Increase In Junctional Resistance Research Articles

Buffer layer-enhanced magnetic field effect in manganite-based heterojunctions

Influence of magnetic field on the rectifying property of the La0.67Ca0.33MnO3/LaMnO3/SrTiO3:0.05 wt %Nb heterojunctions has been studied. In addition to an enhanced magnetic response of the current-voltage characteristics, a field-induced increase in junction resistance, which is an effect different from that in the junctions without the LaMnO3 layer, is observed. The positive magnetoresistance is further found to show a systematic variation with the thickness of the LaMnO3 layer (t), growing rapidly with the increase of layer thickness and getting a maximum of ∼91% at t=4 nm (T=50 K and ΔH=5 T). Analysis of the current-voltage and capacitance-voltage characteristics indicates a field-induced growth of interfacial barrier, which is responsible for the abnormal effect observed here.

MgO-based double barrier magnetic tunnel junctions with thin free layers

The free layer thickness (tfree) in double barrier magnetic tunnel junctions (DMTJs) based on crystalline MgO barriers and CoFeB ferromagnetic layers has been varied from 0.5 to 3.0 nm in order to investigate its effect on the magnetic and electrical properties. One obvious feature of DMTJs with tfree≤1 nm is the absence of sharp free layer switching in the TMR curves, which can be explained by the superparamagnetic nature of discontinuous CoFeB layer, which breaks into nanodots when it is very thin. Normal free layer switch is observed when tfree=2.0 and 3.0 nm. Another difference is a rapid increase in junction resistance and tunnel magnetoresistance at low temperature for DMTJs with thin tfree, which is attributed to the Coulomb blockade effect. We also observed a small conductance peak in the dI/dV curve at low bias only in the parallel configuration and at temperatures below 100 K. This is related to the Kondo scattering process on the nanodots, which constitutes the discontinuous free layer. We found no Coulomb staircase existing in the I-V curves; this may be due to the microsize of the junctions.

Carbon/Molecule/Metal and Carbon/Molecule/Metal Oxide Molecular Electronic Junctions

Molecular junctions were fabricated on the basis of a 1.7−4.5 nm thick layer of fluorene (FL) or nitroazobenzene (NAB) covalently bonded to a graphitic pyrolyzed photoresist film (PPF) substrate. The junction was completed with a top contact consisting of metallic Cu, TiO2, or aluminum(III) oxide (AlOx) and a final layer of Au. The current/voltage behavior of the junctions depended strongly both on the nature of the metal or metal oxide top layers and on the structure of the molecular layer. PPF/NAB/Cu/Au and PPF/FL/Cu/Au junctions were highly conducting, with resistances of 0.3−1.7 Ω cm2, depending on the identity and thickness of the molecular layer. Substitution of Cu with either AlOx or TiO2 caused a large increase in junction resistance by 2−4 orders of magnitude, but also yielded rectifying junctions in the case of PPF/NAB(4.5)/TiO2(3.1)/Au. For a positive bias (PPF relative to Au) above +2 V, the NAB(4.5)/TiO2(3.1) junction became highly conductive, apparently due to injection of electrons into the...

Analysis of the Causes of Variance in Resistance Measurements on Metal−Molecule−Metal Junctions Formed by Conducting-Probe Atomic Force Microscopy

Alkanethiol tunnel junctions were studied using conducting-probe atomic force microscopy to determine causes of variability in measured resistance behavior. Measurements were made on Au/decanethiol/Au monolayer junctions, and effects of substrate roughness, tip chemistry, presence of solvent, extensive tip usage, applied load, and tip radius were examined. Resistance measurements yielded log-normal distributions under a variety of conditions, indicating that the origin of the variance is likely to be either changes in tunneling length or electronic overlap. Spreads in resistance values for a given tip were much less when flat, template-stripped Au substrates were used rather than rough, evaporated Au substrates. Chemical modification of tips with ethanethiol (C2) or butanethiol (C4) and performing measurements under cyclohexane were also found to reduce variance by a factor of about 2-4. Experiments performed with unmodified tips showed an increase in junction resistance over the course of hundreds of consecutive measurements, whereas junctions made with modified tips or under cyclohexane did not. Attempts to ascribe variance between tips to varying tip radii failed; however, decreases in resistance with increasing applied load on the tip contact were observed and could be interpreted in terms of conventional contact mechanics models.

