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Showing papers on "Conductance published in 1997"


Journal ArticleDOI
10 Oct 1997-Science
TL;DR: In this paper, benzene-1,4-dithiol molecules were self-assembled onto the two facing gold electrodes of a mechanically controllable break junction to form a statically stable gold-sulfur-aryl-solfur-gold system, allowing for direct observation of charge transport through the molecules.
Abstract: Molecules of benzene-1,4-dithiol were self-assembled onto the two facing gold electrodes of a mechanically controllable break junction to form a statically stable gold-sulfur-aryl-sulfur-gold system, allowing for direct observation of charge transport through the molecules. Current-voltage measurements at room temperature demonstrated a highly reproducible apparent gap at about 0.7 volt, and the conductance-voltage curve showed two steps in both bias directions. This study provides a quantative measure of the conductance of a junction containing a single molecule, which is a fundamental step in the emerging area of molecular-scale electronics.

3,114 citations


Journal ArticleDOI
28 Mar 1997-Science
TL;DR: In this paper, the electrical properties of individual bundles of single-walled carbon nanotubes have been measured and the results are interpreted in terms of singleelectron charging and resonant tunneling through the quantized energy levels of the nanotube composing the rope.
Abstract: The electrical properties of individual bundles, or “ropes,” of single-walled carbon nanotubes have been measured. Below about 10 kelvin, the low-bias conductance was suppressed for voltages less than a few millivolts. In addition, dramatic peaks were observed in the conductance as a function of a gate voltage that modulated the number of electrons in the rope. These results are interpreted in terms of single-electron charging and resonant tunneling through the quantized energy levels of the nanotubes composing the rope.

1,173 citations


01 Mar 1997
TL;DR: The electrical properties of individual bundles, or “ropes,” of single-walled carbon nanotubes have been measured, and dramatic peaks were observed in the conductance as a function of a gate voltage that modulated the number of electrons in the rope.
Abstract: The electrical properties of individual bundles, or “ropes,” of single-walled carbon nanotubes have been measured. Below about 10 kelvin, the low-bias conductance was suppressed for voltages less than a few millivolts. In addition, dramatic peaks were observed in the conductance as a function of a gate voltage that modulated the number of electrons in the rope. These results are interpreted in terms of single-electron charging and resonant tunneling through the quantized energy levels of the nanotubes composing the rope.

1,049 citations


Journal ArticleDOI
TL;DR: Results for a cholera toxin B-subunit crystal structure agree well with the measured values, and the difficulty in interpreting such studies is discussed, with the conclusion that measurements on channels of known structure are required.

210 citations


Journal ArticleDOI
TL;DR: In this paper, the authors present experimental results concerning conductance quantization at room temperature in metallic nanowires, both at ambient presure and in ultrahigh vacuum usingscanning tunneling microscopy ~STM!-like devices.
Abstract: Laboratorio de Fi´sica de Sistemas Pequen˜os (CSIC-UAM), Universidad Auto´noma de Madrid,C-IX, Cantoblanco, E-28049-Madrid, Spain~Received 16 November 1995; revised manuscript received 4 October 1996!In this work we present experimental results concerning conductance quantization at room temperature inmetallic nanowires. The experiments are performed both at ambient presure and in ultrahigh vacuum usingscanning tunneling microscopy ~STM!-like devices. Both microscopic and macroscopic electrodes are used.The obtained results are independent of the electrode size, indicating that nanowires form at the last stages ofthe contact-breakage process, independently of the initial size and shape of the contact. In addition, we study~a! the stability of the nanowires, finding their lifetimes remarkably long, on the order of minutes; ~b! theI-V curves for different quantum channels; ~c! nanowires formed and visualized using a STM inside a scanningelectron microscope ~SEM!; ~d! the statistical behavior of the conductance, sampling tens of thousands ofconsecutive conductance experiments, showing clear quantized peaks up to five quanta of conductance; and~e!the length of the conductance plateaus, that might indicate the chaotic behavior of the quantum states formedat the nanowire. The statistical study of the conductance plateau lengths shows a broad Wigner-like distributionwith an average value of 0.13 nm for the first quantum channel. This is at variance with a constant length,predicted and experimentally observed, in the plateaus between force jumps. @S0163-1829~97!00207-5#I. INTRODUCTION

