310 research outputs found
Revealing the role of molecular rigidity on the fragility evolution of glass-forming liquids
If quenched fast enough, a liquid is able to avoid crystallization and will
remain in a metastable supercooled state down to the glass transition, with an
important increase in viscosity upon further cooling. There are important
differences in the way liquids relax as they approach the glass transition,
rapid or slow variation in dynamic quantities under moderate temperature
changes, and a simple means to quantify such variations is provided by the
concept of "fragility". Here, we report molecular dynamics simulations of a
typical network-forming glass, Ge-Se, and find that the relaxation behaviour of
the supercooled liquid is strongly correlated to the variation of rigidity with
temperature and the spatial distribution of the corresponding topological
constraints which, ultimately connect to fragility minima. This permits
extending the fragility concept to aspects of topology/rigidity, and to the
degree of homogeneity of the atomic-sale interactions for a variety of
structural glasses.Comment: 22 pages, 8 figure
Understanding amorphous phase-change materials from the viewpoint of Maxwell rigidity
Phase-change materials (PCMs) are the subject of considerable interest
because they have been recognized as potential active layers for
next-generation non-volatile memory devices, known as Phase Change Random
Access Memories (PRAMs). By analyzing First Principles Molecular Dynamics
simulations we develop a new method for the enumeration of mechanical
constraints in the amorphous phase and show that the phase diagram of the most
popular system (Ge-Sb-Te) can be split into two compositional regions having a
well-defined mechanical character: a Tellurium rich flexible phase, and a
stressed rigid phase that encompasses the known PCMs. This sound atomic scale
insight should open new avenues for the understanding of PCMs and other complex
amorphous materials from the viewpoint of rigidity.Comment: 5 pages, 4 figures in EP
Laboratory course on Streptomyces genetics and secondary metabolism
The "Streptomyces genetics and secondary metabolism" laboratory course gives an introduction to the versatile soil dwelling Gram-positive bacteria Streptomyces and their secondary metabolism. The course combines genetic modification of Streptomyces; growing of the strain and protoplast preparation, plasmid isolation by alkaline lysis and phenol precipitation, digestions, and ligations prior to protoplast transformation, as well as investigating the secondary metabolites produced by the strains. Thus, the course is a combination of microbiology, molecular biology, and chemistry. After the course the students should understand the relationship between genes, proteins, and the produced metabolites. 2016 by The International Union of Biochemistry and Molecular Biology, 44(5):492-499, 2016. 2016 The International Union of Biochemistry and Molecular Biology.</p
Can models for forest attributes based on airborne laser scanning be generalized for different silvicultural management systems?
In Finland, interest in continuous cover forestry (CCF) has increased rapidly in recent years. During those years CCF has been examined from various viewpoints but not from the perspective of forest inventories. This holds especially true for applications based on remote sensing. Conversely, airborne laser scanning (ALS) data have been widely used to predict forest characteristics such as size distribution and vertical forest structure, which are closely related to the forest information needs of CCF. In this study we used the area-based approach to predict a set of stand attributes from ALS data (5 pulses per m2) in a CCF forest management experiment in Katajama & BULL;ki, eastern Finland. In addition to the CCF stands, the experiment included shelterwood stands and untreated stands. The predicted attributes included volume, biomass, basal area, number of stems, mean diameter, Lorey's height, dominant height, standing dead wood volume, parameters of the theoretical stem diameter distribution model, understory height and number of understory stems. Our main aim was to test whether the same model could be used across different management systems. The accuracy of the attributes predicted for the CCF stands was compared with the predictions for the other management systems in the same experiment. We also compared and discussed our results in relation to the even-aged stand attribute predictions that were conducted by using separate operational forest data collected from sites surrounding Katajama & BULL;ki. The results showed that forest data from the different management systems could be combined into a single model of a stand attribute, i.e., ALS metrics were found to be suitable for comparing different management systems in regard to differences in forest structure. The accuracy of the predicted attributes in the CCF plots was comparable to that of the other management alternatives in the experiment. The accuracy was also comparable to that of even-aged forests. The results of this study were promising; the stand attributes of CCF-managed forests could be predicted analogously to those of other management systems. This indicates that for the purposes of forest inventories there may not be a need to stratify forest lands by management system. It should be noted, however, that the study area was relatively small, that the forest stands were harvested in the 1980 s, and that the attributes may not have been completely exhaustive for CCF
Discovering electron transfer driven changes in chemical bonding in lead chalcogenides (PbX, where X = Te, Se, S, O)
Understanding the nature of chemical bonding in solids is crucial to
comprehend the physical and chemical properties of a given compound. To explore
changes in chemical bonding in lead chalcogenides (PbX, where X = Te, Se, S,
O), a combination of property-, bond breaking- and quantum-mechanical bonding
descriptors have been applied. The outcome of our explorations reveals an
electron transfer driven transition from metavalent bonding in PbX (X = Te, Se,
S) to iono-covalent bonding in beta-PbO. Metavalent bonding is characterized by
adjacent atoms being held together by sharing about a single electron and small
electron transfer (ET). The transition from metavalent to iono-covalent bonding
manifests itself in clear changes in these quantum-mechanical descriptors (ES
and ET), as well as in property-based descriptors (i.e. Born effective charge,
dielectric function, effective coordination number (ECON) and mode-specific
Grueneisen parameter, and in bond breaking descriptors (PME). Metavalent
bonding collapses, if significant charge localization occurs at the ion cores
(ET) and/or in the interatomic region (ES). Predominantly changing the degree
of electron transfer opens possibilities to tailor materials properties such as
the chemical bond and electronic polarizability, optical band gap and optical
interband transitions characterized by the imaginary part of the dielectric
function. Hence, the insights gained from this study highlight the
technological relevance of the concept of metavalent bonding and its potential
for materials design
Transparent dense sodium
Under pressure, metals exhibit increasingly shorter interatomic distances.
