76 research outputs found
Dislocation networks in helium-4 crystals
The mechanical behavior of crystals is dominated by dislocation networks,
their structure and their interactions with impurities or thermal phonons.
However, in classical crystals, networks are usually random with impurities
often forming non-equilibrium clusters when their motion freezes at low
temperature. Helium provides unique advantages for the study of dislocations:
crystals are free of all but isotopic impurities, the concentration of these
can be reduced to the ppb level, and the impurities are mobile at all
temperatures and therefore remain in equilibrium with the dislocations. We have
achieved a comprehensive study of the mechanical response of 4He crystals to a
driving strain as a function of temperature, frequency and strain amplitude.
The quality of our fits to the complete set of data strongly supports our
assumption of string-like vibrating dislocations. It leads to a precise
determination of the distribution of dislocation network lengths and to
detailed information about the interaction between dislocations and both
thermal phonons and 3He impurities. The width of the dissipation peak
associated with impurity binding is larger than predicted by a simple Debye
model, and much of this broadening is due to the distribution of network
lengths.Comment: accepted by Phys. Rev.
Observation of metastable hcp solid helium
We have produced and observed metastable solid helium-4 below its melting
pressure between 1.1 K and 1.4 K. This is achieved by an intense pressure wave
carefully focused inside a crystal of known orientation. An accurate density
map of the focal zone is provided by an optical interferometric technique.
Depending on the sample, minimum density achieved at focus corresponds to
pressures between 2 and 4 bar below the static melting pressure. Beyond, the
crystal undergoes an unexpected instability much earlier than the predicted
spinodal limit. This opens a novel opportunity to study this quantum crystal in
an expanded metastable state and its stability limits.Comment: deuxi\`eme versio
Direct Domino Synthesis of Azido-Dienoic Acids: Potential Linker Units
We report an atom-economical domino synthesis of functionalized and stereodefined dienes. This method hinges on an allylic alkylation–electrocyclic ring-opening sequence and allows direct access to doubly vinylogous azido-dienoic acids bearing challenging substitution patterns. This class of compounds represents useful linkers in chemical biology by virtue of the orthogonality between the azido and carboxylic acid moieties
Direct observation of homogeneous cavitation in nanopores
We report on the evaporation of hexane from porous alumina and silicon
membranes. These membranes contain billions of independent nanopores tailored
to an ink-bottle shape, where a cavity several tens of nanometers in diameter
is separated from the bulk vapor by a constriction. For alumina membranes with
narrow enough constrictions, we demonstrate that cavity evaporation proceeds by
cavitation. Measurements of the pressure dependence of the cavitation rate
follow the predictions of the bulk, homogeneous, classical nucleation theory,
definitively establishing the relevance of homogeneous cavitation as an
evaporation mechanism in mesoporous materials. Our results imply that porous
alumina membranes are a promising new system to study liquids in a deeply
metastable state.Comment: 14 pages , 4 figures. Source files also contain Supplemental Material
(Doebele_HomogeneousCavitationMembranes_SM.pdf
An Atom-Economical and Stereoselective Domino Synthesis of Functionalised Dienes
Open sesame: A direct synthesis of functionalised and stereodefined dienes, relying on a domino allylic alkylation/electrocyclic ring-opening sequence, is reported. This method allows concise access to doubly vinylogous esters. A further systematic study of ring-opening rates of carbon-substituted cyclobutenes allowed the design of substrates amenable to sequential pericyclic reactions
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