178 research outputs found
Living without Oxygen: Anoxia-Responsive Gene Expression and Regulation
Many species of marine mollusks demonstrate exceptional capacities for long term survival without oxygen. Analysis of gene expression under anoxic conditions, including the subsequent translational responses, allows examination of the functional mechanisms that support and regulate natural anaerobiosis and permit noninjurious transitions between aerobic and anoxic states. Identification of stress-specific gene expression can provide important insights into the metabolic adaptations that are needed for anoxia tolerance, with potential applications to anoxia-intolerant systems. Various methods are available to do this, including high throughput microarray screening and construction and screening of cDNA libraries. Anoxia-responsive genes have been identified in mollusks; some have known functions in other organisms but were not previously linked with anoxia survival. In other cases, completely novel anoxia-responsive genes have been discovered, some that show known motifs or domains that hint at function. Selected genes are expressed at different times over an anoxia-recovery time course with their transcription and translation being actively regulated to ensure protein expression at the optimal time. An examination of transcript status over the course of anoxia exposure and subsequent aerobic recovery identifies genes, and the proteins that they encode, that enhance cell survival under oxygen-limited conditions. Analysis of data generated from non-mainstream model systems allows for insight into the response by cells to anoxia stress
I-V characteristics and differential conductance fluctuations of Au nanowires
Electronic transport properties of Au nano-structure are investigated using
both experimental and theoretical analysis. Experimentally, stable Au nanowires
were created using mechanically controllable break junction in air, and
simultaneous current-voltage (I-V) and differential conductance data were measured. The atomic device scale structures are
mechanically very stable up to bias voltage and have a life time
of a few . Facilitated by a shape function data analysis technique
which finger-prints electronic properties of the atomic device, our data show
clearly differential conductance fluctuations with an amplitude at room
temperature, and a nonlinear I-V characteristics. To understand the transport
features of these atomic scale conductors, we carried out {\it ab initio}
calculations on various Au atomic wires. The theoretical results demonstrate
that transport properties of these systems crucially depend on the electronic
properties of the scattering region, the leads, and most importantly the
interaction of the scattering region with the leads. For ideal, clean Au
contacts, the theoretical results indicate a linear I-V behavior for bias
voltage . When sulfur impurities exist at the contact junction,
nonlinear I-V curves emerge due to a tunnelling barrier established in the
presence of the S atom. The most striking observation is that even a single S
atom can cause a qualitative change of the I-V curve from linear to nonlinear.
A quantitatively favorable comparison between experimental data and theoretical
results is obtained. We also report other results concerning quantum transport
through Au atomic contacts.Comment: 11 pages and 9 figures, submitted to PR
De ChĂ©ticamp Ă Kâjipuktuk : learning and living in translation
1 online resource (iii, 20 pages)Includes abstract in English and French.Includes bibliographical references (page 20).The Third Space, a term coined by postcolonial critic Homi K. Bhabha, is a space in which binary conceptions of identity cease to exist. For a bilingual person, this space therefore offers the possibility of a hybrid conception of language and, consequently, of identity. Instead of demanding that a choice be made between English and French, for example, the Third Space makes it possible to embrace these two linguistic identities simultaneously. In this vein, Halifax is a Third Space which allows me to experience the triple paradox that is the anglo-franco-acadien and the corresponding aspects of culture and identity. My experiences as an Acadian, francophone and anglophone have profoundly influenced my conceptualization of language and, consequently, my translation practices. This led to the development of a feminist ethics of translation during the translation of the intercultural studies textbook. Like Bhabhaâs Third Space, my translation rejected all binary conceptions of gender to instead use neutral terms. Some of these words and expressions are not often used in the target language, but it is only by using new innovative terminology that language can evolve and be more inclusive
Current rectification by simple molecular quantum dots: an ab-initio study
We calculate a current rectification by molecules containing a conjugated
molecular group sandwiched between two saturated (insulating) molecular groups
of different length (molecular quantum dot) using an ab-initio non-equilibrium
Green's function method. In particular, we study S-(CH2)m-C10H6-(CH2)n-S
dithiol with Naphthalene as a conjugated central group. The rectification
current ratio ~35 has been observed at m = 2 and n = 10, due to resonant
tunneling through the molecular orbital (MO) closest to the electrode Fermi
level (lowest unoccupied MO in the present case). The rectification is limited
by interference of other conducting orbitals, but can be improved by e.g.
