102,879 research outputs found
Transformation of hexagonal Lu to cubic LuH single-crystalline films
With the recent report of near ambient superconductivity at room temperature
in the N-doped lutetium hydride (Lu-H-N) system, the understanding of cubic
Lu-H compounds has attracted worldwide attention. Generally, compared to
polycrystal structures with non-negligible impurities, the single-crystalline
form of materials with high purity can provide an opportunity to show their
hidden properties. However, the experimental synthesis of single-crystalline
cubic Lu-H compounds has not been reported thus far. Here, we developed an easy
way to synthesize highly pure LuH single-crystalline films by the
post-annealing of Lu single-crystalline films (purity of 99.99%) in the H
atmosphere. The crystal and electronic structures of films were characterized
by X-ray diffraction, Raman spectroscopy, and electrical transport.
Interestingly, Lu films are silver-white and metallic, whereas their
transformed LuH films become purple-red and insulating, indicating the
formation of an unreported electronic state of Lu-H compounds possibly. Our
work provides a novel route to synthesize and explore more single-crystalline
Lu-H compounds
Lu-H-N phase diagram from first-principles calculations
Using a comprehensive structure search and high-throughput first-principles
calculations of 1483 compounds, this study presents the phase diagram of
Lu-H-N. The formation energy landscape of Lu-H-N was derived and utilized to
assess the thermodynamic stability of compounds. Results indicate that there
are no stable Lu-H-N ternary structures in this system, but metastable ternary
structures, such as Lu20H2N17 (C2/m), Lu2H2N (P3-m1), were observed with small
Ehull (< 100 meV/atom). Moreover, applying hydrostatic pressure up to 10 GPa
causes the energy convex hull of the Lu-H-N to shift its shape and stabilizes
binary phases such as LuN9 and Lu10H21. Additionally, interstitial empty sites
in LuH2 were noted, which may explain the formation of Lu10H21 and LuH3-xNy. To
provide a basis for comparison, X-ray diffraction patterns and electronic
structures of some compounds are also presented
Phase separation, crystallization and interactions in biopolymer blends studied in situ by macro ATR-FTIR spectroscopic imaging
Ubiquitous applications of biopolymer blends require a broader range of accurate information about their behaviour. For this purpose, macro attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy and spectroscopic imaging are the main characterisation methods of choice to provide reliable information for both qualitative and quantitative analysis. The main objective of this project is to realise their full potential to investigate biopolymers and biopolymer blends with and without high-pressure CO2.
In situ high-pressure ATR-FTIR spectroscopic imaging was applied to visualize the dynamic process of phase separation in biopolymer Polycaprolactone (PCL)/Poly (lactic acid) (PLA) blends under high-pressure CO2. It was demonstrated that the extent of phase separation in PCL/PLA blends under high-pressure CO2 is enhanced with increasing exposure time, CO2 pressure and temperature.
The mechanisms of polymer-polymer interactions were investigated using 2D correlation analysis and 2D disrelation mapping. Evidence of the break of some of the existing inter- and intramolecular dipole-dipole interaction (C=O···C=O) between polymer molecules under high-pressure CO2 was provided.
The simultaneous visualization of crystallization and phase separation in Poly (3-hydroxybutyrate) PHB/PLA blends has been realized for the first time through utilizing in situ ATR-FTIR spectroscopic imaging. Decreasing the annealing temperature is proved to be an effective method to constrain phase separation between these two blend components and tailor the final morphology in upper critical solution temperature (UCST) crystallizable polymer blends for practical applications.
Spectroscopic images based on the band position were successfully used to investigate changes in intra- and intermolecular interactions in PHB/PLA blends. During the isothermal melt crystallization, the disappearance of the intermolecular interaction between PHB and PLA and the appearance of the inter- and intramolecular interactions within the PHB crystal were both visualized.
