Apparent Conductance Research Articles

Direct electrode-electrode tunneling in break-junction measurements of molecular conductance

In break-junction experiments, the electrical contact between two metal electrodes is broken. When opening the junction, one or more molecules can be trapped between the electrodes and thereby contacted which allows their transport properties to be investigated. Statistical analysis of data plotted in histograms is generally used to determine the values of molecular conductance and the variations therefrom. In this paper, the effect of direct electron tunneling between the metal electrodes on the molecular conductance measurement is discussed. It is shown that this tunneling forms a parallel conductance channel that can influence both the position and the distribution of the apparent molecular conductance extracted from histogram peaks. Criteria to recognize the effect of this form of parallel tunneling path in comparison to the molecular transport path in the experimental data are established. With the help of analytic models and simulations, it will be shown how to disentangle the contributions of several physical variation sources on the resulting position, height, and width of the histogram peaks. Finally, the plotting of 2D histograms is described as a practical method to help distinguish the effects of molecular conductance from those of direct electrode-electrode tunneling.

How Chemistry, Nanoscale Roughness, and the Direction of Heat Flow Affect Thermal Conductance of Solid–Water Interfaces

We quantify the Kapitza thermal conductance of solid–liquid interfaces between self-assembled monolayers (SAMs) and liquid water using nonequilibrium molecular dynamics simulations. We focus on understanding how surface chemistry, nanoscale roughness, and the direction of heat flow affect interfacial thermal conductance. In agreement with calculations by Shenogina et al. (Phys. Rev. Lett., 2009, 102, 156101) for SAMs with homogeneous headgroup chemistries, we find that for mixed −CF3/–OH SAMs, thermal conductance increases roughly linearly with the fraction of −OH groups on the surface. Increasing nanoscale roughness increases solid–water contact area, and therefore the apparent thermal conductance. However, the inherent thermal conductance, which accounts for the increased contact area, shows only small and subtle variations. These variations are consistent with expectations based on recent work on the effects of nanoscale roughness on interfacial tension (Mittal and Hummer, Faraday Disc., 2010, 146, 341). Finally, we find that SAM–water interfaces show thermal rectification. Thermal conductance is larger when heat flows from the ordered SAM phase to the disordered liquid water phase, and the magnitude of rectification increases with surface hydrophilicity.

Seasonal variation in plant hydraulic traits of two co‐occurring desert shrubs, Tamarix ramosissima and Haloxylon ammodendron, with different rooting patterns

AbstractEffective hydraulic responses to varying soil moisture and evaporative demand are crucial to plant survival in arid ecosystems. This study was carried out during two growing seasons (2004–2005) on two typical desert shrub species, Tamarix ramosissima and Haloxylon ammodendron, co‐occurring in the Gurbantonggut Desert (Central Asia), to investigate their hydraulic responses to seasonal variations in water availability. The root distribution was studied by excavating the intact root systems. Leaf‐specific apparent hydraulic conductance (Kl) for the two species was calculated based on leaf water potential (ψl) and transpiration rate (Tr), which were monitored during the growing seasons. T. ramosissima had a deeper taproot (3.1 vs. 2.6 m) and a larger root surface area (3.02 vs. 1.28 m2) than H. ammodendron. Combined with a higher ψl, this meant that it maintained a better water status in general. For the deep‐rooting T. ramosissima, the seasonal pattern of its predawn leaf water potential (ψpd) was in high accordance with the seasonal changes in soil moisture at a depth of 2.6–2.8 m, which was largely influenced by the upflow of groundwater through capillary ascent, and barely responded to rain events. For the shallow‐rooting H. ammodendron, the seasonal pattern of ψpd was closely related to soil moisture in the upper layer at a depth of 0–0.2 and 0.6–0.8 m, which was recharged periodically by rain events and responded acutely to rainfall above 5 mm. The two species differed in their maximum transpiration rates (Trmax) and ψpd: in T. ramosissima, Trmax gradually dropped with decreasing ψpd; in H. ammodendron, Trmax showed no significant response to ψpd. For T. ramosissima, the major water resource was groundwater and vadose zone water, and ψl contributed significantly to transpiration regulation. For H. ammodendron, the primary water resource was precipitation input, which was not sufficient to keep the ψl for this species outside the critical range of leaf shedding in summer. Thus, for these two representative species that share the same habitat, contrasting response strategies to water limitation were observed in relation to water acquisition and root distribution characteristics.

