Quantum Hall effect in exfoliated graphene affected by charged impurities: metrological measurements

Metrological investigations of the quantum Hall effect (QHE) completed by transport measurements at low magnetic field are carried out in a-few-$\mu\mathrm{m}$-wide Hall bars made of monolayer (ML) or bilayer (BL) exfoliated graphene transferred on $\textrm{Si/SiO}_{2}$ substrate. From the charge carrier density dependence of the conductivity and from the measurement of the quantum corrections at low magnetic field, we deduce that transport properties in these devices are mainly governed by the Coulomb interaction of carriers with a large concentration of charged impurities. In the QHE regime, at high magnetic field and low temperature ($T<1.3 \textrm{K}$), the Hall resistance is measured by comparison with a GaAs based quantum resistance standard using a cryogenic current comparator. In the low dissipation limit, it is found quantized within 5 parts in $10^{7}$ (one standard deviation, $1 \sigma$) at the expected rational fractions of the von Klitzing constant, respectively $R_{\mathrm{K}}/2$ and $R_{\mathrm{K}}/4$ in the ML and BL devices. These results constitute the most accurate QHE quantization tests to date in monolayer and bilayer exfoliated graphene. It turns out that a main limitation to the quantization accuracy, which is found well above the $10^{-9}$ accuracy usually achieved in GaAs, is the low value of the QHE breakdown current being no more than $1 \mu\mathrm{A}$. The current dependence of the longitudinal conductivity investigated in the BL Hall bar shows that dissipation occurs through quasi-elastic inter-Landau level scattering, assisted by large local electric fields. We propose that charged impurities are responsible for an enhancement of such inter-Landau level transition rate and cause small breakdown currents.Comment: 14 pages, 9 figure

The Role of GABA Signalling in Lung Macrophage Immune Response

Lung macrophages (LMϕs) play a key role in pulmonary innate immunity. They polarize into different phenotypes adapting to the needs of the immediate pulmonary environment. Studies in our laboratory suggest that murine LMϕs are endowed with an autocrine gamma-aminobutyric acid (GABA) signaling system. My honors thesis study found that antagonizing the autocrine GABA signaling in alveolar macrophages (AMϕs) increased secretion of the M1 cytokine tumor necrosis factor-alpha (TNF-α), suggesting a role for GABA signaling in immune response. This project explored whether GABA signaling plays a role in LMϕ polarization. Results from this study confirmed that bacterial toxin lipopolysaccharide (LPS) and the Th1 cytokine interferon gamma (IFNγ) shifted LMϕs to the pro-inflammatory M1 phenotype, marked by increased expression of inducible nitric oxide synthase (iNOS). On the other hand, the Th2 cytokines interleukin (IL)-4 and IL-13 shifted LMϕs toward the M2 phenotype marked by increased arginase-1. Importantly, in both RAW 264.7 cell line and primary LMϕs, LPS and IFNγ treatment increased iNOS expression while decreasing glutamic acid decarboxylase (GAD) and A-type GABA receptor α2-subunit (α2-GABAAR). Conversely, treatment with IL4/13 induced an upregulation of arginase-1, GAD, and α2-GABAAR. Moreover, treatment of primary LMϕs with IL4/13 and GABAAR antagonist picrotoxin decreased arginase-1 and GAD expression, and increased iNOS levels. These results suggest the autocrine GABA signaling system in LMϕs dynamically changes with their phenotypic polarization. This signaling system functions to limit the M1 response, but facilitate M2 reaction, and thus a change in the GABA signaling may alter the inflammatory responses of these cells

Practical quantum realization of the ampere from the electron charge

One major change of the future revision of the International System of Units (SI) is a new definition of the ampere based on the elementary charge \emph{e}. Replacing the former definition based on Amp\`ere's force law will allow one to fully benefit from quantum physics to realize the ampere. However, a quantum realization of the ampere from \emph{e}, accurate to within $10^{-8}$ in relative value and fulfilling traceability needs, is still missing despite many efforts have been spent for the development of single-electron tunneling devices. Starting again with Ohm's law, applied here in a quantum circuit combining the quantum Hall resistance and Josephson voltage standards with a superconducting cryogenic amplifier, we report on a practical and universal programmable quantum current generator. We demonstrate that currents generated in the milliampere range are quantized in terms of $ef_\mathrm{J}$ ($f_\mathrm{J}$ is the Josephson frequency) with a measurement uncertainty of $10^{-8}$. This new quantum current source, able to deliver such accurate currents down to the microampere range, can greatly improve the current measurement traceability, as demonstrated with the calibrations of digital ammeters. Beyond, it opens the way to further developments in metrology and in fundamental physics, such as a quantum multimeter or new accurate comparisons to single electron pumps.Comment: 15 pages, 4 figure

How does the chromatin fiber deal with topological constraints?

In the nuclei of eukaryotic cells, DNA is packaged through several levels of compaction in an orderly retrievable way that enables the correct regulation of gene expression. The functional dynamics of this assembly involves the unwinding of the so-called 30 nm chromatin fiber and accordingly imposes strong topological constraints. We present a general method for computing both the twist and the writhe of any winding pattern. An explicit derivation is implemented for the chromatin fiber which provides the linking number of DNA in eukaryotic chromosomes. We show that there exists one and only one unwinding path which satisfies both topological and mechanical constraints that DNA has to deal with during condensation/decondensation processes.Comment: Presented in Nature "News and views in brief" Vol. 429 (13 May 2004). Movies available at http://www.lptl.jussieu.fr/recherche/operationE_fichiers/Page_figurePRL.htm