adding an electron withdrawing group to the naphthalene.Comment: 8 pages, 9 figure
First-principles investigation of spin polarized conductance in atomic carbon wire
We analyze spin-dependent energetics and conductance for one dimensional (1D)
atomic carbon wires consisting of terminal magnetic (Co) and interior
nonmagnetic (C) atoms sandwiched between gold electrodes, obtained employing
first-principles gradient corrected density functional theory and Landauer's
formalism for conductance. Wires containing an even number of interior carbon
atoms are found to be acetylenic with sigma-pi bonding patterns, while cumulene
structures are seen in wires containing odd number of interior carbon atoms, as
a result of strong pi-conjugation. Ground states of carbon wires containing up
to 13 C atoms are found to have anti-parallel spin configurations of the two
terminal Co atoms, while the 14 C wire has a parallel Co spin configuration in
the ground state. The stability of the anti-ferromagnetic state in the wires is
ascribed to a super-exchange effect. For the cumulenic wires this effect is
constant for all wire lengths. For the acetylenic wires, the super-exchange
effect diminishes as the wire length increases, going to zero for the atomic
wire containing 14 carbon atoms. Conductance calculations at the zero bias
limit show spin-valve behavior, with the parallel Co spin configuration state
giving higher conductance than the corresponding anti-parallel state, and a
non-monotonic variation of conductance with the length of the wires for both
spin configurations.Comment: revtex, 6 pages, 5 figure
Ballistic conductance of Ni nanowire with a magnetization reversal
The approach proposed by Choi and Ihm for calculating the ballistic
conductance of open quantum systems is generalized to deal with magnetic
transition metals. The method has been implemented with ultrasoft
pseudopotentials and plane wave basis set in a DFT-LSDA ab-initio scheme. We
present the quantum-mechanical conductance calculations for monatomic Ni
nanowire with a single spin reversal. We find that a spin reversal blocks the
conductance of electrons at the Fermi energy of the Ni nanowire. On the
other hand, two electrons (one per each spin) are perfectly transmitted in
the whole energy window giving for the total conductance. The relevance
of these results in connection with recent experimental data is discussed.Comment: 4 pages, 1 figure, to be published in Surface Scienc
Effect of impurities on transport through organic molecular films from first principles
We calculate electron transport through molecular monolayers of saturated
alkanes with point defects from first principles. Single defects (incorporated
Au ions, kinks, dangling bonds) produce deep localized levels in the gap
between occupied and unoccupied molecular levels. Single defects produce steps
on the I-V curve, whereas pairs of (unlike and like) defects give negative
differential resistance peaks. The results are discussed in relation to the
observed unusual transport behavior of organic monolayers and compared with
transport through conjugated polythiophenes.Comment: 5 pages, 2 figures. Small typos corrected and text slightly
shortened. To appear in Phys. Rev.
First-Principles Analysis of Molecular Conduction Using Quantum Chemistry Software
We present a rigorous and computationally efficient method to do a
parameter-free analysis of molecular wires connected to contacts. The
self-consistent field approach is coupled with Non-equilibrium Green's Function
(NEGF) formalism to describe electronic transport under an applied bias.
Standard quantum chemistry software is used to calculate the self-consistent
field using density functional theory (DFT). Such close coupling to standard
quantum chemistry software not only makes the procedure simple to implement but
also makes the relation between the I-V characteristics and the chemistry of
the molecule more obvious. We use our method to interpolate between two extreme
examples of transport through a molecular wire connected to gold (111)
contacts: band conduction in a metallic (gold) nanowire, and resonant
conduction through broadened, quasidiscrete levels of a phenyl dithiol
molecule. We obtain several quantities of interest like I-V characteristic,
electron density and voltage drop along the molecule.Comment: Accepted for publication in J. Chem. Phys. (Special issue on
molecular electronics, Ed. Mark Ratner
First-Principles Based Matrix-Green's Function Approach to Molecular Electronic Devices: General Formalism
Transport in molecular electronic devices is different from that in
semiconductor mesoscopic devices in two important aspects: (1) the effect of
the electronic structure and (2) the effect of the interface to the external
contact. A rigorous treatment of molecular electronic devices will require the
inclusion of these effects in the context of an open system exchanging particle
and energy with the external environment. This calls for combining the theory
of quantum transport with the theory of electronic structure starting from the
first-principles. We present a rigorous yet tractable matrix Green's function
approach for studying transport in molecular electronic devices, based on the
Non-Equilibrium Green's Function Formalism of quantum transport and the
density-functional theory of electronic structure using local orbital basis
sets. By separating the device rigorously into the molecular region and the
contact region, we can take full advantage of the natural spatial locality
associated with the metallic screening in the electrodes and focus on the
physical processes in the finite molecular region. This not only opens up the
possibility of using the existing well-established technique of molecular
electronic structure theory in transport calculations with little change, but
also allows us to use the language of qualitative molecular orbital theory to
interpret and rationalize the results of the computation. For the device at
equilibrium, our method provides an alternative approach for solving the
molecular chemisorption problem. For the device out of equilibrium, we show
that the calculation of elastic current transport through molecules, both
conceptually and computationally, is no more difficult than solving the
chemisorption problem.Comment: To appear in Chemical Physic
- âŠ