98 research outputs found
Virial theorem and exact properties of density functionals for periodic systems
No full textIn the framework of density functional theory, scaling and the virial theorem are essential tools for deriving the exact properties of density functionals. Preexisting mathematical difficulties in deriving the virial theorem via scaling for periodic systems are resolved via a particular scaling technique. This methodology is employed to derive universal properties of the exchange-correlation energy functional for periodic systems.Fil: Mirhosseini, H.. Max Planck Institute of Microstructure Physics; AlemaniaFil: Cangi, A.. Max Planck Institute of Microstructure Physics; AlemaniaFil: Baldsiefen, T.. Max Planck Institute of Microstructure Physics; AlemaniaFil: Sanna, A.. Max Planck Institute of Microstructure Physics; AlemaniaFil: Proetto, Cesar Ramon. Max Planck Institute of Microstructure Physics; Alemania. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Gross, E. K. U.. Max Planck Institute of Microstructure Physics; Alemani
Concepts of highly excited electronic systems
No full textJamal Berakdar is an outstanding young researcher who has accomplished in less than ten years remarkable contributions to the field. He is well acknowledged by the scientific community. PhD-thesis at University of Freiburg (Prof. Klar, 1994), Humboldt- and Research fellow at Atomic & Molecular Physics Laboratories, Australian National University; since 1998 researcher at Max-Planck Institute for Microstructure Physics in Halle, Germany
Ab Initio Nonadiabatic Dynamics with Coupled Trajectories: A Rigorous Approach to Quantum (De)Coherence
No full textWe report the first nonadiabatic molecular dynamics study based on the exact factorization of the electron nuclear wave function. Our approach (a coupled-trajectory mixed quantum classical, CT-MQC, scheme) is based on the quantum classical limit derived from systematic and controlled approximations to the full quantum-mechanical problem formulated in the exact-factorization framework. Its strength is the ability to correctly capture quantum (de)coherence effects in a trajectory-based approach to excited-state dynamics. We show this by benchmarking CT-MQC dynamics against a revised version of the popular fewest-switches surface-hopping scheme that is able to fix its well-documented overcoherence issue. The CT-MQC approach is successfully applied to investigation of the photochemistry (ring-opening) of oxirane in the gas phase, analyzing in detail the role of decoherence. This work represents a significant step forward in the establishment of the exact factorization as a powerful tool to study excited-state dynamics, not only for interpretation purposes but mainly for nonadiabatic ab initio molecular dynamics simulations
Calibration and measurement uncertainties of a continuous-flow cloud condensation nuclei counter (DMT-CCNC): CCN activation of ammonium sulfate and sodium chloride aerosol particles in theory and experiment
No full textExperimental and theoretical uncertainties in the measurement of cloud condensation nuclei (CCN) with a continuous-flow thermal-gradient CCN counter from Droplet Measurement Technologies (DMT-CCNC) have been assessed by model calculations and calibration experiments with ammonium sulfate and sodium chloride aerosol particles in the diameter range of 20&ndash;220 nm. Experiments have been performed in the laboratory and during field measurement campaigns, covering a wide range of instrument operating conditions (650&ndash;1020 hPa pressure, 293&ndash;303 K inlet temperature, 4&ndash;34 K m<sup>&minus;1</sup> temperature gradient, 0.5&ndash;1.0 L min<sup>&minus;1</sup> flow rate). For each set of conditions, the effective water vapor supersaturation (<I>S</I><sub>eff</sub>, 0.05&ndash;1.4%) was determined from the measured CCN activation spectra (dry particle activation diameters) and Köhler model calculations. High measurement precision was achieved under stable laboratory conditions, where the relative standard deviations of <I>S</I><sub>eff</sub> were as low as &plusmn;1%. During field measurements, however, the relative deviations increased to about &plusmn;5%, which can be mostly attributed to variations of the CCNC column top temperature with ambient temperature. The observed dependence of <I>S</I><sub>eff</sub> on temperature, pressure, and flow rate was compared to the CCNC flow model of Lance et al. (2006). At high <I>S</I><sub>eff</sub> the relative deviations between flow model and experimental results were mostly less than 10%, but at <I>S</I><sub>eff</sub>&le;0.1% they exceeded 40%. Thus, careful experimental calibration is required for high-accuracy CCN measurements &ndash; especially at low <I>S</I><sub>eff</sub>. A comprehensive comparison and uncertainty analysis of the various Köhler models and thermodynamic parameterizations commonly used in CCN studies showed that the relative deviations between different approaches are as high as 25% for (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub> and 12% for NaCl. The deviations were mostly caused by the different parameterizations for the activity of water