A complete identification of lithium sites in a model of LiPO3_3 glass: effects of the local structure and energy landscape on ionic jump dynamics

We perform molecular dynamics simulations to study lithium dynamics in a model of LiPO$_3$ glass at temperatures below the glass transition. A straightforward analysis of the ionic trajectories shows that lithium diffusion results from jumps between sites that are basically unmodified on the time scale of the lithium ionic relaxation. This allows us a detailed identification and characterization of the sites. The results indicate that the number of lithium sites is only slightly bigger than the number of lithium ions so that the fraction of vacant sites is very limited at every instant. Mapping the ionic trajectories onto series of jumps between the sites provides direct access to lithium jump dynamics. For each site, we determine the mean residence time $\tau_s$ and the probability $p_s^b$ that a jump from this site to another site is followed by a direct backjump. While a broad distribution $G(\lg \tau_s)$ shows that different sites feature diverse lithium dynamics, high values of $p_s^b$ give direct evidence for back-and-forth jumps. We further study how the local glass structure and the local energy landscape affect lithium jump dynamics. We observe substantial effects due to the energy landscape, which are difficult to capture within single-particle approaches.Comment: 10 pages, 8 figure

Conformational phase diagram for polymers adsorbed at ultrathin nanowires

We study the conformational behavior of a polymer adsorbed at an attractive nanostring and construct the complete structural phase diagram in dependence of the binding strength and effective thickness of the string. For this purpose, Monte Carlo optimization techniques are employed to identify lowest-energy structures for a coarse-grained hybrid polymer-wire model. Among the representative conformations in the different phases are, for example, compact droplets attached to the string and also nanotube-like monolayer films wrapping the string in a very ordered way. We here systematically analyze low-energy shapes and structural order parameters to elucidate the transitions between the structural phases

Accurate modeling approach for the structural comparison between monolayer polymer tubes and single-walled nanotubes

In a recent computational study, we found highly structured ground states for coarse-grained polymers adsorbed to ultrathin nanowires in a certain model parameter region. Those tubelike configurations show, even at a first glance, exciting morphological similarities to known atomistic nanotubes such as single-walled carbon nanotubes. In order to explain those similarities in a systematic way, we performed additional detailed and extensive simulations of coarse-grained polymer models with various parameter settings. We show this here and explain why standard geometrical models for atomistic nanotubes are not suited to interpret the results of those studies. In fact, the general structural behavior of polymer nanotubes, as well as specific previous observations, can only be explained by applying recently developed polyhedral tube models.Comment: Proceedings of the 24th Workshop on Recent Developments in Computer Simulation Studies in Condensed Matter Physics, Feb 21-25, 2011, Athens, Georgia, US

Thermodynamics of tubelike flexible polymers

In this work we present the general phase behavior of short tubelike flexible polymers. The geometric thickness constraint is implemented through the concept of the global radius of curvature. We use sophisticated Monte Carlo sampling methods to simulate small bead-stick polymer models with Lennard-Jones interaction among non-bonded monomers. We analyze energetic fluctuations and structural quantities to classify conformational pseudophases. We find that the tube thickness influences the thermodynamic behavior of simple tubelike polymers significantly, i.e., for given temperature, the formation of secondary structures strongly depends on the tube thickness

Synthesis of aluminum nitride thin films utilizing magnetron sputtering on diamond for potential pseudo surface acoustic wave applications

