Quantum resistance metrology in graphene

We have performed a metrological characterization of the quantum Hall resistance in a 1 $\mu$m wide graphene Hall-bar. The longitudinal resistivity in the center of the $\nu=\pm 2$ quantum Hall plateaus vanishes within the measurement noise of 20 m$\Omega$ upto 2 $\mu$A. Our results show that the quantization of these plateaus is within the experimental uncertainty (15 ppm for 1.5$\mu$A current) equal to that in conventional semiconductors. The principal limitation of the present experiments are the relatively high contact resistances in the quantum Hall regime, leading to a significantly increased noise across the voltage contacts and a heating of the sample when a high current is applied

Import of cytochrome c into mitochondria

The import of cytochrome c into mitochondria can be resolved into a number of discrete steps. Here we report on the covalent attachment of heme to apocytochrome c by the enzyme cytochrome c heme lyase in mitochondria from Neurospora crassa. A new method was developed to measure directly the linkage of heme to apocytochrome c. This method is independent of conformational changes in the protein accompanying heme attachment. Tryptic peptides of [35S]cysteine-labelled apocytochrome c, and of enzymatically formed holocytochrome c, were resolved by reverse-phase HPLC. The cysteine-containing peptide to which heme was attached eluted later than the corresponding peptide from apocytochrome c and could be quantified by counting 35S radioactivity as a measure of holocytochrome c formation. Using this procedure, the covalent attachment of heme to apocytochrome c, which is dependent on the enzyme cytochrome c heme lyase, could be measured. Activity required heme (as hemin) and could be reversibly inhibited by the analogue deuterohemin. Holocytochrome c formation was stimulated 5–10-fold by NADH > NADPH > glutathione and was independent of a potential across the inner mitochondrial membrane. NADH was not required for the binding of apocytochrome c to mitochondria and was not involved in the reduction of the cysteine thiols prior to heme attachment. Holocytochrome c formation was also dependent on a cytosolic factor that was necessary for the heme attaching step of cytochrome c import. The factor was a heat-stable, protease-insensitive, low-molecular-mass component of unknown function. Cytochrome c heme lyase appeared to be a soluble protein located in the mitochondrial intermembrane space and was distinct from the previously identified apocytochrome c binding protein having a similar location. A model is presented in which the covalent attachment of heme by cytochrome c heme lyase also plays an essential role in the import pathway of cytochrome c

Immobilized photocatalyst on stainless steel woven meshes assuring efficient light distribution in a solar reactor

An immobilized TiO₂ photocatalyst with a high specific surface area was prepared on stainless steel woven meshes in order to be used packed in layers for water purification. Immobilization of such a complex shape needs a special coating technique. For this purpose, dip coating and electrophoretic deposition (EPD) techniques were used. The EPD technique gave the TiO₂ coating films a better homogeneity and adhesion, fewer cracks, and a higher ^·OH formation than the dip coating technique. The woven mesh structure packed in layers guaranteed an efficient light-penetration in water treatment reactor. A simple equation model was used to describe the distribution of light through the mesh layers in the presence of absorbing medium (e.g., colored water with humic acids). Maximum three or four coated meshes were enough to harvest the solar UV light from 300 nm to 400 nm with a high penetration efficiency. The separation distance between the mesh layers played an important role in the efficiency of solar light penetration through the coated mesh layers, especially in case of colored water contaminated with high concentrations of humic acid

Accurate measurements of small currents using a CCC with DC SQUID read out

The continuous miniaturisation in the semiconductor industry increases the importance of accurate measurement and control of very small currents. In the field of precision electrical measurements, cryogenic current comparators (CCCs) are used as extremely sensitive and accurate instruments for scaling currents. In an international co-operation project, a special CCC system is being developed, optimised for the measurement of extremely small currents to less than 1 pA (10−12 A). DC superconducting quantum interference device (SQUID) readout is used for monitoring the ampere-turn unbalance of the CCC. In this paper, we report on the accurate determination of the CCC and SQUID input coil inductances. Matching of these inductances is required for obtaining ultimate current resolution with the CCC. We find very good agreement between measurements of the CCC inductance at room temperature and at 4.2 K, and results from numerical calculations. The measured values for the input and mutual inductance of the SQUID sensor are in good agreement with the design values when the effect of the slit in the SQUID washer is correctly taken into account. Final current resolution of our measurement system is expected to be better than 1×10−15 A/√Hz for a CCC with 20,000 primary windings

