Giant Terahertz Photoconductance of Quantum Point Contacts in the Tunneling Regime

We report on the observation of the giant photoconductance of a quantum point contact (QPC) in the tunneling regime excited by terahertz radiation. Studied QPCs are formed in a GaAs/(Al, Ga) As heterostructure with a high-electron-mobility two-dimensional electron gas. We demonstrate that irradiation of strongly negatively biased QPCs by laser radiation with frequency f = 0.69 THz and intensity 50mW/cm(2) results in two orders of magnitude enhancement of the QPC conductance. The effect increases with the dark conductivity decrease. It is also characterized by a strong polarization dependence and a drastic reduction of the signal by increasing the radiation frequency to 1.63 THz. We demonstrate that all experimental findings can be well explained by the photon-assisted tunneling through the QPC. Corresponding calculations are in good agreement with the experiment

High frequency of BRCA1, but not CHEK2 or NBS1 (NBN), founder mutations in Russian ovarian cancer patients

<p>Abstract</p> <p>Background</p> <p>A significant portion of ovarian cancer (OC) cases is caused by germ-line mutations in BRCA1 or BRCA2 genes. BRCA testing is cheap in populations with founder effect and therefore recommended for all patients with OC diagnosis. Recurrent mutations constitute the vast majority of BRCA defects in Russia, however their impact in OC morbidity has not been yet systematically studied. Furthermore, Russian population is characterized by a relatively high frequency of CHEK2 and NBS1 (NBN) heterozygotes, but it remains unclear whether these two genes contribute to the OC risk.</p> <p>Methods</p> <p>The study included 354 OC patients from 2 distinct, geographically remote regions (290 from North-Western Russia (St.-Petersburg) and 64 from the south of the country (Krasnodar)). DNA samples were tested by allele-specific PCR for the presence of 8 founder mutations (BRCA1 5382insC, BRCA1 4153delA, BRCA1 185delAG, BRCA1 300T>G, BRCA2 6174delT, CHEK2 1100delC, CHEK2 IVS2+1G>A, NBS1 657del5). In addition, literature data on the occurrence of BRCA1, BRCA2, CHEK2 and NBS1 mutations in non-selected ovarian cancer patients were reviewed.</p> <p>Results</p> <p>BRCA1 5382insC allele was detected in 28/290 (9.7%) OC cases from the North-West and 11/64 (17.2%) OC patients from the South of Russia. In addition, 4 BRCA1 185delAG, 2 BRCA1 4153delA, 1 BRCA2 6174delT, 2 CHEK2 1100delC and 1 NBS1 657del5 mutation were detected. 1 patient from Krasnodar was heterozygous for both BRCA1 5382insC and NBS1 657del5 variants.</p> <p>Conclusion</p> <p>Founder BRCA1 mutations, especially BRCA1 5382insC variant, are responsible for substantial share of OC morbidity in Russia, therefore DNA testing has to be considered for every OC patient of Russian origin. Taken together with literature data, this study does not support the contribution of CHEK2 in OC risk, while the role of NBS1 heterozygosity may require further clarification.</p

Decade-long study of degassing at Kudriavy volcano, Iturup, Kurile Islands (1990–1999): Gas temperature and composition variations, and occurrence of 1999 phreatic eruption

A high-temperature (up to 940°C) fumarolic activity at Kudriavy volcano had been studied during 1990–1999. The maximum gas temperatures of the fumaroles were measured in 1992 as 940°C, then gradually decreased with time and reached to 907°C in 1999. Gas composition of the high-temperature fumarole became enriched in H2O and depleted in other gas components, in particular in CO2. Hydrogen isotopic compositions of the high-temperature fumarolic gases were gradually depleted in deuterium. The gradual and continuous decrease in temperature and changes in gas composition observed during the last 10-year suggest that a magmatic melt have been degassing in a relatively steady-state manner from a single magma chamber. The detail investigations in 1998 and 1999 revealed short-term changes in gas composition characterized by sporadic increases in H2, CO2, and Stotal after intense precipitations. Small-scale eruptions occurred on October 7, 1999 at the summit. The ratios of major gas components (C/S, C/Cl, S/Cl, C/F, S/F, and Cl/F) significantly increased just prior to the eruption. The eruption at the Kudriavy volcano in 1999 was likely a phreatic eruption as a result of the intense precipitations after unusually long dry period. Meteoric water penetrated into the hot zone of volcano edifice and rapidly boiled causing the eruption