Hydraulic efficiency and coordination with xylem resistance to cavitation, leaf function, and growth performance among eight unrelated Populus deltoides×Populus nigra hybrids

Tests were carried out to determine whether variations in the hydraulic architecture of eight Populus deltoides×Populus nigra genotypes could be related to variations in leaf function and growth performance. Measurements were performed in a coppice plantation on 1-year-old shoots under optimal irrigation. Hydraulic architecture was characterized through estimates of hydraulic efficiency (the ratio of conducting sapwood area to leaf area, A(X):A(L); leaf- and xylem-specific hydraulic conductance of defoliated shoots, k(SL) and k(SS), respectively; apparent whole-plant leaf-specific hydraulic conductance, k(plant)) and xylem safety (water potential inducing 50% loss in hydraulic conductance). The eight genotypes spanned a significant range of k(SL) from 2.63 kg s(-1) m(-2) MPa(-1) to 4.18 kg s(-1) m(-2) MPa(-1), variations being mostly driven by k(SS) rather than A(X):A(L). There was a strong trade-off between hydraulic efficiency and xylem safety. Values of k(SL) correlated positively with k(plant), indicating that high-pressure flowmeter (HPFM) measurements of stem hydraulic efficiency accurately reflected whole-plant water transport efficiency of field-grown plants at maximum transpiration rate. No clear relationship could be found between hydraulic efficiency and either net CO(2) assimilation rates, water-use efficiency estimates (intrinsic water-use efficiency and carbon isotope discrimination against (13)C), or stomatal characteristics (stomatal density and stomatal pore area index). Estimates of hydraulic efficiency were negatively associated with relative growth rate. This unusual pattern, combined with the trade-off observed between hydraulic efficiency and xylem safety, provides the rationale for the positive link already reported between relative growth rate and xylem safety among the same eight P. deltoides×P. nigra genotypes.

Single-Molecule Mapping of Long-range Electron Transport for a Cytochrome b562 Variant

Cytochrome b(562) was engineered to introduce a cysteine residue at a surface-exposed position to facilitate direct self-assembly on a Au(111) surface. The confined protein exhibited reversible and fast electron exchange with a gold substrate over a distance of 20 Å between the heme redox center and the gold surface, a clear indication that a long-range electron-transfer pathway is established. Electrochemical scanning tunneling microscopy was used to map electron transport features of the protein at the single-molecule level. Tunneling resonance was directly imaged and apparent molecular conductance was measured, which both show strong redox-gated effects. This study has addressed the first case of heme proteins and offered new perspectives in single-molecule bioelectronics.

Ca2+ release from the sarcoplasmic reticulum is required for sustained TRPM4 activity in cerebral artery smooth muscle cells

The melastatin transient receptor potential (TRP) channel TRPM4 is a critical regulator of vascular smooth muscle cell membrane potential and contractility. Activation of the channel is Ca(2+)-dependent, but prolonged exposure to high (>1 microM) levels of intracellular Ca(2+) causes rapid (within approximately 2 min) desensitization of TRPM4 currents under conventional whole cell and inside-out patch-clamp conditions. The goal of the present study was to establish a novel method to record sustained TRPM4 currents in smooth muscle cells under near-physiological conditions. Using the amphotericin B-perforated patch-clamp technique, we recorded and characterized sustained (up to 30 min) transient inward cation currents (TICCs) in freshly isolated cerebral artery myocytes. In symmetrical cation solutions, TICCs reversed at 0 mV and had an apparent unitary conductance of 25 pS. Replacement of extracellular Na(+) with the nonpermeable cation N-methyl-d-glucamine abolished the current. TICC activity was attenuated by the TRPM4 blockers fluflenamic acid and 9-phenanthrol. Selective silencing of TRPM4 expression using small interfering RNA diminished TICC activity, suggesting that the molecular identity of the responsible ion channel is TRPM4. We used the perforated patch-clamp method to test the hypothesis that TRPM4 is activated by intracellular Ca(2+) signaling events. We found that TICC activity is independent of Ca(2+) influx and ryanodine receptor activity but is attenuated by sarco(endo)plasmic reticulum Ca(2+)-ATPase inhibition and blockade of inositol 1,4,5-trisphosphate receptor-mediated Ca(2+) release from the sarcoplasmic reticulum. Our findings suggest that TRPM4 channels in cerebral artery myocytes are regulated by Ca(2+) release from inositol 1,4,5-trisphosphate receptor on the sarcoplasmic reticulum.

