659 research outputs found
Mapping of mutants resistant to p-fluorophenylalanine in diploid Aspergillus nidulans, lethal in haploids
In a previous paper (Babudri and Morpurgo 1990 Curr. Genet. 17:519-522) we described a new class of para-fluorophenylalanine (FPA) resistant mutants in Aspergillus nidulans. These mutants were obtained by plating UV irradiated diploid conidia on minimal medium (MM) supplemented with FPA (0.188 mg/ml)
Nanometer-spaced platinum electrodes with calibrated separation
We have fabricated pairs of platinum electrodes with separation between 20
and 3.5 nm. Our technique combines electron beam lithography and chemical
electrodeposition. We show that the measurement of the conductance between the
two electrodes through the electrolyte provides an accurate and reproducible
way to control their separation. We have tested the robustness of the
electrodes by applying large voltages across them and by using them to measure
the transport properties of Au nano-clusters. Our results show that the
technique reliably produces metallic electrodes with a separation that bridges
the minimum scale accessible by electron beam lithography with the atomic
scale.Comment: 4 pages, 4 figure
Marginal topological properties of graphene: a comparison with topological insulators
The electronic structures of graphene systems and topological insulators have
closely-related features, such as quantized Berry phase and zero-energy edge
states. The reason for these analogies is that in both systems there are two
relevant orbital bands, which generate the pseudo-spin degree of freedom, and,
less obviously, there is a correspondence between the valley degree of freedom
in graphene and electron spin in topological insulators. Despite the
similarities, there are also several important distinctions, both for the bulk
topological properties and for their implications for the edge states --
primarily due to the fundamental difference between valley and spin. In view of
their peculiar band structure features, gapped graphene systems should be
properly characterized as marginal topological insulators, distinct from either
the trivial insulators or the true topological insulators.Comment: This manuscript will be published on the Proceedings of the 2010
Nobel Symposium on Graphene and Quantum Matte
Aharonov-Bohm effect and broken valley-degeneracy in graphene rings
We analyze theoretically the electronic properties of Aharonov-Bohm rings
made of graphene. We show that the combined effect of the ring confinement and
applied magnetic flux offers a controllable way to lift the orbital degeneracy
originating from the two valleys, even in the absence of intervalley
scattering. The phenomenon has observable consequences on the persistent
current circulating around the closed graphene ring, as well as on the ring
conductance. We explicitly confirm this prediction analytically for a circular
ring with a smooth boundary modelled by a space-dependent mass term in the
Dirac equation. This model describes rings with zero or weak intervalley
scattering so that the valley isospin is a good quantum number. The tunable
breaking of the valley degeneracy by the flux allows for the controlled
manipulation of valley isospins. We compare our analytical model to another
type of ring with strong intervalley scattering. For the latter case, we study
a ring of hexagonal form with lattice-terminated zigzag edges numerically. We
find for the hexagonal ring that the orbital degeneracy can still be controlled
via the flux, similar to the ring with the mass confinement.Comment: 7 pages, 7 figures, replaced with considerably extended new versio
Current saturation and Coulomb interactions in organic single-crystal transistors
Electronic transport through rubrene single-crystal field effect transistors
(FETs) is investigated experimentally in the high carrier density regime (n ~
0.1 carrier/molecule). In this regime, we find that the current does not
increase linearly with the density of charge carriers, and tends to saturate.
At the same time, the activation energy for transport unexpectedly increases
with increasing n. We perform a theoretical analysis in terms of a well-defined
microscopic model for interacting Frohlich polarons, that quantitatively
accounts for our experimental observations. This work is particularly
significant for our understanding of electronic transport through organic FETs.Comment: Extended version with 1 additional figure and an appendix explaining
the consistency of the theoretical calculatio
Quantitative analysis of electronic transport through weakly-coupled metal/organic interfaces
Using single-crystal transistors, we have performed a systematic experimental
study of electronic transport through oxidized copper/rubrene interfaces as a
function of temperature and bias. We find that the measurements can be
reproduced quantitatively in terms of the thermionic emission theory for
Schottky diodes, if the effect of the bias-induced barrier lowering is
included. Our analysis emphasizes the role of the coupling between metal and
molecules, which in our devices is weak due to the presence of an oxide layer
at the surface of the copper electrodes.Comment: 4 pages, 3 figure
Inferring biochemical reaction pathways: the case of the gemcitabine pharmacokinetics.
Background
The representation of a biochemical system as a network is the precursor of any mathematical model of the processes driving the dynamics of that system. Pharmacokinetics uses mathematical models to describe the interactions between drug, and drug metabolites and targets and through the simulation of these models predicts drug levels and/or dynamic behaviors of drug entities in the body. Therefore, the development of computational techniques for inferring the interaction network of the drug entities and its kinetic parameters from observational data is raising great interest in the scientic community of pharmacologists. In fact, the network inference is a set of mathematical procedures deducing the structure of a model from the experimental data associated to the nodes of the network of interactions. In this paper, we deal with the inference of a pharmacokinetic network from the concentrations of the drug and its metabolites observed at discrete time points.
Results
The method of network inference presented in this paper is inspired by the theory of time-lagged correlation inference with regard to the deduction of the interaction network, and on a maximum likelihood approach with regard to the estimation of the kinetic parameters of the network. Both network inference and parameter estimation have been designed specically to identify systems of biotransformations, at the biochemical level, from noisy time-resolved experimental data. We use our inference method to deduce the metabolic pathway of the gemcitabine. The inputs to our inference algorithm are the experimental time series of the concentration of gemcitabine and its metabolites. The output is the set of reactions of the metabolic network of the gemcitabine.
Conclusions
Time-lagged correlation based inference pairs up to a probabilistic model of parameter inference from metabolites time series allows the identication of the microscopic pharmacokinetics and pharmacodynamics of a drug with a minimal a priori knowledge. In fact, the inference model presented in this paper is completely unsupervised. It takes as input the time series of the concetrations of the parent drug and its metabolites. The method, applied to the case study of the gemcitabine pharmacokinetics, shows good accuracy and sensitivit
Shot Noise in Ballistic Graphene
We have investigated shot noise in graphene field effect devices in the
temperature range of 4.2--30 K at low frequency ( = 600--850 MHz). We find
that for our graphene samples with large width over length ratio , the
Fano factor reaches a maximum 1/3 at the
Dirac point and that it decreases strongly with increasing charge density. For
smaller , the Fano factor at Dirac point is significantly lower. Our
results are in good agreement with the theory describing that transport at the
Dirac point in clean graphene arises from evanescent electronic states.Comment: Phys. Rev. Lett. 100, 196802 (2008
Reflectionless tunneling in ballistic normal-metal--superconductor junctions
We investigate the phenomenon of reflectionless tunneling in ballistic
normal-metal--superconductor (NS) structures, using a semiclassical formalism.
It is shown that applied magnetic field and superconducting phase difference
both impair the constructive interference leading to this effect, but in a
qualitatively different way. This is manifested both in the conductance and in
the shot noise properties of the system considered. Unlike diffusive systems,
the features of the conductance are sharp, and enable fine spatial control of
the current, as well as single channel manipulations. We discuss the
possibility of conducting experiments in ballistic semiconductor-superconductor
structures with smooth interfaces and some of the phenomena, specific to such
structures, that could be measured. A general criterion for the barrier at NS
interfaces, though large, to be effectively transparent to pair current is
obtained.Comment: published versio
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