166 citations


Journal ArticleDOI
TL;DR: In this article, the electronic conductance of metal nanocontacts is analyzed in terms of eigenchannels for the transmission, and the transmission through individual eigen channels is calculated numerically for realistic models of gold point contacts based on molecular-dynamics simulation of the elongation of a contact.
Abstract: The electronic conductance of metal nanocontacts is analyzed in terms of eigenchannels for the transmission. The transmission through individual eigenchannels is calculated numerically for realistic models of gold point contacts based on molecular-dynamics simulation of the elongation of a contact. The conductance as a function of contact elongation exhibits a step structure. For the smallest contact areas of one or a few atom diameters, the conductance is typically quantized, and a specific number of almost open eigenchannels can be ascribed. For larger contact areas the scattering leads to partly open channels, but plateaus in the conductance can still be present. We also show that the finite stiffness of the experimental setup can significantly affect the step structure of the conductance curves. @S0163-1829~97!05047-9# Electrical transport in structures with a size on the order of the Fermi wavelength (l F) has been studied intensely in the last decade. For these systems, the conductance is determined by the quantum-mechanical transmission. This was demonstrated clearly by the conductance quantization ~CQ!, which was first observed in fabricated semiconductor structures. 1 More recently the tendency for CQ in nanocontacts of certain metals, especially the single s-valence metals Au, 2,3 Na, 4 Cu, and Ag, 5 was established. In contrast to the former case, the atomic discreteness of the latter systems is of importance in two respects: ~i! The contact narrows in ‘‘jumps’’ upon elongation due to atomic rearrangements. 6 This gives rise to jumps in the conductance and tensile force. 7,3,8‐11 ~ii! The potential confining the electrons inside the contact is corrugated on an atomic length scale which is close to l F . Thus the relevance of the adiabatic picture, 12 with well-defined quantized transmission channels, is far from obvious. We introduce and calculate numerically the general eigenchannel transmissions of realistic models of gold metal nanocontacts. The eigenchannels are defined such that an unambiguous transmission can be assigned to each individual eigenchannel. Thus the conductance is uniquely split into eigenchannel contributions. As an input into the calculations we use molecular-dynamics ~MD! simulations of the elongation of nanocontacts. Our results explicitly show a change in character of the steps in the conductance curves. For very small contact areas, plateaus in the conductance are essentially due to a certain number of open eigenchannels. For a single atom contact, for example, a single eigenchannel is seen to carry almost all the transport. For larger contact areas the picture changes: Due to scattering, partly open channels contribute to the conductance, and the plateaus must be associated with mechanical properties of the contact. The scattering is almost entirely determined by the one-electron potential at the narrowest point. Finally, we show that the observation of plateaus in the conductance curve for the larger contacts will depend significantly on the stiffness of the experimental setup. The conductance can be calculated from the multichannel

165 citations


Journal ArticleDOI
TL;DR: Mapping of functional channels indicates that the IP3R tends to aggregate into microscopic as well as macroscopic clusters, and channel clustering may contribute to complex [Ca2+] signals in cells.
Abstract: Single-channel properties of the Xenopus inositol trisphosphate receptor (IP3R) ion channel were examined by patch clamp electrophysiology of the outer nuclear membrane of isolated oocyte nuclei. With 140 mM K+ as the charge carrier (cytoplasmic [IP3] = 10 microM, free [Ca2+] = 200 nM), the IP3R exhibited four and possibly five conductance states. The conductance of the most-frequently observed state M was 113 pS around 0 mV and approximately 300 pS at 60 mV. The channel was frequently observed with high open probability (mean P(o) = 0.4 at 20 mV). Dwell time distribution analysis revealed at least two kinetic states of M with time constants tau 1 s. Higher cytoplasmic potential increased the relative frequency and tau of the longest closed state. A novel "flicker" kinetic mode was observed, in which the channel alternated rapidly between two new conductance states: F1 and F2. The relative occupation probability of the flicker states exhibited voltage dependence described by a Boltzmann distribution corresponding to 1.33 electron charges moving across the entire electric field during F1 to F2 transitions. Channel run-down or inactivation (tau approximately 30 s) was consistently observed in the continuous presence of IP3 and the absence of change in [Ca2+]. Some (approximately 10%) channel disappearances could be reversed by an increase in voltage before irreversible inactivation. A model for voltage-dependent channel gating is proposed in which one mechanism controls channel opening in both the normal and flicker modes, whereas a separate independent mechanism generates flicker activity and voltage-reversible inactivation. Mapping of functional channels indicates that the IP3R tends to aggregate into microscopic (<1 microm) as well as macroscopic (approximately 10 microm) clusters. Ca2+-independent inactivation of IP3R and channel clustering may contribute to complex [Ca2+] signals in cells.