Photosynthetic acclimation to different light levels in the brown marine macroalga, Hizikia fusiformis (Sargassaceae, Phaeophyta)

Hizikia fusiformis thalli experience dynamic incident light conditions during the period of growth. The present study was designed to examine how changing photon irradiance affects the photosynthesis both in the short and long terms by culturing H. fusiformis under three different light levels: 35 μmol photons m-2 s-1 (low light, LL), 85 μmol photons m-2 s-1 (intermediate light, IL), and 165 μmol photons m-2 s-1 (high light, HL). A similar relative growth rate was observed between IL- and HL-grown algae, but the growth rate was significantly reduced in LL-grown algae. The photosynthetic rates (Pn) measured at their respective growth light levels were found to be lowest in the thalli grown at LL and highest at HL. However, LL-grown algae exhibited much higher Pn in comparison with IL- and the HL-grown thalli at the same measuring photosynthetic photon flux density, indicating the photosynthetic acclimation to low growth light in H. fusiformis. The photosynthesis–light curves showed that LL-grown algae had a highest light-saturating maximum Pn (Pmax) in comparison with IL- or HL-grown algae when the photosynthetic rates were expressed on the biomass basis. However, Pmax was highest in HL-grown algae compared to IL- or LL-grown algae when the rates were normalized to chlorophyll a. The photosynthesis–inorganic carbon (Ci) response curves were also significantly affected by the growth light conditions. The highest value of apparent photosynthetic conductance occurred in LL-grown algae while the lowest value in HL-grown algae. Additionally, the activity of external carbonic anhydrase (CA) tended to increase while the total CA activity inclined to decrease in H. fusiformis thalli when the growth light level altered from 35 to 165 μmol photons per square meter per second. The external CA inhibitors showed a higher inhibition in HL-grown algae compared with LL-grown algae. It was proposed that photosynthetic acclimation to low light condition in H. fusiformis was achieved through an increase in the number of reaction centers and increased capacities of electron transport and of Ci transport within cells. The ability of photosynthetic acclimation to low light confers H. fusiformis thalli to overcome the environmental low light condition as a result of the attenuation of seawater or self-shading through enhancing its photosynthetic performance and carbon assimilation necessary for growth.

Physiological and morphological responses of young mahogany ( Swietenia macrophylla King) plants to drought

Young mahogany ( Swietenia macrophylla King) plants were grown under either well-watered (pre-dawn leaf water potential, Ψ pd, ca. −0.40 MPa) or drought ( Ψ pd, ca. −3.52 MPa) conditions to examine some physiological strategies that allow the maintenance of leaf turgor. In well-watered plants, stomatal conductance ( g s) was nearly constant (440 mmol m −2 s −1) between 7:00 and 13:00 h. This was accomplished by significant increases in transpiration ( E) and apparent total hydraulic conductance ( K T), in which averages were higher at 13:00 h. From 13:00 to 17:00 h, g s, E, and K T decreased sharply, reaching their lower values at 17:00 h. In these plants, significant increases in height (116%), stem diameter (50%) and leaf area (200%) were registered over the experimental period (20 days). Analyses of linear regression between g s or E and leaf-to-air vapor pressure deficit ( Δ w) were not significant. In water-stressed plants, g s and E were higher at 7:00 h and lower from 9:00 to 17:00 h, while K T was higher in early morning (7:00 h) and in late afternoon (17:00 h) than between 9:00 and 15:00 h. Moreover, both g s and E decreased potentially ( P < 0.001) with the diurnal increases on Δ w. Drought also decreased leaf and leaflet numbers and reduced total leaf area, but had no effect on stem height and diameter. Leaf proline was higher (ca. 400%, between 13:00 and 15:00 h) in water-stressed plants, suggesting osmotic adjustment under drought. Twelve hours after resumption of irrigation, Ψ pd was similar ( P > 0.05) between well-watered and drought-stressed plants, suggesting an ability of plants to recover turgor after stress cessation. Altogether, our data support the hypothesis that young mahogany plants have the ability to satisfactorily tolerate or postpone drought.