In der vorliegenden Dissertation wurden qualitativ hochwertige, piezoelektrische AlN-Schichtenhergestellt und deren Eignung als SAW-Sensormaterial untersucht. Für die Synthese der Schichten wurde das reaktive Radiofrequenz-Magnetron-Sputtern von hochreinem Aluminium unter Beimischung von N2 verwendet. Zur Bestimmung der optimalen Wachstumsbedingungen und zur Strukturaufklärung wurden Beschichtungsparameter-Studienim Zusammenhang mit umfangreichen Charakterisierungsmethoden durchgeführt. Die Parameterstudien umfassten die RF-Leistung, Substrattemperatur, Gaszusammensetzung (Ar:N2-Verhältnis) und den Prozessdruck. Qualitativ hochwertige, (002)-fasertexturierte AlNSchichten wurden bei Raumtemperatur in reiner N2-Atmosphäre hergestellt. Die Schichten sind vollständig c-Achsen orientiert mit einer Halbwertsbreite der (002)-AlN-Rockingkurve von 0,1°. Es sind Schichtdicken bis 6,6 µm mit einer Oberflächenrauheit von weniger als 2 nm hergestellt worden. Mittels Nanoindentierung bestimmt, weisen die Schichten eine Härte von 21,8 GPa und einen E-Modul von 338 GPa auf. Die SAW-, SLAW- und Transversal-Geschwindigkeiten wurden mit 5,7 km / s, 10,9 km / s bzw. 6,7 km / s mittels Rasterultraschallmikroskopie bestimmt. Für den Aufbau einer PSAW-Teststruktur wurden mittels Mikrowellenplasma-unterstützter CVD hergestellte, polierte, nanokristalline Diamantschichten (NCD) als Substratmaterial für die AlN-Abscheidung verwendet. Die nötige AlN-Schichtdicke für die Anregung von PSAW-Moden wurde simuliert und erfolgreich realisiert. Zur Erzeugung der Oberflächenwellen wurden IDT-Strukturen mittels FIB-SEM sowohl durch Pt-Abscheidung als auch mittels Elektronenstrahllithographie hergestellt. Es wurde eine Symmetrische Kammstruktur mit einer Fingerbreite und –abstand von jeweils 2 µm gewählt, was einer Wellenlänge von 8 µm entspricht. Gemäß Simulation ergibt sich daraus eine AlN-Schichtdicke von 720 nm zum Erreichen der ersten PSAW-Mode. Die als SAW-Filter angeordneten IDTs zeigten bei den Messungen mittels Frequenzgeber und Oszilloskop eine Resonanzfrequenz von 1 GHz im AlN-Si-System und 2 GHz im AlN-NCD-System. Die dazugehörige Phasengeschwindigkeit der PSAWs ist 8,2 km / s und 16,1 km / s im Falle des AlN-Si- bzw. AlN-NCD-Systems. Die Simulation stimmt mit den hier gemessenen Werten mit einer Abweichung von nur 1 % überein und bestätigt die erfolgreiche Anregung von PSAWModen. Die hier gefundenen Ergebnisse zeigen die Überlegenheit der Materialkombination AlN-NCD gegenüber anderen Piezomaterialien. Zusammen mit der Möglichkeit beide Materialien in Form von Schichten herzustellen, erweitert das deren Einsatzgebiete immens, da die Substrat-Schichtkombination nahezu beliebig ist.In this dissertation high quality, piezoelectrical AlN coatings were synthesized and their suitability as SAW sensor material have been tested. For thin film synthesis reactive radiofrequency magnetron sputtering of high purity aluminum in a N2 atmosphere was utilized. For the optimization of the growth mechanisms and to elucidate the coatings’ structure the process parameters in correlation to comprehensive materials characterization studies were carried out. In detail, the investigated process parameters were rf-power, substrate temperature, composition of process gases and the process pressure. High quality (002)-fibretextured AlN coatings were synthesized at room temperature in pure N2 atmosphere. These films were fully c-axis oriented with a (002)-AlN-rocking-curve exhibiting 0.1° of full width at half maximum. The thickness and surface roughness of the synthesized coatings were 6.6 µm and less than 2 nm, respectively. Measurements by means of nanoindentation displayed hardness values of 21.8 GPa and Young’s modulus of 338 GPa. According to scanning acoustic microscopy the SAW-, SLAW-, and transversal-velocities were 5.7 km / s, 10.9 km / s and 6.7 km / s, respectively. To test the feasibility of PSAW devices based on AlN coatings, nanocrystalline, polished diamond substrates, synthesized by microwave-plasma-enhanced CVD were utilized as substrate material. The AlN coatings’ thickness was simulated and tested successfully to match the needs for a successful PSAW stimulation. To generate surface waves IDT structures were produced in a FIB-SEM by Platinum deposition and electron beam lithography. A symmetrical comb structure with finger thickness of 2 µm each have been chosen corresponding to a wavelength of 8 µm. As per simulation the AlN film thickness should be 720 nm for excitation of the first PSAW mode. The IDTs were arranged to work as a SAW filter showing a resonance frequency at 1 GHz in the case of AlN-Si-systems and 2 GHz in the case of AlN-NCD. Measurements were conducted by employing a frequency generator and an oscilloscope. The corresponding phase velocity of measured PSAWs were 8.2 km / s and 16.1 km / s in the case of AlN-Si and AlN-NCD, respectively. Simulation matched the measurements with a deviation of 1 % only, thus confirming the successful stimulation of the PSAW modes. The results found in this study present the advantage of AlN-NCD compared to other piezoelectric materials. Moreover, the possibility of combining the piezoelectric thin-film material (AlN) with a variety of substrate materials tremendously extends its applications

Theism and Psychological Science: A Call for Rapprochement

The authors offer two arguments for the inclusion of theism in natural science. First, an argument against excluding theism is offered. Though early roots of science promoted a view that it is a way to accumulate knowledge that is untainted by presuppositions and traditions, postmodern critiques call this into question. Scientists have sometimes rejected religion as a context-dependent, tradition-based way of knowing, yet science itself is also context-dependent and tradition-based. Second, an argument for including theism in psychological is offered. Theistic beliefs are relevant insofar as they are part of human experience for many, they represent a form of human diversity, and they have been associated with some positive health outcomes

Characterization of the complex ion dynamics in lithium silicate glasses via computer simulations

We present results of molecular dynamics simulations on lithium metasilicate over a broad range of temperatures for which the silicate network is frozen in but the lithium ions can still be equilibrated. The lithium dynamics is studied via the analysis of different correlation functions. The activation energy for the lithium mobility agrees very well with experimental data. The correlation of the dynamics of adjacent ions is weak. At low temperatures the dynamics can be separated into local vibrational dynamics and hopping events between adjacent lithium sites. The derivative of the mean square displacement displays several characteristic time regimes. They can be directly mapped onto respective frequency regimes for the conductivity. In particular it is possible to identify time regimes dominated by localized dynamics and long-range dynamics, respectively. The question of time-temperature superposition is discussed for the mean square displacement and the incoherent scattering function.Comment: to be published in Phys. Chem. Chem. Phy