161 citations


Journal ArticleDOI
03 Jul 1997-Nature
TL;DR: It is shown that the benzodiazepine diazepam can also increase the conductance of GABAAchannels activated by low concentrations of GABA in rat cultured hippocampal neurons, which may be an important way in which these drugs depress excitation in the central nervous system.
Abstract: Benzodiazepines, which are widely used clinically for relief of anxiety and for sedation, are thought to enhance synaptic inhibition in the central nervous system by increasing the open probability of chloride channels activated by the inhibitory neurotransmitter gamma-aminobutyric acid (GABA). Here we show that the benzodiazepine diazepam can also increase the conductance of GABAA channels activated by low concentrations of GABA (0.5 or 5 microM) in rat cultured hippocampal neurons. Before exposure to diazepam, chloride channels activated by GABA had conductances of 8 to 53pS. Diazepam caused a concentration-dependent and reversible increase in the conductance of these channels towards a maximum conductance of 70-80 pS and the effect was as great as 7-fold in channels of lowest initial conductance. Increasing the conductance of GABAA channels tonically activated by low ambient concentrations of GABA in the extracellular environment may be an important way in which these drugs depress excitation in the central nervous system. That any drug has such a large effect on single channel conductance has not been reported previously and has implications for models of channel structure and conductance.

159 citations


Journal ArticleDOI
TL;DR: The data quantitatively demonstrate that the rCx43 gap junction channel is permeable to monovalent atomic and organic cations and anions and the relative permeability sequences are consistent with an Eisenman sequence II or I, respectively.
Abstract: The relative permeability sequences of the rat connexin 43 (rCx43) gap junction channel to seven cations and chloride were examined by double whole cell patch clamp recording of single gap junction channel currents in rCx43 transfected neuroblastoma 2A (N2A) cell pairs. The measured maximal single channel slope conductances (gammaj, in pS) of the junctional current-voltage relationships in 115 mM XCI were RbC1 (103) > or = CsC1 (102) > KC1 (97) > NaC1 (79) > or = LiC1 (78) > TMAC1 (65) > TEAC1 (53) and for 115 mM KY were KBr (105) > KC1 (97) > Kacetate (77) > Kglutamate (61). The single channel conductance- aqueous mobility relationships for the test cations and anions were linear. However, the predicted minimum anionic and cationic conductances of these plots did not accurately predict the rCx43 channel conductance in 115 mM KC1. Instead, the conductance of the rCx43 channel in 115 mM KC1 was accurately predicted from cationic and anionic conductance-mobility plots by applying a mobility scaling factor Dx/Do, which depends upon the relative radii of the permeant ions to an estimated pore radius. Relative permeabilities were determined for all of the monovalent catious and anions tested from asymmetric salt reversal potential measurements and the Goldman-Hodgkin-Katz voltage equation. These experiments estimate the relative chloride to potassium permeability to be 0.13. The relationship between the relative cation permeability and hydrated radius was modeled using the hydrodynamic equation assuming a pore radius of 6.3 +/- 0.4 A. Our data quantitatively demonstrate that the rCx43 gap junction channel is permeable to monovalent atomic and organic cations and anions and the relative permeability sequences are consistent with an Eisenman sequence II or I, respectively. These predictions about the rCx43 channel pore provide a useful basis for future investigations into the structural determinants of the conductance and permeability properties of the connexin channel pore.

128 citations


Journal ArticleDOI
TL;DR: In this article, the authors show that conductance quantization at room temperature is a physical and reliable observation, which is demonstrated by conductance histograms taking all (12 000) consecutive nanowire conductance experiments in Au at room-temperature.
Abstract: This paper shows that conductance quantization at room temperature is a physical and reliable observation. This is demonstrated by conductance histograms taking all (12 000) consecutive nanowire conductance experiments in Au at room temperature. On the other hand, conductance curves in Ni, a room-temperature ferromagnet, show staircase-quantized behavior, but the histograms do not show quantized peaks, most probably due to the lifting of the spin degeneracy.