Effects of temperature, deposition conditions, and magnetic field on Ni/sub 80/Fe/sub 20//Fe/sub 50/Mn/sub 50//Ni/sub 80/Fe/sub 20//Al/sub 2/O/sub 3//Co and Ni/sub 80/Fe/sub 20//Al/sub 2/O/sub 3//Co magnetic tunnel junctions

We fabricated magnetic tunnel junctions (MTJs) of Ni/sub 80/Fe/sub 20//Fe/sub 50/Mn/sub 50//Ni/sub 80/Fe/sub 20//Al/sub 2/O/sub 3//Co exchange-biased structures and Ni/sub 80/Fe/sub 20//Al/sub 2/O/sub 3//Co nonexchange-biased structures using shadow masks, with and without an in situ magnetic field. We magnetically annealed the junctions at 230/spl deg/C for 15 min after deposition. Low-temperature measurements revealed an increase in junction resistance and tunneling magnetoresistance and enhancement of the exchange field for exchange-biased junctions. Post-deposition magnetic annealing at an optimum predicted temperature did not improve the quality of the Al/sub 2/O/sub 3/ but instead degraded it, highlighting the importance of other contributing factors. MTJs fabricated with an in situ magnetic field without any post-deposition magnetic annealing produced the most desirable results from the perspective of magnetic application technology.

Impedance measurements and connexin expression in human detrusor muscle from stable and unstable bladders.

Three of this month's Scientific Discovery papers highlight the importance of collaboration in delivering high quality scientific research. As scientific technology increases in power and cost, and specific areas of interest become more specialized, it is becoming more difficult to cover all aspects of a completeresearch story. Collaborating with other experts in the field or other fields, including industry, allows strong scientific proof to be generated for the hypothesis and aims. Building strong collaborative,inter-disciplinary, multi-institutional, international groups with academic and industrial partners is the way forward for all discovery. We look forward to publishing more of these collaborative papersin future issues of the BJU International. To test the hypothesis that intercellular electrical coupling is altered in human detrusor smooth muscle from patients with unstable bladders. Human detrusor biopsy samples were obtained from patients with stable and unstable bladders. Intracellular electrical impedance was measured with alternating current (20 Hz-300 kHz) across the ends of detrusor strips in an oil-gap, after correcting for extracellular space resistance. Gap junctions were identified by localization of connexins (Cx), specifically Cx45, Cx43 and Cx40 transcripts, using immunoconfocal microscopy. Total intracellular resistivity was greater in strips from unstable than from stable bladders (median 1246 vs 817 Omega.cm). The increase was attributed to an increase in junctional resistance; cytoplasmic resistance was unchanged. Cx43 was localized to a submucosal layer and to connective tissue; Cx40 label was confined to endothelial cells of blood vessels. Cx45 labelling was localized to detrusor bundles and appeared to be less marked in samples from unstable bladders. Semi-quantitative analysis of Northern blots showed that Cx45 expression in unstable was less than that in stable bladders. These data suggest that intercellular coupling is reduced in detrusor from unstable bladders. Cx45 was localized to the detrusor layer, with Cx 43 more evident in the suburothelial mucosa. Cx45 labelling was less intense in detrusor samples from unstable bladders. These results are consistent with reduced gap junction coupling in detrusor from unstable bladders.

Characterization of the fabrication process of Nb/Al–AlNx/Nb tunnel junctions with low RnA values up to 1 Ω μm2

We discuss and characterize the fabrication process of superconductor–insulator–superconductor (SIS) junctions based on a Nb/Al–AlNx/Nb tri-layer. Utilization of the AlNx tunnel barrier, produced by Al nitridation in a nitrogen glow discharge, enables us to produce high-quality SIS junctions with low RnA values (a product of junction resistance and area). We characterize the tunnel barrier formation and investigate the correlation of plasma characteristics and junction properties. The experiment shows that an increase in nitridation time and applied power results in an increase in junction resistance, while variation in nitrogen pressure has almost no influence on the junction characteristics. Analysing the correlation of junction resistance and plasma properties, it is concluded that the mechanism of tunnel barrier formation is based on nitrogen implantation into the Al layer with subsequent diffusion of nitrogen, stimulated by plasma heating.

