1,600 research outputs found
Thermal rectification in carbon nanotube intramolecular junctions: Molecular dynamics calculations
We study heat conduction in (n, 0)/(2n, 0) intramolecular junctions by using
molecular dynamics method. It is found that the heat conduction is asymmetric,
namely, heat transports preferably in one direction. This phenomenon is also
called thermal rectification. The rectification is weakly dependent on the
detailed structure of connection part, but is strongly dependent on the
temperature gradient. We also study the effect of the tube radius and
intramolecular junction length on the rectification. Our study shows that the
tensile stress can increase rectification. The physical mechanism of the
rectification is explained
The Attitudes and Behaviors of Current Medical Students towards Business Education and Training
Medical students often look into owning their own practice as an option for their career. However, many of them do not acquire the proper training for owning and operating a business, and ultimately end up making costly mistakes. Thus, the purpose of this study is to gauge how medical students perceive the importance of business education and training. Based on survey data from 137 current medical students from across the United States, our study found that the vast majority of medical students believe that business education and training is valuable. However, today’s MD/MBA programs do not appear to meet the current needs of medical students, as a broad MBA degree may not be the best fit for most medical students
Electrical conductivity measured in atomic carbon chains
The first electrical conductivity measurements of monoatomic carbon chains
are reported in this study. The chains were obtained by unraveling carbon atoms
from graphene ribbons while an electrical current flowed through the ribbon
and, successively, through the chain. The formation of the chains was
accompanied by a characteristic drop in the electrical conductivity. The
conductivity of carbon chains was much lower than previously predicted for
ideal chains. First-principles calculations using both density functional and
many-body perturbation theory show that strain in the chains determines the
conductivity in a decisive way. Indeed, carbon chains are always under varying
non-zero strain that transforms its atomic structure from cumulene to polyyne
configuration, thus inducing a tunable band gap. The modified electronic
structure and the characteristics of the contact to the graphitic periphery
explain the low conductivity of the locally constrained carbon chain.Comment: 21 pages, 9 figure
Probing the electron-phonon coupling in ozone-doped graphene by Raman spectroscopy
We have investigated the effects of ozone treatment on graphene by Raman
scattering. Sequential ozone short-exposure cycles resulted in increasing the
doping levels as inferred from the blue shift of the 2 and peak
frequencies, without introducing significant disorder. The two-phonon 2 and
2 Raman peak intensities show a significant decrease, while, on the
contrary, the one-phonon G Raman peak intensity remains constant for the whole
exposure process. The former reflects the dynamics of the photoexcited
electrons (holes) and, specifically, the increase of the electron-electron
scattering rate with doping. From the ratio of 2 to 2 intensities, which
remains constant with doping, we could extract the ratio of electron-phonon
coupling parameters. This ratio is found independent on the number of layers up
to ten layers. Moreover, the rate of decrease of 2 and 2 intensities
with doping was found to slowdown inversely proportional to the number of
graphene layers, revealing the increase of the electron-electron collision
probability
Test of a Jastrow-type wavefunction for a trapped few-body system in one dimension
For a system with interacting quantum mechanical particles in a
one-dimensional harmonic oscillator, a trial wavefunction with simple structure
based on the solution of the corresponding two-particle system is suggested and
tested numerically. With the inclusion of a scaling parameter for the distance
between particles, at least for the very small systems tested here the ansatz
gives a very good estimate of the ground state energy, with the error being of
the order of ~1% of the gap to the first excited state
Climate variability of Southern Chile since the Last Glacial Maximum: a continuous sedimentological record from Lago Puyehue (40°S)