120 citations


Journal ArticleDOI
TL;DR: The voltage-activated H+ selective conductance of rat alveolar epithelial cells was studied using whole-cell and excised-patch voltage-clamp techniques and the effects of substituting deuterium oxide for water, H2O, on both the conductance and the pH dependence of gating were explored.
Abstract: The voltage-activated H+ selective conductance of rat alveolar epithelial cells was studied using whole-cell and excised-patch voltage-clamp techniques. The effects of substituting deuterium oxide, D2O, for water, H2O, on both the conductance and the pH dependence of gating were explored. D+ was able to permeate proton channels, but with a conductance only about 50% that of H+. The conductance in D2O was reduced more than could be accounted for by bulk solvent isotope effects (i.e., the lower mobility of D+ than H+), suggesting that D+ interacts specifically with the channel during permeation. Evidently the H+ or D+ current is not diffusion limited, and the H+ channel does not behave like a water-filled pore. This result indirectly strengthens the hypothesis that H+ (or D+) and not OH- is the ionic species carrying current. The voltage dependence of H- channel gating characteristically is sensitive to pH0 and pHi and was regulated by pD0 and pDi in an analogous manner. shifting 40 mV/U change in the pD gradient. The time constant of H+ current activation was about three times slower (T(act) was larger) in D2O than in H2O. The size of the isotope effect is consistent with deuterium isotope effects for proton abstraction reactions, suggesting that H+ channel activation requires deprotonation of the channel. In contrast, deactivation (T(tail)) was slowed only by a factor < or = 1.5 in D2O. The results are interpreted within the context of a model for the regulation of H+ channel gating by mutually exclusive protonation at internal and external sites (Cherny, V.V., V.S. Markin, and T.E. DeCoursey. 1995. J. Gen. Physiol. 105:861-896). Most of the kinetic effects of D2O can be explained if the pKa of the external regulatory site is approximately 0.5 pH U higher in D2O.

Journal ArticleDOI
TL;DR: It is proposed that CFTR displays a “weak field strength” anion selectivity sequence and may influence I− permeation and be responsible for the wide range of PI/PCl ratios that have been reported for the CFTR channel.
Abstract: Permeation of cystic fibrosis transmembrane conductance regulator (CFTR) Cl channels by halide ions was studied in stably transfected Chinese hamster ovary cells by using the patch clamp technique. In cell-attached patches with a high Cl pipette solution, the CFTR channel displayed outwardly rectifying currents and had a conductance near the membrane potential of 6.0 pS at 22 degrees C or 8.7 pS at 37 degrees C. The current-voltage relationship became linear when patches were excised into symmetrical, -tris(hydroxymethyl)methyl-2-aminomethane sulfonate (TES)-buffered solutions. Under these conditions, conductance increased from 7.0 pS at 22 degrees C to 10.9 pS at 37 degrees C. The conductance at 22 degrees C was approximately 1.0 pS higher when TES and HEPES were omitted from the solution, suggesting weak, voltage-independent block by pH buffers. The relationship between conductance and Cl activity was hyperbolic and well fitted by a Michaelis-Menten-type function having a of approximately 38 mM and maximum conductance of 10 pS at 22 degrees C. Dilution potentials measured with NaCl gradients indicated high anion selectivity (P/P = 0.003-0.028). Biionic reversal potentials measured immediately after exposure of the cytoplasmic side to various test anions indicated P(1.8) > P(1. 3) > P(1.0) > P(0.17), consistent with a "weak field strength" selectivity site. The same sequence was obtained for external halides, although inward F flow was not observed. Iodide currents were protocol dependent and became blocked after 1-2 min. This coincided with a large shift in the (extrapolated) reversal potential to values indicating a greatly reduced I/Cl permeability ratio (P/P< 0.4). The switch to low I permeability was enhanced at potentials that favored Cl entry into the pore and was not observed in the R347D mutant, which is thought to lack an anion binding site involved in multi-ion pore behavior. Interactions between Cl and I ions may influence I permeation and be responsible for the wide range of P/P ratios that have been reported for the CFTR channel. The low P/P ratio usually reported for CFTR only occurred after entry into an altered permeability state and thus may not be comparable with permeability ratios for other anions, which are obtained in the absence of iodide. We propose that CFTR displays a "weak field strength" anion selectivity sequence.