Risk of preterm birth after prior term cesarean

Peer reviewedPublisher PD

Phenomena induced by powerful HF pumping towards magnetic zenith with a frequency near the F-region critical frequency and the third electron gyro harmonic frequency

Multi-instrument observational data from an experiment on 13 October 2006 at the EISCAT/HEATING facility at Tromsø, Norway are analysed. The experiment was carried out in the evening hours when the electron density in the F-region dropped, and the HF pump frequency <I>f<sub>H</sub></I> was near and then above the critical frequency of the F2 layer. The distinctive feature of this experiment is that the pump frequency was just below the third electron gyro harmonic frequency, while both the HF pump beam and UHF radar beam were directed towards the magnetic zenith (MZ). The HF pump-induced phenomena were diagnosed with several instruments: the bi-static HF radio scatter on the London-Tromsø-St. Petersburg path, the CUTLASS radar in Hankasalmi (Finland), the European Incoherent Scatter (EISCAT) UHF radar at Tromsø and the Tromsø ionosonde (dynasonde). The results show thermal electron excitation of the HF-induced striations seen simultaneously from HF bi-static scatter and CUTLASS radar observations, accompanied by increases of electron temperature when the heater frequency was near and then above the critical frequency of the F2 layer by up to 0.4 MHz. An increase of the electron density up to 25% accompanied by strong HF-induced electron heating was observed, only when the heater frequency was near the critical frequency and just below the third electron gyro harmonic frequency. It is concluded that the combined effect of upper hybrid resonance and gyro resonance at the same altitude gives rise to strong electron heating, the excitation of striations, HF ray trapping and extension of HF waves to altitudes where they can excite Langmuir turbulence and fluxes of electrons accelerated to energies that produce ionization

A comparison of overshoot modelling with observations of polar mesospheric summer echoes at radar frequencies of 56 and 224 MHz

We have compared radar observations of polar mesospheric summer echoes (PMSEs) modulated by artificial electron heating, at frequencies of 224 MHz (EISCAT VHF) and 56 MHz (MORRO). We have concentrated on 1 day of observation, lasting ∼ 3.8 h. The MORRO radar, with its much wider beam, observes one or more PMSE layers all the time while the VHF radar observes PMSEs in 69% of the time. Statistically there is a clear difference between how the MORRO and the VHF radar backscatter reacts to the heater cycling (48 s heater on and 168 s heater off). While MORRO often reacts by having its backscatter level increased when the heater is switched on, as predicted by Scales and Chen (2008), the VHF radar nearly always sees the "normal" VHF overshoot behaviour with an initial rapid reduction of backscatter. However, in some heater cycles we do see a substantial recovery of the VHF backscatter after its initial reduction to levels several times above that just before the heater was switched on. For the MORRO radar a recovery during the heater-on phase is much more common. The reaction when the heater was switched off was a clear overshoot for nearly all VHF cases but less so for MORRO. A comparison of individual curves for the backscatter values as a function of time shows, at least for this particular day, that in high layers above ∼ 85 km height, both radars see a reduction of the backscatter as the heater is switched on, with little recovery during the heater-on time. These variations are well described by present models. On the other hand, the backscatter in low layers at 81-82 km can be quite different, with modest or no reduction in backscatter as the heater is switched on, followed by a strong recovery for both radars to levels several times above that of the undisturbed PMSEs. This simultaneous, nearly identical behaviour at the two very different radar frequencies is not well described by present modelling

First modulation of high-frequency polar mesospheric summer echoes by radio heating of the ionosphere

The first high-frequency (HF, 8 MHz) observations of the modulation of polar mesospheric summer echoes (PMSE) by artificial radio heating of the ionosphere are presented and compared to observations at 224 MHz and model predictions. The experiments were performed at the European Incoherent Scatter facility in northern Norway. It is shown that model results are in qualitative and partial quantitative agreement with the observations, supporting the prediction that with certain ranges of ice particle radii and concentration, PMSE at HF radar wavelengths can be enhanced by heating due to the dominance of dust charging over plasma diffusion