Intuitively, this response is expected to be accompanied by an increase in the
widths of the valence and conduction bands and hence a more pronounced
free-electron-like behaviour. But at the densities that can now be achieved
experimentally, compression can be so substantial that core electrons overlap.
This effect dramatically alters electronic properties from those typically
associated with simple free-electron metals such as lithium and sodium, leading
in turn to structurally complex phases and superconductivity with a high
critical temperature. But the most intriguing prediction - that the seemingly
simple metals Li and Na will transform under pressure into insulating states,
owing to pairing of alkali atoms - has yet to be experimentally confirmed. Here
we report experimental observations of a pressure-induced transformation of Na
into an optically transparent phase at 200 GPa (corresponding to 5.0-fold
compression). Experimental and computational data identify the new phase as a
wide bandgap dielectric with a six-coordinated, highly distorted
double-hexagonal close-packed structure. We attribute the emergence of this
dense insulating state not to atom pairing, but to p-d hybridizations of
valence electrons and their repulsion by core electrons into the lattice
interstices. We expect that such insulating states may also form in other
elements and compounds when compression is sufficiently strong that atomic
cores start to overlap strongly.Comment: Published in Nature 458, 182-185 (2009
Spatially-resolved electronic and vibronic properties of single diamondoid molecules
Diamondoids are a unique form of carbon nanostructure best described as
hydrogen-terminated diamond molecules. Their diamond-cage structures and
tetrahedral sp3 hybrid bonding create new possibilities for tuning electronic
band gaps, optical properties, thermal transport, and mechanical strength at
the nanoscale. The recently-discovered higher diamondoids (each containing more
than three diamond cells) have thus generated much excitement in regards to
their potential versatility as nanoscale devices. Despite this excitement,
however, very little is known about the properties of isolated diamondoids on
metal surfaces, a very relevant system for molecular electronics. Here we
report the first molecular scale study of individual tetramantane diamondoids
on Au(111) using scanning tunneling microscopy and spectroscopy. We find that
both the diamondoid electronic structure and electron-vibrational coupling
exhibit unique spatial distributions characterized by pronounced line nodes
across the molecular surfaces. Ab-initio pseudopotential density functional
calculations reveal that the observed dominant electronic and vibronic
properties of diamondoids are determined by surface hydrogen terminations, a
feature having important implications for designing diamondoid-based molecular
devices.Comment: 16 pages, 4 figures. to appear in Nature Material
Nordic mcl2-3 trials: mirna-18b overexpression identifies a mantle cell lymphoma subgroup with poor survival and improves mipi-b prediction of prognosis
Nuclear Magnetic Resonance Study of Ultrananocrystalline Diamonds
We report on a nuclear magnetic resonance (NMR) study of ultrananocrystalline
diamond (UNCD) materials produced by detonation technique. Analysis of the 13C
and 1H NMR spectra, spin-spin and spin-lattice relaxation times in purified
UNCD samples is presented. Our measurements show that UNCD particles consist of
a diamond core that is partially covered by a sp2-carbon fullerene-like shell.
The uncovered part of outer diamond surface comprises a number of hydrocarbon
groups that saturate the dangling bonds. Our findings are discussed along with
recent calculations of the UNCD structure. Significant increase in the
spin-lattice relaxation rate (in comparison with that of natural diamond), as
well as stretched exponential character of the magnetization recovery, are
attributed to the interaction of nuclear spins with paramagnetic centers which
are likely fabrication-driven dangling bonds with unpaired electrons. We show
that these centers are located mainly at the interface between the diamond core
and shell.Comment: 25 pages, 7 figure
Assessing the benefits and usefulness of Schwartz Centre Rounds in Second-Year Medical Students using Clinical Educator-Facilitated Group Work Session: not just “A Facilitated Moan”!
Background
An experiential curriculum exposing medical students to the clinic early has many benefits but comes with the emotional stress this environment engenders. Schwartz rounds (SR) are an effective means to combat emotional stress and increasingly used in UK and USA hospitals. Recent studies show that the SR format may also provide benefits for medical students. This study aimed to investigate whether the guidance of SR in second year medical students provides the same benefits as to healthcare professionals.
Methods
SR assessment involved 83 second year MBChB students in facilitated groupwork sessions. Topics discussed were “change and resilience” and “duty of candour”. Students completed a Likert Scale questionnaire evaluating outcomes proffered by the Point of Care Foundation in collaboration with the Schwartz Foundation, with freeform feedback.
Results
There was an 86% completion rate with 25% providing written feedback. Participants were more likely to agree than disagree that SR were beneficial. SR effectiveness in enhancing students’ working relationship awareness and skills was strongly correlated with understanding the purpose of, and engagement with, the SR (P<0.001). Similarly, engagement with the SR was strongly correlated with self-reporting of enhanced patient-centredness (P < 0.001). Freeform feedback could be grouped into five themes that revolved around understanding of the SR and engagement with the process. Many positive comments regarded the SR as a forum not only to “learn experientially” but to so in a “safe environment”. Many negative comments stemmed from students not seeing any benefits of engagement with the SR, in that sharing experiences was “unbeneficial”, “empathy is inherent and not learnt”, or that sharing emotional problems is simply “moaning”.
Conclusion
SRs are an effective way of fostering empathy and understanding towards patients and colleagues. However, for the students to benefit fully from the SR it is necessary for them to engage and understand the process. Therefore, for the successful implementation of SR into pre-clinical medical education, it is important to help students realise that SR are not merely a “facilitated whinge”
- …