in aqueous solutions of the two salts. To ensure comparability of results, we suggest that CCN studies should always report exactly which Köhler model equations and parameters were used. Provided that the Aerosol Inorganics Model (AIM) can be regarded as an accurate source of water activity data for highly dilute solutions of (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub> and NaCl, only Köhler models that are based on the AIM or yield similar results should be used in CCN studies involving these salts and aiming at high accuracy. Experiments with (NH<sub>4</sub>)<sub>2</sub>SO<sub>4</sub> and NaCl aerosols showed that the conditions of particle generation and the shape and microstructure of NaCl particles are critical for their application in CCN activation experiments (relative deviations up to 18%)
The Chisholm firestorm: observed microstructure, precipitation and lightning activity of a pyro-cumulonimbus
No full textA fire storm that occurred on 28 May 2001 and devastated the town of Chisholm, ~150 km north of Edmonton, Alberta, induced a violent fire-invigorated cumulonimbus cloud. This pyro-cumulonimbus (pyro-Cb) had overshooting tops of 2.5–3 km above the tropopause, and injected massive amounts of smoke into the lower stratosphere. Fortunately, this event occurred under good coverage of radar, rain gauge, lightning and satellite measurements, which allowed in-depth documentation of the event, and gave us an opportunity to study the cloud top morphology and microstructure, precipitation and cloud electrification of the pyro-Cb. The combination of heat and smoke created a cloud with extremely small drops, which ascended rapidly in violent updrafts. There appeared to be little freezing up to the homogeneous freezing isotherm level of −38°C. A cloud with such small and short-lived highly supercooled drops is incapable of producing precipitation except for few large graupel and hail, which produced the observed radar echoes and charged the cloud with positive lightning. The small cloud drops froze homogeneously to equally small ice particles, for which there is no mechanism to aggregate into precipitation particles, and which hence remain in the anvil. The lack of significant precipitation implies that only a small fraction of the smoke is scavenged, so that most of it is exhausted through the anvil to the upper troposphere and lower stratosphere. Comparisons with other cases suggest that a pyro-Cb does not have to be as violent as the Chisholm case for precipitation to be strongly suppressed. However, this level of convective vigor is necessary to create the overshooting updraft that injects the smoke into the lower stratosphere
Growth, magnetic properties and electronic structure of Co/Pd (111) : a doctoral dissertation performed at the Max Planck Institute of Microstructure Physics Halle, Germany and submitted to the Faculty of Physics and Applied Computer Science of the AGH University of Science and Technology Cracow, Poland /
No full textRecenzenci pracy: Józef Korecki, Mieczysław Jałochowski.Praca doktorska. Akademia Górniczo-Hutnicza im. Stanisława Staszica (Kraków), 2006.Doctoral dissertation. Max-Planck-Institut für Mikrostrukturphysik, 2006.Bibliogr. przy rozdz..Atomic processes during growth by MBE, tunneling effect, STM, magnetic properties of thin films and MOKE, properties of cobalt and palladium, experimental details, UHV system, STM/STS technique, variable-temperature, VT-STM, low-temperature, LT-STM, tunneling spectroscopy experiments, sample preparation, tip, nucleation, mode of growth, island densities, surface diffusion barrier, shapes for edge diffusion, stacking faults in Cobalt islands on Pd(111), film growth at 300 K, 180 K, 550 K, annealing of samples prepared, dislocation network, magnetic properties, growth temperature dependence of magnetic anisotropy, Co/Pd(111), annealing effect, inverted loops-magneto-optical properties, comparison with other studies, study of the electronic structure by STS, submonolayer coverage at 210 K, pure Co, regime at 300 K, intermixed islands, electronic structure of DL islands, moiré pattern, out-of-plane spin contrast, annealed islands, surface alloying, capping, electronic structure of fcc and hcp islands as seen with STM, triangular Co islands, characterization of thick Co films prepared at 300 K, coalescence, stacking sequence in the Moiré patterns of higher coverage, thickness dependent spectroscop
Using Spinodal Decomposition to Investigate Diffusion Enhancement and Vacancy Population