Journal ArticleDOI
TL;DR: In this paper, the electrical conductance through breaking point contacts in commercial and home-built electromechanical relays under ambient conditions is measured using an automated setup, which makes it possible to record the conductance in a breaking contact 25 000 times during 24 hours.
Abstract: We present measurements of the electrical conductance through breaking point contacts in commercial and home-built electromechanical relays under ambient conditions. Inspired by an experiment published by Costa-Kr\"amer et al. [Surf. Sci. 342, L1144 (1995)], we constructed an automated setup, which makes it possible to record the conductance in a breaking contact 25 000 times during 24 h. Quantization of the conductance in units of ${G}_{0}{=2e}^{2}/h$ is shown to be intimately correlated with the choice of contact material and experimental conditions. This and previous studies clearly demonstrate that Au is a benchmark system, which exhibits quantized conductance independent of the conditions. Peaks in the Au histogram are shown to be asymmetric, which is interpreted as another indicator of conductance quantization. For the more reactive metals Ag and Cu, quantization is also observed, but not as clearly as in Au. In contrast, we find no evidence of quantization in a number of transition metals. It is tentatively suggested that the presence and/or absence of quantization is correlated with impurities in the nanowires. Other deviations from perfect quantization are discussed. It is revealed how differential nonlinearity of analog-to-digital converters introduces significant noise in the corresponding conductance histograms unless a proper compensation is made.


Journal ArticleDOI
TL;DR: In this article, the qualitative influence of patchy stomatal conductance distributions on the values of photosynthesis and intercellular CO 2 concentration as determined by gas-exchange measurements were investigated using computer modelling.
Abstract: The qualitative influence of patchy stomatal conductance distributions on the values of photosynthesis (A) and intercellular CO 2 concentration (c i ) as determined by gas-exchange measurements were investigated using computer modelling. Gas-exchange measurements were simulated for different conductance distributions by modelling photosynthesis explicitly for each patch, summing these rates, and inferring c i from a diffusion equation. Qualitative relationships are presented between conductance distribution features and the difference between assimilation rates measured for patchy and homogeneous leaves at the same c i (A P and A h , respectively). These data show that, although most conductance distributions have little effect on the value of A measured for a given c i , some distribution features (which we have termed 'bimodality', 'position', 'skewness' and 'range') play a key role in controlling the magnitude of these effects. Distributions that are more nearly bimodal, span regions of lower conductance, are right-skewed, or have broader conductance ranges are associated with larger effects on the A(c i ) relationship. To clarify our mathematical analysis and illustrate some of the trends it predicts, we present conductance distributions and gas-exchange data from leaves of Malus dolgo var. Spring Snow that were treated with ABA. The results are discussed in the light of recent controversy over the effect of patchy stomatal conductance on gas-exchange data.

Journal ArticleDOI
07 Mar 1997-Science
TL;DR: The conductance observed in Xenopus laevis oocytes injected with aquaporin1 complementary RNA represented an intrinsic property of the protein, and studies of the AQP1 protein have reevaluated this property.
Abstract: A J Yool et al studied membrane conductance in Xenopus laevis oocytes injected with aquaporin1 complementary RNA ( AQP1 cRNA) and concluded that the conductance observed represented an intrinsic property of the protein ([1][1]) We have reevaluated our own studies of the AQP1 protein and have

Journal ArticleDOI
Yuji Nakajima1, Sakura Takeda1, Tadaaki Nagao1, Shuji Hasegawa1, Xiao Tong 
TL;DR: In this paper, it was shown that the surface state acts as a surface conduction band and the electron mobility in this band is estimated to be on the order of 10 cm{}^{2}$/V s.
Abstract: Photoemission spectroscopy has shown that each Ag atom in its two-dimensional adatom gas (2DAG) phase deposited on the Si(111)-$\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$-Ag surface at room temperature donates one electron into an antibonding surface-state band of this substrate, resulting in a steep increase in electrical conductance through the band. The surface space-charge layer makes no contribution to the conductance increase by the 2DAG adsorption, estimated from the band-bending measurements. When the 2DAG nucleates into three-dimensional Ag microcrystals by further deposition beyond a critical supersaturation coverage, the carrier-doping effect vanishes, returning to a lower conductance. These results reveal that the surface state acts as a surface conduction band. The electron mobility in this band is estimated to be on the order of 10 cm${}^{2}$/V s.

Journal ArticleDOI
TL;DR: In this paper, a flow conductance cell at temperatures between 603 K and 674 K and pressures between 15 MPa and 28 MPa (water densities from 650 to 200 kg·m-3).
Abstract: Conductivities of LiCl(aq), NaCl(aq), NaBr(aq), and CsBr(aq) solutions have been measured in the range of concentrations from 0.013 to 4 × 10-8 mol·dm-3 using a flow conductance cell at temperatures between 603 K and 674 K and pressures between 15 MPa and 28 MPa (water densities from 650 to 200 kg·m-3). Limiting equivalent conductances and ionization equilibrium constants calculated from the concentration dependence of the equivalent conductance are reported. Previous measurements of equilibrium constants in the low-density supercritical region, although of lesser accuracy, agree well with the present results. Even at the critical density and 2.5 K above the critical temperature there is no evidence for the critical effects which make the Debye−Huckel limiting law invalid at the critical isotherm−isobar. No critical scaling of Λ0 is observed. Walden's rule is not obeyed and the Stokes radius increases by 70% near the critical region as the density goes from 700 to 200 kg·m-3, suggesting an increase in the...