Psalmotoxin-1 Docking to Human Acid-sensing Ion Channel-1

Acid-sensing ion channel-1 (ASIC-1) is a proton-gated ion channel implicated in nociception and neuronal death during ischemia. Recently the first crystal structure of a chicken ASIC was obtained. Expanding upon this work, homology models of the human ASICs were constructed and evaluated. Energy-minimized structures were tested for validity by in silico docking of the models to psalmotoxin-1, which potently inhibits ASIC-1 and not other members of the family. The data are consistent with prior radioligand binding and functional assays while also explaining the selectivity of PcTX-1 for homomeric hASIC-1a. Binding energy calculations suggest that the toxin and channel create a complex that is more stable than the channel alone. The binding is dominated by the coulombic contributions, which account for why the toxin-channel interaction is not observed at low pH. The computational data were experimentally verified with single channel and whole-cell electrophysiological studies. These validated models should allow for the rational design of specific and potent peptidomimetic compounds that may be useful for the treatment of pain or ischemic stroke.

Residues Important for Nitrate/Proton Coupling in Plant and Mammalian CLC Transporters

Members of the CLC gene family either function as chloride channels or as anion/proton exchangers. The plant AtClC-a uses the pH gradient across the vacuolar membrane to accumulate the nutrient NO(3)(-) in this organelle. When AtClC-a was expressed in Xenopus oocytes, it mediated NO(3)(-)/H(+) exchange and less efficiently mediated Cl(-)/H(+) exchange. Mutating the "gating glutamate" Glu-203 to alanine resulted in an uncoupled anion conductance that was larger for Cl(-) than NO(3)(-). Replacing the "proton glutamate" Glu-270 by alanine abolished currents. These could be restored by the uncoupling E203A mutation. Whereas mammalian endosomal ClC-4 and ClC-5 mediate stoichiometrically coupled 2Cl(-)/H(+) exchange, their NO(3)(-) transport is largely uncoupled from protons. By contrast, the AtClC-a-mediated NO(3)(-) accumulation in plant vacuoles requires tight NO(3)(-)/H(+) coupling. Comparison of AtClC-a and ClC-5 sequences identified a proline in AtClC-a that is replaced by serine in all mammalian CLC isoforms. When this proline was mutated to serine (P160S), Cl(-)/H(+) exchange of AtClC-a proceeded as efficiently as NO(3)(-)/H(+) exchange, suggesting a role of this residue in NO(3)(-)/H(+) exchange. Indeed, when the corresponding serine of ClC-5 was replaced by proline, this Cl(-)/H(+) exchanger gained efficient NO(3)(-)/H(+) coupling. When inserted into the model Torpedo chloride channel ClC-0, the equivalent mutation increased nitrate relative to chloride conductance. Hence, proline in the CLC pore signature sequence is important for NO(3)(-)/H(+) exchange and NO(3)(-) conductance both in plants and mammals. Gating and proton glutamates play similar roles in bacterial, plant, and mammalian CLC anion/proton exchangers.