Overall, this thesis has demonstrated further the use of ATR-FTIR spectroscopy and macro ATR-FTIR spectroscopic imaging as a valuable analytical approach to investigate biopolymers and biopolymer blends with and without high-pressure CO2, which can aid the design of biopolymer products and biopolymer process.Open Acces
Higher-spin strings and W minimal models
We study the spectrum of physical states for higher-spin generalisations of
string theory, based on two-dimensional theories with local spin-2 and spin-
symmetries. We explore the relation of the resulting effective Virasoro string
theories to certain minimal models. In particular, we show how the
highest-weight states of the minimal models decompose into Virasoro
primaries.Comment: 13 pages, CTP TAMU-43/93, KUL-TF-93/9
Bouncing Braneworlds Go Crunch!
Recently, interesting braneworld cosmologies in the Randall-Sundrum scenario
have been constructed using a bulk spacetime which corresponds to a charged AdS
black hole. In particular, these solutions appear to `bounce', making a smooth
transition from a contracting to an expanding phase. By considering the
spacetime geometry more carefully, we demonstrate that generically in these
solutions the brane will encounter a singularity in the transition region.Comment: 17 pages, 4 figures, ref adde
On Winding Branes and Cosmological Evolution of Extra Dimensions in String Theory
We consider evolution of compact extra dimensions in cosmology and try to see
whether wrapped branes can prevent the expansion of the internal space. Some
difficulties of Brandenberger and Vafa mechanism for decompactification are
pointed out. In both pure Einstein and dilaton gravities, we study cosmology of
winding brane gases in a continuum approximation. The energy momentum tensor is
obtained by coupling the brane action to the gravity action and we present
several exact solutions for various brane configurations. T-duality invariance
of the solutions are established in dilaton gravity. Our results indicate that
phenomenologically the most viable scenario can be realized when there is only
one brane wrapping over all extra dimensions.Comment: 17 pages, RevTex4, 2 figures, a paragraph added, to appear in
Classical and Quantum Gravit
Oxidation = group theory
Dimensional reduction of theories involving (super-)gravity gives rise to
sigma models on coset spaces of the form G/H, with G a non-compact group, and H
its maximal compact subgroup. The reverse process, called oxidation, is the
reconstruction of the possible higher dimensional theories, given the lower
dimensional theory. In 3 dimensions, all degrees of freedom can be dualized to
scalars. Given the group G for a 3 dimensional sigma model on the coset G/H, we
demonstrate an efficient method for recovering the higher dimensional theories,
essentially by decomposition into subgroups. The equations of motion, Bianchi
identities, Kaluza-Klein modifications and Chern-Simons terms are easily
extracted from the root lattice of the group G. We briefly discuss some aspects
of oxidation from the E_{8(8)}/SO(16) coset, and demonstrate that our formalism
reproduces the Chern-Simons term of 11-d supergravity, knows about the
T-duality of IIA and IIB theory, and easily deals with self-dual tensors, like
the 5-tensor of IIB supergravity.Comment: LaTeX, 8 pages, uses IOP style files; Talk given at the RTN workshop
``The quantum structure of spacetime and the geometric nature of fundamental
interactions'', Leuven, September 200
Influence of Hydrodynamic Interactions on Mechanical Unfolding of Proteins
We incorporate hydrodynamic interactions in a structure-based model of
ubiquitin and demonstrate that the hydrodynamic coupling may reduce the peak
force when stretching the protein at constant speed, especially at larger
speeds. Hydrodynamic interactions are also shown to facilitate unfolding at
constant force and inhibit stretching by fluid flows.Comment: to be published in Journal of Physics: Condensed Matte
Baldness/delocalization in intersecting brane systems
Marginally bound systems of two types of branes are considered, such as the
prototypical case of Dp+4 branes and Dp branes. As the transverse separation
between the two types of branes goes to zero, different behaviour occurs in the
supergravity solutions depending on p; no-hair theorems result for p<=1 only.
Within the framework of the AdS/CFT correspondence, these supergravity no-hair
results are understood as dual manifestations of the Coleman-Mermin-Wagner
theorem. Furthermore, the rates of delocalization for p<=1 are matched in a
scaling analysis. Talk given at ``Strings '99''; based on hep-th/9903213 with
D. Marolf.Comment: LaTeX, 10 pages, 1 figure; contribution to Strings'99 proceeding
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