Pattern Formation in Semiconductors

In semiconductors, nonlinear generation and recombination processes of free carriers and nonlinear charge transport can give rise to non-equilibrium phase transitions. At low temperatures, the basic nonlinearity is due to the autocatalytic generation of free carriers by impact ionization of shallow impurities. The electric field accelerates free electrons, causing an abrupt increase in free carrier density at a critical electric field. In static electric fields, this nonlinearity is known to yield complex filamentary current patterns bound to electric contacts

Theory of traveling filaments in bistable semiconductor structures

We present a generic nonlinear model for current filamentation in semiconductor structures with S-shaped current-voltage characteristics. The model accounts for Joule self-heating of a current density filament. It is shown that the self-heating leads to a bifurcation from static to traveling filament. Filaments start to travel when increase of the lattice temperature has negative impact on the cathode-anode transport. Since the impact ionization rate decreases with temperature, this occurs for a wide class of semiconductor systems whose bistability is due to the avalanche impact ionization. We develop an analytical theory of traveling filaments which reveals the mechanism of filament motion, find the condition for bifurcation to traveling filament, and determine the filament velocity.Comment: 13 pages, 5 figure

The Influence of Chemical Modification on Linker Rotational Dynamics in Metal–Organic Frameworks

The robust synthetic flexibility of metal–organic frameworks (MOFs) offers a promising class of tailorable materials, for which the ability to tune specific physicochemical properties is highly desired. This is achievable only through a thorough description of the consequences for chemical manipulations both in structure and dynamics. Magic angle spinning solid‐state NMR spectroscopy offers many modalities in this pursuit, particularly for dynamic studies. Herein, we employ a separated‐local‐field NMR approach to show how specific intraframework chemical modifications to MOF UiO‐66 heavily modulate the dynamic evolution of the organic ring moiety over several orders of magnitude.Ringrotationen in MOFs wurden in Festkörper‐NMR‐Experimenten unter Probenrotation um den magischen Winkel durch dipolare Dephasierung über die Rotorperiode detektiert. Informationen zur Dynamik in Metall‐organischen Gerüsten sind wichtig, weil die Geschwindigkeit der Rotationsbewegung des Linkers die Sorptions‐ und Trenneigenschaften von MOFs beeinflusst.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/144665/1/ange201805004_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/144665/2/ange201805004-sup-0001-misc_information.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/144665/3/ange201805004.pd

Deuteron NMR and modelling in solid polymers

Deuteron NMR techniques are described and some recent applications to the study of rotational motions in solid polymers are reviewed. The information content and the domain of applicability of each technique are presented. Ultra-slow motions are studied in real time without any motional model consideration. For very fast motions, computer molecular dynamics simulations are shown to complement the NMR results. Experimental examples deal with the chain motion in the crystalline α-phase of poly (vinylidenefluoride) and nylon 6,6

NMR of transition metals in clusters

The NMR of 195Pt and 59Co on different clusters is used to proof the potentialities of this spectroscopy in the chemistry of transition metals. Platinum NMR, on five platinum atom clusters, leads to complex spectra with second order effect and interesting structural parameters. The resolution of cobalt spectra allows the measurement of all the resonances of clusters on high field spectrometers. The application of Werbelow’s theory allows the interpretation of phosphorus 31 spectra. 59Co solid state NMR spectra is also possible and gives all the necessary parameters to interpret the relaxation time measurements of this nucleus in solution

Quantitative analysis by

The technique of cross-polarization (CP/MAS) offers many possibilities in the study of insoluble molecules such as the eumelanins. A dipolar dephasing of 43-μs permitted a determination of the resonances due to quaternary carbons. Moreover, after an extremely short polarization transfer time (35-μs), it is possible to selectively differentiate the protonated carbons. The spectra obtained by these two methods associated with those resulting from a cross-polarization experiment for a duration of 2-ms permitted a quantitative measurement of the ratio of protonated to non- protonated carbons in natural and synthetic melanins. The quantitative aspect of CP/MAS was verified by a comparison of MAS and CP/MAS spectra

Que peut-on faire avec un noyau quadrupolaire en RMN ? Un exemple avec le cobalt 59

We have observed cobalt 59 NMR on tetrahedric clusters in solution and in the solid state. From the relaxation times in solution, we are able to deduce the quadrupolar coupling constants for 99Ru and to simulate the 59Co spectrum in the solid state for the basal atoms of Co4(CO)12 which has never been observed. This short review shows the interest of high fields applied to quadrupolar nuclei

Stoßionisationsinduzierte Strukturbildung in Halbleitern mittels Mikrowellen

In gering dotierten Halbleiterschichten kann bei elektrischen Feldern von wenigen V/cm ein Nichtgleichgewichtsphasenübergang von einem niedrigleitenden in einen hochleitenden Zustand beobachtet werden. Bei Beschaltung mit elektrischen Gleichfeldern ist dieser Übergang mit der Ausbildung von Stromfilamenten verbunden. Erstmals wurde auch ohne elektrische Kontakte, im elektrischen Wechselfeld von Mikrowellen, die Ausbildung kreisförmiger räumlichen Strukturen in der Elektronendichte gefunden. In Abhängigkeit von der Mikrowellenleistung tritt eine deutliche Hysterese im Entstehungs- und Verlöschprozeß der mikrowelleninduzierten Strukturen auf. Die Abstände der Strukturen werden mit den Ergebnissen zweidimensionaler Berechnungen der Mikrowellenfeldverteilung verglichen

High-Resolution Solid-State Nmr: a Versatile Tool for the Study of AI-O-P Clusters

International audienc

Microwave induced patterns in n-GaAs and their photoluminescence imaging

Using the technique of photoluminescence imaging, self-organized patterns of high-electron density in homogeneous n-GaAs layers under homogeneous microwave irradiation are studied. The structures are shown to be analogous to current filaments in a static electric field. The symmetry of the microwave induced patterns is not constrained by the current feeding electrodes. It is, however, concluded that a feedback mechanism exists between the formation of high-conducting structures and the homogeneity of the incident microwave irradiation