Conversion of the 2 Cl−/1 H+ antiporter ClC-5 in a NO3−/H+ antiporter by a single point mutation

Several members of the CLC family are secondary active anion/proton exchangers, and not passive chloride channels. Among the exchangers, the endosomal ClC-5 protein that is mutated in Dent's disease shows an extreme outward rectification that precludes a precise determination of its transport stoichiometry from measurements of the reversal potential. We developed a novel imaging method to determine the absolute proton flux in Xenopus oocytes from the extracellular proton gradient. We determined a transport stoichiometry of 2 Cl(-)/1 H+. Nitrate uncoupled proton transport but mutating the highly conserved serine 168 to proline, as found in the plant NO3(-)/H+ antiporter atClCa, led to coupled NO3(-)/H+ exchange. Among several amino acids tested at position 168, S168P was unique in mediating highly coupled NO3(-)/H+ exchange. We further found that ClC-5 is strongly stimulated by intracellular protons in an allosteric manner with an apparent pK of approximately 7.2. A 2:1 stoichiometry appears to be a general property of CLC anion/proton exchangers. Serine 168 has an important function in determining anionic specificity of the exchange mechanism.

Leaf water status and stem xylem flux in relation to soil drought in five temperate broad-leaved tree species with contrasting water use strategies

• Five temperate broad-leaved tree species were compared with respect to their water consumption strategies under ample and restricted water supply. We measured synchronously leaf conductance (gL) in the sun canopy, xylem sap flux (Js) and leaf water potential (predawn, Ψpd and noon, Ψnoon) in adult trees in a mixed stand and related them to the fluctuations in vapor pressure deficit (D )a nd soil moisture. • Maximum gL was particularly high in F. excelsior, C. betulus and T. cordata and revealed a higher D sensitivity. Ψpd remained constantly high in A. pseudoplatanus, C. betulus and F. excelsior ,b ut decreased in T. cordata and F. sylvatica with decreasing soil moisture. • Jsd decreased linearly with decreasing soil matrix potential in all species except for F. excelsior. Apparent hydraulic conductance in the soil-to-leaf flow path (Lc) was higher in A. pseudoplatanus than in the other species. • F. sylvatica maintained a low maximum gL and reduced Jsd markedly upon drought, but faced severe decreases in Ψpd and Ψnoon. F. excelsior represents an opposite strategy with high maximum gL and stable Ψpd. • The species drought sensitivity increases in the sequence F. excelsior < C. betulus < T. cordata < A. pseudoplatanus < F. sylvatica.

Drought during canopy development has lasting effect on annual carbon balance in a deciduous temperate forest

* Climate change projections predict an intensifying hydrologic cycle and an increasing frequency of droughts, yet quantitative understanding of the effects on ecosystem carbon exchange remains limited. * Here, the effect of contrasting precipitation and soil moisture dynamics were evaluated on forest carbon exchange using 2 yr of eddy covariance and microclimate data from a 50-yr-old mixed oak woodland in northern Ohio, USA. * The stand accumulated 40% less carbon in a year with drought between bud-break and full leaf expansion (354 +/- 81 g C m(-2) yr(-1) in 2004 and 252 +/- 45 g C m(-2) yr(-1) in 2005). This was caused by greater suppression of gross ecosystem productivity (GEP; 16% = 200 g) than of ecosystem respiration (ER; 11% = 100 g) by drought. Suppressed GEP was traced to lower leaf area, lower apparent quantum yield and lower canopy conductance. The moisture sensitivity of ER may have been mediated by GEP. * The results highlight the vulnerability of the ecosystem to even a moderate drought, when it affects a critical aspect of development. Although the drought was preceded by rain, the storage capacity of the soil seemed limited to 1-2 wk, and therefore droughts longer than this are likely to impair productivity in the region.

Possible heat‐tolerant wheat cultivar improvement through the use of flag leaf gas exchange traits in a Mediterranean environment