Journal ArticleDOI
TL;DR: In this paper, the authors developed a detailed microscopic analysis of electron transport in normal diffusive conductors in the presence of proximity-induced superconducting correlations and calculated the linear conductance of the system, the profile of the electric field and the densities of states.
Abstract: We develop a detailed microscopic analysis of electron transport in normal diffusive conductors in the presence of proximity-induced superconducting correlations. We calculated the linear conductance of the system, the profile of the electric field, and the densities of states. In the case of transparent metallic boundaries the temperature-dependent conductance has a nonmonotonic ``reentrant'' structure. We argue that this behavior is due to nonequilibrium effects occurring in the normal metal in the presence of both superconducting correlations and the electric field. Low transparent tunnel barriers suppress nonequilibrium effects and destroy the reentrant behavior of the conductance. If the wire contains a loop, the conductance shows Aharonov-Bohm oscillations with a period ${\mathrm{\ensuremath{\Phi}}}_{0}$=h/2e as a function of the magnetic flux \ensuremath{\Phi} inside the loop. The amplitude of these oscillations also demonstrates the reentrant behavior. It vanishes at T=0 and decays as 1/T at relatively large temperatures. The latter behavior is due to low-energy correlated electrons which penetrate deep into the normal metal and ``feel'' the effect of the magnetic flux \ensuremath{\Phi}. We point out that the density of states and thus the ``strength'' of the proximity effect can be tuned by the value of the flux inside the loop. Our results are fully consistent with recent experimental findings.

Journal ArticleDOI
10 Apr 1997-EPL
TL;DR: In this paper, the fluctuations and the distribution of the conductance peak spacings of a quantum dot in the Coulomb blockade regime are studied and compared with the predictions of random matrix theory (RMT).
Abstract: The fluctuations and the distribution of the conductance peak spacings of a quantum dot in the Coulomb-blockade regime are studied and compared with the predictions of random matrix theory (RMT). The experimental data were obtained in transport measurements performed on a semiconductor quantum dot fabricated in a GaAs-AlGaAs heterostructure. It is found that the fluctuations in the peak spacings are considerably larger than the mean level spacing in the quantum dot. The distribution of the spacings appears to be Gaussian both for zero and for non-zero magnetic field and deviates strongly from the RMT predictions.

Journal ArticleDOI
TL;DR: In this paper, the Coulomb blockade of conductance is progressively shifted at lower temperatures, and the experimental results agree quantitatively with the available $1/T$ expansion at high temperature, and qualitatively with predictions of an effective two-state model at low temperature.
Abstract: We have investigated the suppression of single-electron charging effects in metallic single-electron transistors when the conductance of the tunnel junctions becomes larger than the conductance quantum ${e}^{2}/h$. We find that the Coulomb blockade of the conductance is progressively shifted at lower temperatures. The experimental results agree quantitatively with the available $1/T$ expansion at high temperature, and qualitatively with the predictions of an effective two-state model at low temperature, which predicts at $T\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0$ a blockade of conductance for all gate voltages.

Journal ArticleDOI
TL;DR: In this paper, an improved statistical study is presented, where histograms built with thousands of consecutive gold contact breakage experiments at 4 K show up to seven conductance peaks, pointing to a conductance quantization behavior that is the same at low, intermediate, and high (RT) temperatures.
Abstract: Driving a scanning tunneling microscope (STM) tip into a metallic surface and pulling it out afterwards results in the formation of a nanometer-sized wire (nanowire) between the electrodes. The electrical conduction measured during this process shows signs of quantized conductance in units of ${\mathrm{G}}_{0}$=${2\mathrm{e}}^{2}$/h. Due to the inherent irreproducibility of the measured conductance curves, the standard technique has been to construct histograms with a few hundred selected curves. These histograms, for gold nanowires at room temperature, have shown three to four peaks at integer values of ${2\mathrm{e}}^{2}$/h, while in a low-temperature mechanically controlled break junction study, with statistics using only 65 curves, only the first peak has been reported. A proposed explanation for this apparent experimental discrepancy has been a higher nanowire temperature arising from the higher retraction speed used in scanning tunneling microscopy measurements. However, our simple estimation using macroscopic heat transport theory produces a very low temperature rise, less than 1 \ensuremath{\mu}K. In this work, an improved statistical study is presented, where histograms built with thousands of consecutive gold contact breakage experiments at 4 K show up to seven conductance peaks. Thus, no significant differences with our previous room-temperature (RT) studies are observed, pointing to a conductance quantization behavior that is the same at low, intermediate, and high (RT) temperatures.