The Influence Of Vegetation On Reflected Solar Radiation In Arid And Extra-Arid Zone Of Mongolian Gobi

Vegetation cover has a noticeable effect on surface reflectivity and local microclimate in arid areas of Mongolian Gobi. Over the past decades, various shrub species (Haloxylon ammodendron and Calligonum mongolicum) have appeared on the previously unvegetated hamada. The climatic consequences of bush encroachment are still poorly understood. Using the experimental data, this article estimates the reflectance of plants dominating in Mongolia’s dry steppe, desert-steppe, and desert ecosystems. The average values of the total reflection coefficient at visible and infrared wavelengths range from 19.7% ± 1.4% to 20.1% ± 1.7% in plants growing in desert-steppe ecosystems, and from 25.0% ± 0.9% to 24.8% ± 1.5% on the bare surface. The difference between the reflectance of vegetated and unvegetated surfaces reaches 5%. Therefore, in daylight hours, the vegetated day surface loses less solar energy than the non-vegetated surface does. This phenomenon may be defined as a quasi- or secondary greenhouse effect – in daylight hours, solar energy is retained on the surface by vegetation and this contributes to the temperature increase. Such an impulse, which seems to be insignificantly small at first glance, triggers a series of climatic variations leading to a change in the structure of the radiation and heat balance as well as the climate not only in the desert-steppe and desert ecosystems but also in Central Asia as a whole. All this may explain the 1.2-2.3°C increase in air temperatures in the Gobi observed over the last 25 years

Inhibition of NADH oxidation by 1-methyl-4-phenylpyridinium analogs as the basis for the prediction of the inhibitory potency of novel compounds

Inhibition of NADH dehydrogenase (Complex I) of the mitochondrial respiratory chain by 1-methyl-4-phenylpyridinium (MPP+) and its analogs results in dopaminergic cell death. In the present study, the inhibition of mitochondrial respiration and of NADH oxidation in inverted inner membrane preparations by the oxidation products of N-methyl-stilbazoles (N-methyl-styrylpyridiniums) are characterized. These nonflexible MPP+ analogs were found to be considerably more potent inhibitors than the corresponding MPP+ derivatives. The IC50 values for these compounds and previously published figures for MPP+ analogs were then used to select a computer model based on structural parameters to predict the inhibitory potency of other compounds that react at the "rotenone site" in Complex I. A series of 12 novel inhibitors different in structure from the basic set were used to test the predictive capacity of the models selected. Despite major structural differences between the novel test compounds and the MPP+ and styrylpyridinium analogs on which the models were based, substantial agreement was found between the predicted and experimentally determined IC50 values. The value of this technique lies in the potential for the prediction of the inhibitory potency of other drugs and toxins which block mitochondrial respiration by interacting at the rotenone sites.</p

Inhibition of NADH oxidation by 1-methyl-4-phenylpyridinium analogs as the basis for the prediction of the inhibitory potency of novel compounds

Inhibition of NADH dehydrogenase (Complex I) of the mitochondrial respiratory chain by 1-methyl-4-phenylpyridinium (MPP+) and its analogs results in dopaminergic cell death. In the present study, the inhibition of mitochondrial respiration and of NADH oxidation in inverted inner membrane preparations by the oxidation products of N-methyl-stilbazoles (N-methyl-styrylpyridiniums) are characterized. These nonflexible MPP+ analogs were found to be considerably more potent inhibitors than the corresponding MPP+ derivatives. The IC50 values for these compounds and previously published figures for MPP+ analogs were then used to select a computer model based on structural parameters to predict the inhibitory potency of other compounds that react at the "rotenone site" in Complex I. A series of 12 novel inhibitors different in structure from the basic set were used to test the predictive capacity of the models selected. Despite major structural differences between the novel test compounds and the MPP+ and styrylpyridinium analogs on which the models were based, substantial agreement was found between the predicted and experimentally determined IC50 values. The value of this technique lies in the potential for the prediction of the inhibitory potency of other drugs and toxins which block mitochondrial respiration by interacting at the rotenone sites.</p