A key region to study high resolution climate changes of the Southern Hemisphere is undoubtedly the southern part of Chile because it has the advantage to be far removed from the Northern Hemisphere ice sheets and thermohaline circulation influences. In order to reconstruct the regional climate evolution since the Last Glacial Maximum, we investigated the sedimentary infilling of Lago Puyehue (40°S, 164 km2, elevation 185 m) by a multi-proxy analyse of a 11 m long core. Sediments from this core are transported by interflow currents and are made of finely laminated silts, with only small disturbances due to volcanic and seismic activities. Several proxies were measured: grain-size, mineralogy, magnetic susceptibility, major elements geochemistry and biogenic silica concentration. These are used to reconstruct paleo-precipitation and paleo-productivity changes around 40°S. Results evidence that sediment grainsize is highly correlated with the biogenic sediment content and can be used as a proxy for lake paleoproductivity. On the other hand, the magnetic susceptibility signal is highly correlated with the aluminium and titanium concentrations and can be used as a proxy for the terrigenous supply. Temporal variations of sediment composition demonstrate that, since the Last Glacial Maximum, the Chilean Lake District was characterized by 3 abrupt climate changes superimposed on a long term climate evolution. These rapid climate changes are: (1) the end of the Last Glacial Maximum at 17,300 cal. yr. BP; (2) a 13,100-12,300 cal. yr. BP cold event, ending rapidly and interpreted as the local counter part of the European Younger Dryas event, and (3) a 3400-2900 cal. yr. BP climatic instability related to low solar activity. The timing of the 13,100-12,300 cold event is compared with similar records in both hemispheres and demonstrates that this Southern Hemisphere climate change lags behind the Northern Hemisphere Younger Dryas cold period by 500 to 1000 years
Strontium Stable Isotope Composition of Allende Fine-Grained Inclusions
Isotopic anomalies are departures from the laws of mass-dependent fractionation that cannot be explained by radioactive decay, cosmogenic effects, or exotic isotopic fractionation processes such as nuclear field shift or magnetic effects [1 and references therein]. These anomalies often have a nucleosynthetic origin and provide clues on the stellar origin and solar system processing of presolar dust. Anomalies are most often found in refractory elements of relatively low mass, so Sr is a prime target for study. The four stable isotopes of strontium are useful for discerning the various nucleosynthetic origins of early
solar system building blocks and the timing of
accretion processes. Strontium-84 is the least abundant
(0.56%) of these isotopes, but is particularly significant
in being a p-process only nuclide that is produced in
core-collapse or type Ia supernovae [2,3]. The more
abundant isotopes ^(86)Sr (9.86%), ^(87)Sr (7.00%) and ^(88)Sr (82.58%) are produced in s- and r-processes in
asymptotic giant branch stars and other stellar types
[4]. Additionally, ^(87)Sr is produced by ^(87)Rb decay in
proportions that dominate over possible nucleosynthetic variations but provide timings of early solar system processes, most notably volatile element depletion [5-7]. Furthermore, variations in strontium isotopic ratios caused by high-temperature massdependent fractionation [8] are also important [9-12], as they provide insights into nebular and accretionary processes
Three-node zero-thickness hydro-mechanical interface finite element for geotechnical applications
The paper presents the main features of a hydro-mechanical coupled finite
element of interface. The mechanical problem accounts for the detection of contact,
the development of contact pressure, shearing and relative sliding between two solids.
A three-node discretisation of hydraulic problem allows the representation of fluid flow
across and in the plane of the interface. The method involves a drop of pressure between
each side of the interface and the inner medium. Hydro-mechanical couplings result from
1) the definition of the total pressure acting on each side of the interface according to
the Terzaghi’s principle; 2) the dependence of permeability on the gap opening; 3) the
variation of the fluid mass stored within the gap
A generic framework for context-sensitive analysis of modular programs
Context-sensitive analysis provides information which is potentially more accurate than that provided by context-free analysis. Such information can then be applied in order to validate/debug the program and/or to specialize the program obtaining important improvements. Unfortunately, context-sensitive analysis of modular programs poses important theoretical and practical problems. One solution, used in several proposals, is to resort to context-free analysis. Other proposals do address
context-sensitive analysis, but are only applicable when the description domain used satisfies rather restrictive properties. In this paper, we argüe that a general framework for context-sensitive analysis of modular programs, Le., one that allows using all the domains which have proved useful in practice in the non-modular setting, is indeed feasible and very useful. Driven by our experience in the design and implementation of analysis and specialization techniques in the context of CiaoPP, the Ciao
system preprocessor, in this paper we discuss a number of design goals for context-sensitive analysis of modular programs as well as the problems which arise in trying to meet these goals. We also provide a high-level description of a framework for analysis of modular programs which does
substantially meet these objectives. This framework is generic in that it can be instantiated in different ways in order to adapt to different contexts. Finally, the behavior of the different instantiations w.r.t. the design goals that motivate our work is also discussed
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