Journal ArticleDOI
TL;DR: This hypothesis, which is based on measurements of ion selectivity alongside ion current, applies to pores through some synthetic membranes and through channels—such as those created by certain toxins—that remain (at least partially) open in the low conductance state.
Abstract: Fluctuation of ion current, between a high conductance and a low conductance state, through biological ion channels and pores is assumed to arise from conformational changes between an "open" and a "closed" configuration. Here we offer an additional mechanism that arises from changes in ionization of fixed charges within, or at the mouth of, a channel or pore. Our hypothesis, which is based on measurements of ion selectivity alongside ion current, applies to pores through some synthetic membranes and through channels-such as those created by certain toxins-that remain (at least partially) open in the low conductance state. It may also explain the phenomena of "open channel noise" and "substate behavior" that characterize several endogenous ion channels and should be considered when modeling the behavior of such channels.

Journal ArticleDOI
TL;DR: In this paper, tunneling conductance spectra of normal metal/insulator/triplet superconductor junctions are investigated theoretically and the existence of a residual density of states, peculiar to nonunitary states, is shown to have a significant influence on the properties of the tunneling spectra.
Abstract: Tunneling conductance spectra of normal metal/insulator/triplet superconductor junctions are investigated theoretically. As triplet paring states we select several types of symmetries that are promising candidates for the superconducting states in UPt$_{3}$ and in Sr$_{2}$RuO$_{4}$. The calculated conductance spectra are sensitive to the orientation of the junction which reflects the anisotropy of the pairing states. They show either zero-bias conductance peaks or gap-like structures depending on the orientation of the junctions. The existence of a residual density of states, peculiar to nonunitary states, is shown to have a significant influence on the properties of the conductance spectra. Present results serve as a guidefor the experimental determination of the symmetry of the pair potentials in UPt$_{3}$ and Sr$_{2}$RuO$_{4}$.

Journal ArticleDOI
TL;DR: In this paper, the authors observed quantized conductance up to 10 K in epitaxially regrown, heterostructurally defined, 100nm-wide Ga0.25In0.75As/InP quantum wires.
Abstract: We report on the observation of quantized conductance up to 10 K in epitaxially regrown, heterostructurally defined, 100-nm-wide Ga0.25In0.75As/InP quantum wires. In addition to the plateaus at integer steps of 2e2/h, we observe plateaus at 0.2(2e2/h), 0.7(2e2/h), and 1.5(2e2/h), indicating spin polarization at zero magnetic field. Of these, the first two plateaus appear to evolve into one at around 0.5(2e2/h) when the sample is subjected to a magnetic field parallel to the quantum wire. The observation of quantized conductance is made possible by the substantial improvement in the quality of the interface by regrowth.

Journal ArticleDOI
TL;DR: The K+ conductance that was thought to be irreversibly lost at ‐80 mV or more negative holding potentials was fully recovered, however, after a prolonged change in the holding potential to depolarized values (above ‐50 mV).
Abstract: 1. Shaker B K+ channels, expressed in the insect cell line Sf9, were studied in zero K+, Na+ or N-methyl-D-glucamine (NMG)-containing solutions. In the absence of K+ ions on both sides of the membrane, the K+ conductance collapsed with the delivery of short depolarizing pulses that activated the channels. The collapse of the conductance was fully prevented when the channels were kept closed at a holding potential of -80 mV. 2. The fall in K+ conductance had the notable characteristic of being strikingly stable. At -80 mV or more negative holding potentials, the conductance never recovered (cells observed for up to 1 h). 3. The extent of collapse of the K+ conductance depended on the number of depolarizing activating pulses applied in zero K+ solutions. For moderate to low frequencies of pulsing (1 to 0.002 Hz), the extent of the collapse did not depend on the frequency. 4. K+, Rb+, Cs+ and NH4+ added to the external Na+ solution impeded the fall in K+ conductance. 5. TEA added to the external, zero K+, Na(+)-containing solution also precluded the fall of the conductance. The protection by TEA paralleled its block of the outward K+ currents recorded with standard recording solutions. 6. The fall in K+ conductance was prevented by depolarized holding potentials. 7. The K+ conductance that was thought to be irreversibly lost at -80 mV or more negative holding potentials was fully recovered, however, after a prolonged (tens of seconds to minutes) change in the holding potential to depolarized values (above -50 mV). Full recovery could be obtained at any time after the former halt of the conductance.

