abstract: The target of the present thesis was to investigate the nucleophilic adsorption of small molecules at the surface of magnesia. We tried to determine the electronic structure of the bulk as well as of the surface of the crystal, and to compare it with the electronic structure when there is an adsorbed molecule on the surface. We tried to locate the characteristics that affect the ease and geometry of that adsorption. Finally we compared the results of the quantochemical calculations with the results of the solid state calculations. The quantochemical calculations were mainly performed at PM3 level, which is known to provide good results when it comes to geometry and charges. Some calculations at HF and DFT level were also performed. The solid state calculations were performed at LCAO and DFT level. There were also calculations of the cellular method within the tight binding approximation. For that purpose a special program was developed. On the crystal 4 zones are observed. The first one contains one orbital consisting of the s AO of all ions, the second one consists of the s AO of the oxygens, the third one consists of the p AO of the oxygens, and the final unoccupied one consists of the AO of the metal ions. Charges vary between ±0.50 and ±0.65. The molecules approach linearly up to a distance of 2.5 � with no orbital interaction. Some charge transfer from the molecule to the surface takes place though. Some molecules have a second stable distance of about 1.8 �, where chemisorption takes place. Diatomic molecules containing S tend to turn while CO has two stable states of perpendicular and parallel approach. The activity of the surface ions follows the reverse order of their coordination number Mg_{5c} < Mg_{4c} < Mg_{3c} or [100]<[110]<[111]. An increase of the adsorbent�s concentrations affects the adsorption energy negatively. Linear dislocations activate the adsorption near the fissure. Cationic vacancies have almost no effect, while anionic ones do. More specifically, the coordination number of the oxygen removed determines to what extend the charge transfer is inhibited. In most cases removing a 6-coordinated oxygen has the greatest effect. The presence of other ions affects the adsorption according to their electronic content. Substituting Li for Mg eases nucleophilic adsorption on the surface. The symmetry of the MO of the crystal has no effect on the adsorption, due to the accumulation of a great number of orbitals at any energy level.