Effects of caffeine and raynodine on low pHi-induced changes in gap junction conductance and calcium concentration in crayfish septate axons

Electrical uncoupling of crayfish septate axons with acidification has been shown to cause a substantial increase in [Ca2+]i which closely matches in percent the increase in junctional resistance. To determine the origin of [Ca2+]i increase, septate axons have been exposed either to drugs that influence Ca2+ release from internal stores, caffeine and ryanodine, or to treatments that affect Ca2+ entry. A large increase in junctional resistance and [Ca2+]i maxima above controls resulted from addition of caffeine (10-30 mM) to acetate solutions, while a substantial decrease in both parameters was observed when exposure to acetate-caffeine was preceded by caffeine pretreatment. In contrast, ryanodine (1-10 microM) always caused a significant decrease in junctional resistance and [Ca2+]i maxima when applied either together with acetate or both before and with acetate. Calcium channel blockers such as La3+, Cd2+ and nisoldipine had no effect, while an increase in the [Ca2+] of acetate solutions either decreased junctional resistance and [Ca2+]i maxima or had no effect. The data suggest that cytoplasmic acidification causes an increase in [Ca2+]i by releasing Ca2+ from caffeine and ryanodine-sensitive Ca2+ stores. The increase in [Ca2+]i results in a decrease in gap junction conductance.

Increase in gap junction resistance with acidification in crayfish septate axons is closely related to changes in intracellular calcium but not hydrogen ion concentration

Neutral-carrier pH- and Ca-sensitive microelectrodes were used to investigate the relationship between junctional electrical resistance and either pHi or [Ca2+]i in crayfish septate axons uncoupled by acidification. For measuring [Ca2+]i a new neutral carrier sensor sensitive to picomolar [Ca2+] and virtually insensitive to other ions was used. Uncoupling was induced by superfusing the axons with Na-acetate solutions (pH 6.3). With acetate, the time course of changes in junctional resistance differed markedly from that of pHi or [H+]i, and [H+]i peaked 40-90 sec before junctional resistance. The difference in shape and peak time between pHi and junctional resistance curves caused significant hysteresis in the pHi versus junctional resistance relationship. In addition, junctional resistance maxima reached with slow acidification rates were 3-4 times greater than those with fast acidification of similar magnitude. With acetate, [Ca2+]i increased by approximately one order of magnitude from basal values of 0.1-0.3 microM. The curves describing the time course of changes in [Ca2+]i and junctional resistance matched well with each other in shape, peak time and magnitude. Both junctional resistance and [Ca2+]i recovered following a single exponential decay with a time constant of approximately 2 min. Different rates of acidification caused increases in [Ca2+]i and junctional resistance comparable in magnitude. The data indicate that the increase in junctional resistance induced by acidification is more closely related to [Ca2+]i than to [H+]i.

Sodium pump: its importance to intercellular communication in heart fibres.

The effect of ouabain on the electrical coupling of canine Purkinje cells was investigated. It was found that the glycoside decreases cell communication through an increase in junctional resistance, what supports the view that the sodium pump has an important role on the control of cell communication.

Some bio-electric parameters of early Xenopus embryos

ABSTRACT Membrane potential and resistance were measured in eggs, cleavage stages and blastulae of the South African toad Xenopus laevis, using intracellular microelectrodes. The membrane potential increased from – 6·5 ±2 mV in eggs to –57 ±8· 0 mV at the mid-blastula stage. The input resistance of fertile eggs ranged from 0·5 MΩ to 5 ·0 MΩ corresponding to a specific resistance of 20–200 kΩ cm2. During the first two or three division cycles the input resistance usually decreased by a factor of 2– 10 and then subsequently rose during the blastula stages from a mean value of 600 ± 100 kΩ at stage 5 to 2·0 ±0·5 MΩ at stage 8. At all developmental stages examined, point polarization of a surface cell in the embryo by rectangular current pulses of 0·5–6 × 10−8 A produced voltage deflexions in other surface cells. This was seen even when several (7–8) cell junctions intervened between the current passing and voltage recording microelectrodes at distances of more than 1 mm. These measurements suggest that the junctional resistance is low compared with that at the surface, though the geometrical arrangement of cells is not favourable for calculation of absolute values of membrane resistance. Current spread between cells occurred apparently less easily during mid-blastula stages than at earlier stages in development, perhaps indicating an increase in junctional resistance during development. A comparison has been drawn between the present measurements and similar ones made in another amphibian, Triturus.