Determination of Voltage at the Rectifier Installation of the Electric Locomotive VL-80K for Each Position of the Controller Driver's

The object of research is the electrical processes in the control system of the traction drive of the electric locomotive VL-80K (Russia). To improve the accuracy of its mathematical model, it is necessary to use the values of the parameters determined during experimental studies of the traction drive control system. In particular, it is important to use in the traction drive model the value of the voltages on the arms of the rectifier installation of the electric locomotive, taking into account the position of the driver's controller. The complexity of the simulation lies in the fact that the reference does not provide the value of the voltage on the arms of the rectifier for each position of the driver's controller, which makes it difficult to verify the resulting model. A scheme for measuring the voltage value on the arms of the rectifier installation of an electric locomotive for each position of the position of the driver's controller is proposed. On its basis, a simulation model is developed in the MATLAB software environment. The simulation model implements an algorithm for closing the contactors of the electric group contactor for each position of the position of the driver's controller. Approbation of the voltage measurement technique was carried out on an electric locomotive of the VL-80K series during a trip on the Darnytsia-Myronivka section (Ukraine). Comparison of the voltage values on the arms of the rectifier installation, obtained experimentally, with the passport data showed that the measurement error was 0.5 %. In addition, the experimental results showed that the voltages on the arms of the rectifier for paired positions of the position of the driver's controller are the same, for odd ones they are different. Therefore, when simulating the operation of the traction drive control system, the voltage values on different arms of the rectifier installation were taken separately. Comparison of the simulation results for the nominal mode with passport data showed that for this mode the measurement error was 3.78 %. For all others, it did not exceed 5 %

Hydrogen isotope geochemistry and heat balance of a fumarolic system: Kudriavy volcano, Kuriles

The temperature and hydrogen isotope composition of the fumarolic gases have been studied at Kudriavy volcano, Kurile Islands, which is unique for investigating the processes of magma degassing because of the occurrence of numerous easily accessible fumaroles with a temperature range of 100–940°C. There are several local fumarolic fields with a total surface area of about 2600 m2 within the flattened crater of 200×600 m. Each fumarolic field is characterized by the occurrence of high- and low-temperature fumaroles with high gas discharges and steaming areas with lower temperatures. We have studied the thermal budget of the Kudriavy fumarolic system on the basis of the quantitative dependences of the hydrogen isotope ratio (D/H) and tritium concentration on the temperature of fumarolic gases and compared them with the calculated heat balance of mixing between hot magmatic gas and cold meteoric water. Hydrogen isotope composition (δD and 3H) shows a well expressed correlation with the gas temperature. Since D/H ratio and 3H are good indicators of water sources in volcanic areas, it suggests that the thermal budget of the fumarolic system is mostly controlled by the admixing of meteoric waters to magmatic gases. The convective mechanism of heat transfer in the hydrothermal system governs the maximum temperatures of local fumaroles and fumarolic fields. Low-temperature fumaroles at Kudriavy are thermally buffered by the boiling processes of meteoric waters in the mixing zone at pressures of 3–12 bar. These values may correspond to the hydrostatic pressure of water columns about 30–120 m in height in the volcanic edifice and hence to the depth of a mixing/boiling zone. Conductive heat transfer is governed by conductive heat exchange between gases and country rocks and appears to be responsible for the temperature distribution around a local fumarolic vent. The temperature and pressure of shallow degassing magma are estimated to be 1050°C and 2–3 bar, respectively. The length of the ‘main’ fumarolic gas conduit is estimated to be about 80 m from the linear correlation between maximal temperatures of fumarolic fields and distances to the highest-temperature ‘F-940’ fumarole. This value may correspond to the depth of an apical part of the magmatic chamber. The geometry of the crater zone at the Kudriavy summit and the model of convective gas cooling suggest different hydrostatic pressures in the hydrothermal system at the base of high- and low-temperature gas conduits. The depths of gas sources for low-temperature fumaroles are evaluated to be about 200 m at the periphery of the magma chamber