No full textInternational audienceMaterial sustainability requires energy-efficient and rapid strengthening processes. In alloys, strengthening through diffusion-driven precipitation is limited by the low vacancy concentration, with fewer than one vacancy per 100 billion lattice sites at room temperature in metals such as aluminum and iron under thermodynamic equilibrium. Artificially increasing vacancy concentrations by 1 to 7 orders of magnitude above equilibrium levels through quenching, irradiation, or deformation can significantly accelerate material strengthening. However, measuring vacancy concentrations below 10 -7 in alloys and achieving spatial mapping remain challenging. Here, a vacancy-mediated gradient microstructure near grain boundaries is reported and analyzed to investigate diffusion enhancement and the local vacancy population in an Al-Zn system. This method uses cryogenic processes to preserve excess vacancies and halt microstructure evolution, enabling intermittent measurement of compositional fluctuations during ultrafast spinodal decomposition. It allows for the assessment of diffusion enhancement and determination of vacancy supersaturation in sub-micrometer regions. Liquid nitrogen-quenched Al-12.5 at.% Zn alloy shows a vacancy concentration of ≈10 -7 at room temperature, dropping to 10 -9 after 3 h, with significant spatial variation near grain boundaries. This work addresses gaps in understanding the evolution and distribution of vacancies across various measurement scales, advancing the control of vacancies to enhance the strengthening of engineering alloys.</div
Exact factorization-based density functional theory of electron-phonon systems
No full textDensity functional theory is generalized to incorporate electron-phonon coupling. A Kohn-Sham equation yielding the electronic density nU(r), a conditional probability density depending parametrically on the phonon normal mode amplitudes U={Uqλ}, is coupled to the nuclear Schrödinger equation of the exact factorization method. The phonon modes are defined from the harmonic expansion of the nuclear Schrödinger equation. A nonzero Berry curvature on nuclear configuration space affects the phonon modes, showing that the potential energy surface alone is generally not sufficient to define the phonons. An orbital-dependent functional approximation for the nonadiabatic exchange-correlation energy reproduces the leading-order nonadiabatic electron-phonon-induced band structure renormalization in the Fröhlich model.Fil: Requist, Ryan. Max Planck Institute of Microstructure Physics; AlemaniaFil: Proetto, Cesar Ramon. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología - Nodo Bariloche | Comisión Nacional de Energía Atómica. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología - Nodo Bariloche; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; ArgentinaFil: Gross, E. K. U.. Max Planck Institute of Microstructure Physics; Alemani
Orthorhombic-to-tetragonal transition of SrRuO<sub>3</sub> layers in Pr<sub>0.7</sub>Ca<sub>0.3</sub>MnO<sub>3</sub>/SrRuO<sub>3</sub> superlattices
No full textHigh-quality Pr0.7Ca0.3MnO3/SrRuO3 superlattices with ultrathin layers were fabricated by pulsed-laser deposition on SrTiO3 substrates. The superlattices were studied by atomically resolved scanning transmission electron microscopy, high-resolution transmission electron microscopy, resistivity and magnetoresistance measurements. The superlattices grew coherently without growth defects. Viewed along the growth direction, SrRuO3 and Pr0.7Ca0.3MnO3 layers were terminated by RuO2 and MnO2, respectively, which imposes a unique structure to their interfaces. Superlattices with a constant thickness of the SrRuO3 layers, but varying thickness of the Pr0.7Ca0.3MnO3 layers showed a change of crystalline symmetry of the SrRuO3 layers. At a low Pr0.7Ca0.3MnO3 layer thickness of 1.5 nm transmission electron microscopy proved the SrRuO3 layers to be orthorhombic, whereas these were non-orthorhombic for a Pr0.7Ca0.3MnO3 layer thickness of 4.0 nm. Angular magnetoresistance measurements showed orthorhombic (with small monoclinic distortion) symmetry in the first case and tetragonal symmetry of the SrRuO3 layers in the second case. Mechanisms driving this orthorhombic to tetragonal transition are briefly discussed.</jats:p
Design, fabrication, and characterization of deep-etched waveguide gratings
No full textOne-dimensional (1-D) deep-etched gratings on a specially grown AlGaAs wafer were designed and fabricated. The gratings were fabricated using state-of-the-art electron beam lithography and high-aspect-ratio reactive ion etching (RIE) in order to achieve the required narrow deep air slots with good accuracy and reproducibility. Since remarkable etch depths (up to 1.5 /spl mu/m), which completely cut through the waveguide core layer, have been attained, gratings composed of only five periods (and, thus, shorter than 6 /spl mu/m) have a bandgap larger than 100 nm. A defect was introduced by increasing the width of the central semiconductor tooth to create microcavities that exhibit a narrow transmission peak (less than 7 nm) around the wavelength of 1530 nm. The transmission spectra between 1460 and 1580 nm have been systematically measured, and the losses have been estimated for a set of gratings, both with and without a defect, for different periods and air slot dimensions. Numerical results obtained via a bidirectional beam propagation code allowed the evaluation of transmissivity, reflectivity, and diffraction losses. By comparing experimental results with the authors' numerical findings, a clear picture of the role of the grating's geometric parameters in determining its spectral features and diffractive losses is illustrated
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