AbstractBACKGROUND: Flag leaf traits are associated with performance of wheat (Triticum aestivum L.) genotypes in heat‐stress environments. Fifteen cultivars were evaluated during 3 years with two sowing dates under irrigated conditions so as to examine the relationship between the flag leaf traits and performance in a Mediterranean environment.RESULTS: It became possible to assess a heat susceptibility index (HSI) for each cultivar by altering the sowing date, exposing the crops to different temperatures. The cultivars showed significant differences for all measured traits except canopy temperatures and chlorophyll fluorescence, though not all were consistent. Excluding two cultivars that differed in development, yield, grain number, harvest index and leaf An (net CO2 assimilation rate), gs (stomatal conductance to water vapor), mc (apparent mesophyll conductance of CO2) and ash content were negatively associated with HSI, whereas leaf area and width were positively associated. Traits showing the best correlation with HSI were An, followed by area, gs, mc, width and ash content. High An was associated with smaller leaf size.CONCLUSION: It is worthwhile to select high ear grain number and harvest index. In addition, flag leaf gas exchange traits and ash content can be used as complementary selection criteria after the possible pleiotropic associations of these traits with leaf size are clarified. Copyright © 2008 Society of Chemical Industry

Coordination of leaf and stem water transport properties in tropical forest trees

Stomatal regulation of transpiration constrains leaf water potential (Psi(L)) within species-specific ranges that presumably avoid excessive tension and embolism in the stem xylem upstream. However, the hydraulic resistance of leaves can be highly variable over short time scales, uncoupling tension in the xylem of leaves from that in the stems to which they are attached. We evaluated a suite of leaf and stem functional traits governing water relations in individuals of 11 lowland tropical forest tree species to determine the manner in which the traits were coordinated with stem xylem vulnerability to embolism. Stomatal regulation of Psi(L) was associated with minimum values of water potential in branches (Psi(br)) whose functional significance was similar across species. Minimum values of Psi(br) coincided with the bulk sapwood tissue osmotic potential at zero turgor derived from pressure-volume curves and with the transition from a linear to exponential increase in xylem embolism with increasing sapwood water deficits. Branch xylem pressure corresponding to 50% loss of hydraulic conductivity (P (50)) declined linearly with daily minimum Psi(br) in a manner that caused the difference between Psi(br) and P (50) to increase from 0.4 MPa in the species with the least negative Psi(br) to 1.2 MPa in the species with the most negative Psi(br). Both branch P (50) and minimum Psi(br) increased linearly with sapwood capacitance (C) such that the difference between Psi(br) and P (50), an estimate of the safety margin for avoiding runaway embolism, decreased with increasing sapwood C. The results implied a trade-off between maximizing water transport and minimizing the risk of xylem embolism, suggesting a prominent role for the buffering effect of C in preserving the integrity of xylem water transport. At the whole-tree level, discharge and recharge of internal C appeared to generate variations in apparent leaf-specific conductance to which stomata respond dynamically.

Determinants of Anion-Proton Coupling in Mammalian Endosomal CLC Proteins

Many proteins of the CLC gene family are Cl(-) channels, whereas others, like the bacterial ecClC-1 or mammalian ClC-4 and -5, mediate Cl(-)/H(+) exchange. Mutating a "gating glutamate" (Glu-224 in ClC-4 and Glu-211 in ClC-5) converted these exchangers into anion conductances, as did the neutralization of another, intracellular "proton glutamate" in ecClC-1. We show here that neutralizing the proton glutamate of ClC-4 (Glu-281) and ClC-5 (Glu-268), but not replacing it with aspartate, histidine, or tyrosine, rather abolished Cl(-) and H(+) transport. Surface expression was unchanged by these mutations. Uncoupled Cl(-) transport could be restored in the ClC-4(E281A) and ClC-5(E268A) proton glutamate mutations by additionally neutralizing the gating glutamates, suggesting that wild type proteins transport anions only when protons are supplied through a cytoplasmic H(+) donor. Each monomeric unit of the dimeric protein was found to be able to carry out Cl(-)/H(+) exchange independently from the transport activity of the neighboring subunit. NO(3)(-) or SCN(-) transport was partially uncoupled from H(+) countertransport but still depended on the proton glutamate. Inserting proton glutamates into CLC channels altered their gating but failed to convert them into Cl(-)/H(+) exchangers. Noise analysis indicated that ClC-5 switches between silent and transporting states with an apparent unitary conductance of 0.5 picosiemens. Our results are consistent with the idea that Cl(-)/H(+) exchange of the endosomal ClC-4 and -5 proteins relies on proton delivery from an intracellular titratable residue at position 268 (numbering of ClC-5) and that the strong rectification of currents arises from the voltage-dependent proton transfer from Glu-268 to Glu-211.

