73 research outputs found
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 MetalâOrganic Frameworks
The robust synthetic flexibility of metalâorganic 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 solidâstate NMR spectroscopy offers many modalities in this pursuit, particularly for dynamic studies. Herein, we employ a separatedâlocalâfield NMR approach to show how specific intraframework chemical modifications to MOF UiOâ66 heavily modulate the dynamic evolution of the organic ring moiety over several orders of magnitude.Ringrotationen in MOFs wurden in FestkörperâNMRâExperimenten unter Probenrotation um den magischen Winkel durch dipolare Dephasierung ĂŒber die Rotorperiode detektiert. Informationen zur Dynamik in Metallâorganischen GerĂŒsten sind wichtig, weil die Geschwindigkeit der Rotationsbewegung des Linkers die Sorptionsâ 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 Werbelowâs 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-ÎŒs permitted a determination of the resonances due to quaternary carbons. Moreover, after an extremely short polarization transfer time (35-ÎŒs), 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
StoĂionisationsinduzierte Strukturbildung in Halbleitern mittels Mikrowellen
In gering dotierten Halbleiterschichten kann bei elektrischen Feldern von wenigen V/cm ein NichtgleichgewichtsphasenĂŒbergang von einem niedrigleitenden in einen hochleitenden Zustand beobachtet werden. Bei Beschaltung mit elektrischen Gleichfeldern ist dieser Ăbergang mit der Ausbildung von Stromfilamenten verbunden. Erstmals wurde auch ohne elektrische Kontakte, im elektrischen Wechselfeld von Mikrowellen, die Ausbildung kreisförmiger rĂ€umlichen Strukturen in der Elektronendichte gefunden. In AbhĂ€ngigkeit von der Mikrowellenleistung tritt eine deutliche Hysterese im Entstehungs- und VerlöschprozeĂ der mikrowelleninduzierten Strukturen auf. Die AbstĂ€nde 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
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