798 research outputs found
First principles calculation of vibrational Raman spectra in large systems: signature of small rings in crystalline SiO2
We present an approach for the efficient calculation of vibrational Raman
intensities in periodic systems within density functional theory. The Raman
intensities are computed from the second order derivative of the electronic
density matrix with respect to a uniform electric field. In contrast to
previous approaches, the computational effort required by our method for the
evaluation of the intensities is negligible compared to that required for the
calculation of vibrational frequencies. As a first application, we study the
signature of 3- and 4-membered rings in the the Raman spectra of several
polymorphs of SiO2, including a zeolite having 102 atoms per unit cell.Comment: 4 pages, 2 figures, revtex4 Minor corrections; accepted in Phys. Rev.
Let
The Raman Fingerprint of Graphene
Graphene is the two-dimensional (2d) building block for carbon allotropes of
every other dimensionality. It can be stacked into 3d graphite, rolled into 1d
nanotubes, or wrapped into 0d fullerenes. Its recent discovery in free state
has finally provided the possibility to study experimentally its electronic and
phonon properties. Here we show that graphene's electronic structure is
uniquely captured in its Raman spectrum that clearly evolves with increasing
number of layers. Raman fingerprints for single-, bi- and few-layer graphene
reflect changes in the electronic structure and electron-phonon interactions
and allow unambiguous, high-throughput, non-destructive identification of
graphene layers, which is critically lacking in this emerging research area
Formation of p-n junction in polymer electrolyte-top gated bilayer graphene transistor
We show simultaneous p and n type carrier injection in bilayer graphene
channel by varying the longitudinal bias across the channel and the top gate
voltage. The top gate is applied electrochemically using solid polymer
electrolyte and the gate capacitance is measured to be 1.5 , a
value about 125 times higher than the conventional SiO back gate
capacitance. Unlike the single layer graphene, the drain-source current does
not saturate on varying the drain-source bias voltage. The energy gap opened
between the valence and conduction bands using top and back gate geometry is
estimated.Comment: 16 pages, 6 figure
Pullulan for advanced sustainable body- And skin-contact applications
The present review had the aim of describing the methodologies of synthesis and properties of biobased pullulan, a microbial polysaccharide investigated in the last decade because of its interesting potentialities in several applications. After describing the implications of pullulan in nano-technology, biodegradation, compatibility with body and skin, and sustainability, the current applications of pullulan are described, with the aim of assessing the potentialities of this biopolymer in the biomedical, personal care, and cosmetic sector, especially in applications in contact with skin
Phonon dispersion and lifetimes in MgB2
We measure phonon dispersion and linewidth in a single crystal of MgB_2 along
the Gamma-A, Gamma-M and A-L directions using inelastic X-Ray scattering. We
use Density Functional Theory to compute the effect of both electron-phonon
coupling and anharmonicity on the linewidth, obtaining excellent agreement with
experiment. Anomalous broadening of the E_2g phonon mode is found all along
Gamma-A. The dominant contribution to the linewidth is always the
electron-phonon coupling.Comment: 4 pages, 3 figure
Composition dependence of the synergistic effect of nucleating agent and plasticizer in poly(lactic acid): A mixture design study
Blends consisting of commercial poly(lactic acid) (PLA), poly(lactic acid) oligomer (OLA8) as plasticizer and a sulfonic salt of a phthalic ester and poly(D-lactic acid) as nucleating agents were prepared by melt extrusion, following a Mixture Design approach, in order to systematically study mechanical and thermal properties as a function of composition. The full investigation was carried out by differential scanning calorimetry (DSC), dynamic mechanical thermal analysis (DMTA) and tensile tests. The crystallization half-time was also studied at 105 °C as a function of the blends composition. A range of compositions in which the plasticizer and the nucleation agent minimized the crystallization half-time in a synergistic way was clearly identified thanks to the application of the Mixture Design approach. The results allowed also the identification of a composition range to maximize the crystallinity developed during the rapid cooling below glass transition temperature in injection moulding, thus allowing an easier processing of PLA based materials. Moreover the mechanical properties were discussed by correlating them to the chemical structural features and thermal behaviour of blends
Deregulated expression of TCL1 causes T cell leukemia in mice
The TCL1 oncogene on human chromosome 14q32.1 is involved in the development of T cell leukemia in humans. These leukemias are classified either as T prolymphocytic leukemias, which occur very late in life, or as T chronic lymphocytic leukemias, which often arise in patients with ataxia telangiectasia (AT) at a young age. The TCL1 oncogene is activated in these leukemias by juxtaposition to the α or β locus of the T cell receptor, caused by chromosomal translocations t(14:14)(q11:q32), t(7:14)(q35:q32), or by inversions inv(14)(q11:q32). To show that transcriptional alteration of TCL1 is causally involved in the generation of T cell neoplasia we have generated transgenic mice that carry the TCL1 gene under the transcriptional control of the p56(lck) promoter element. The lck-TCL1 transgenic mice developed mature T cell leukemias after a long latency period. Younger mice presented preleukemic T cell expansions expressing TCL1, and leukemias developed only at an older age. The phenotype of the murine leukemias is CD4-CD8+, in contrast to human leukemias, which are predominantly CD4+CD8-. These studies demonstrate that transcriptional activation of the TCL1 protooncogene can cause malignant transformation oft lymphocytes, indicating the role of TCL1 in the initiation of malignant transformation in T prolymphocytic leukemias and T chronic lymphocytic leukemias
Temperature dependent surface relaxations of Ag(111)
The temperature dependent surface relaxation of Ag(111) is calculated by
density-functional theory. At a given temperature, the equilibrium geometry is
determined by minimizing the Helmholtz free energy within the quasiharmonic
approximation. To this end, phonon dispersions all over the Brillouin zone are
determined from density-functional perturbation theory. We find that the
top-layer relaxation of Ag(111) changes from an inward contraction (-0.8 %) to
an outward expansion (+6.3%) as the temperature increases from T=0 K to 1150 K,
in agreement with experimental findings. Also the calculated surface phonon
dispersion curves at room temperature are in good agreement with helium
scattering measurements. The mechanism driving this surface expansion is
analyzed.Comment: 6 pages, 7 figures, submitted to Phys. Rev. B (May 1998). Other
related publications can be found at
http://www.rz-berlin.mpg.de/th/paper.htm
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