Analysing frequency-domain EM data for highly conductive targets

The sensitivity of RESOLVE data to highly conductive targets is examined. Modelling results from synthetic data across horizontal plates suggest that RESOLVE data detect highly conductive structures, but don?t resolve their conductances and time-constants if those values exceed 500 S and 8 ms, respectively. A RESOLVE data set flown initially for kimberlite exploration has been re-analysed for nickel sulphides. Since the latter are highly conductive, the standard processing products such as apparent resistivity grids, deemed useful for mapping kimberlites, did not provide the optimum resolution for data analysis. The survey area is characterized by strong magnetic and EM responses due to the presence of banded-iron formations and pyrrhotite-rich massive sulphides. In order to determine the strongest conductors, time-constants and apparent conductivities were derived from the RESOLVE survey data. Due to the strong magnetic response, correction of the RESOLVE data for magnetic permeability resulted in better-resolved time-constants. This was achieved by inverting the data for the conductivity s and magnetic permeability mr of a layered-earth, followed by the forward modelling of the layer conductivities with mr=1. A comparison of independently derived time-constants and apparent conductances suggests that reliable values do not exceed 0.5 ms and 200 S, respectively. Conductors with values above 200 S / 0.5 ms are detectable, but not resolvable.

Spatial Analysis of Airborne Geophysical Data Applied to Geological Mapping and Mineral Prospecting in the Serra Leste Region, Carajás Mineral Province, Brazil

The Serra Leste region, located in the northeast portion of Carajas Mineral Province (PA), hosts important chrome-PGE, Au–Pd–Pt, Cu–Mo–Au, iron, and manganese deposits and occurrences related to different geological settings. We present a regional prospective model with focus on PGE mineralizations based on airborne geophysics (magnetometry, gamma-ray spectrometry and time domain electromagnetometry) and geological data. Geological and geophysics data analysis indicate a plausible correspondence between areas with meta-mafic–ultramafic rocks and low values in gamma spectrometry, mainly for the thorium channel. The meta-ultramafic units of the complexes, the main host rock of the PGE mineralization, are well defined by analytic signal from the anomalous magnetic field and by the apparent conductance anomalies derived from the GEOTEM system. The combination of the derivative products with updated geological data of the Serra Leste region and PGE-Luanga deposit enable to establish a regional prospective model for PGE and the identification of new targets within the studied area.

Simultaneous sudden changes of electrical behavior at the threshold in laser diodes

Precise ac electrical measurements as well as dc I-V plots at forward bias have been used to characterize multi-quantum-well (MQW) laser diodes. Step offsets of apparent conductance, apparent capacitance, junction voltage, series resistance, and ideality factor at lasing threshold were observed. To compare the electrical characteristics of different diodes numerical simulations of the IdV∕dI-I curve of the double-heterostructure (DH) laser were performed. We find that the conventional model used to explain this curve is not very satisfactory. Our simulations demonstrate that the junction voltage, series resistance, and ideality factor for both DH and MQW lasers exhibit the same sudden changes at threshold. Perhaps the electrical behavior near threshold for all laser diodes is very similar.

Sudden Change of Electrical Characteristics at Lasing Threshold of a Semiconductor Laser

Accurate forward electrical characteristics of multiquantum-well (MQW) lasers have been measured using ac admittance measurements together with dc I-V plot. The synchronous step offsets of apparent conductance, apparent capacitance, junction voltage, series resistance, and ideality factor at the onset of lasing were observed for the first time. With this effect, the lasing threshold can be deduced immediately by the LCR Meter. It is also found that the junction voltage jumps abruptly to a saturated value at the onset of lasing, and the starting and end points of jumping exactly correspond to the maximum of the second derivative of lasing power with respect to current and the kink point of the first derivative, respectively. All of the phenomena may help to verify and improve existing semiconductor laser models. In addition, negative capacitance effect in LDs was observed