Journal ArticleDOI
TL;DR: In this paper, the authors measured the transport properties of quasi-one-dimensional ballistic constrictions defined on a high-mobility two-dimensional hole gas and found that the conductance quantization of the constriction is changed from even to odd multiples of ${\mathrm{e}}^{2}$/h as a function of the magnetic field in the plane of the heterojunction, as spin splitting causes the subbands of the 1D channel to cross.
Abstract: We have measured the transport properties of quasi-one-dimensional ballistic constrictions defined on a high-mobility two-dimensional hole gas. The conductance quantization of the constrictions is changed from even to odd multiples of ${\mathrm{e}}^{2}$/h as a function of the magnetic field in the plane of the heterojunction, as spin splitting causes the subbands of the one-dimensional (1D) channel to cross. We calculate the in-plane g factors of the 1D subbands using the magnetic fields at which crossing occurs and find that they increase as the number of occupied 1D subbands decreases. We attribute this enhancement of the g factor to Coulomb interactions.

Journal ArticleDOI
TL;DR: The number of ATP‐gated ion channels on individual OHCs, presumed to be localized to the stereocilia, increases from approximately 6000 in turn 4 cells to 16,500 in turn 1 cells, based on estimates of unitary conductance and average maximum ATP‐activated OHC conductance (2 mM ATP).
Abstract: 1. Whole-cell patch-clamp recordings were used to determine the variation in the P2X receptor conductance, activated by extracellular ATP, in outer hair cells (OHCs) isolated from each of the four turns of the guinea-pig cochlea. 2. In standard solution (containing 1.5 mM Ca2+) slope conductances were determined in OHCs of known origin from current-voltage relationships obtained from voltage ramps applied between -100 and +50 mV. Membrane conductance throughout this voltage range was greatest in OHCs originating from the basal (high frequency encoding) region of the cochlea. This gradient in OHC conductance from apex to base of the cochlea can be attributed to variation in expression of both a negatively activated K+ conductance and a TEA-sensitive outwardly rectifying K+ conductance. OHC slope conductance measured about a membrane potential of -75 mV increased from a mean of 33.5 nS in the apical region (turn 4) to 96.8 nS in the basal region (turn 1) of the cochlea. 3. Removal of external Ca2+ reduced OHC conductance by an average of 25%, reflecting a Ca2+ dependence of the background conductances in these cells. In zero external Ca2+ the mean slope conductance measured at -75 mV in the apical turn was 25.0 nS compared with 73.8 nS in the basal turn. 4. In Ca(2+)-free solution both 2 mM and 4 microM ATP produced inward currents that were progressively larger in OHCs originating from more basal regions of the cochlea. The steady-state inward current elicited by 2 mM extracellular ATP increased from -1.44 to -3.26 nA for turns 4 and 1, respectively. 5. The P2X receptor conductance was determined between -100 and +50 mV by comparing voltage ramps in the presence and absence of extracellular ATP in Ca(2+)-free solution. The conductance was inwardly rectifying with a reversal potential close to 0 mV. Measured close to the resting membrane potential of the cells (-75 mV), 2 mM ATP elicited an average 300% increase in conductance in parallel with the systematic increase in background conductance which occurs in OHCs originating from the more basal regions of the cochlea. The conductance at -75 mV activated by 2 mM ATP increased from a mean of 59.6 nS in turn 4 OHCs to a mean of 166.2 nS in turn 1 OHCs. The conductance activated by 4 microM ATP was also greater in the basal turn OHCs (45.3 nS) than in the apical region OHCs (5.9 nS). 6. The number of ATP-gated ion channels on individual OHCs, presumed to be localized to the stereocilia, increases from approximately 6000 in turn 4 cells to 16,500 in turn 1 cells, based on estimates of unitary conductance and average maximum ATP-activated OHC conductance (2 mM ATP).

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TL;DR: In this article, the quantum transport properties of a chain of Si atoms connected to the outside through long leads were investigated and the Si wires were found to be metallic and they observed quantized conductance in units of 2e − 2 − 2 /h.
Abstract: We have performed first-principles pesudopotential calculations of the quantum transport properties of a chain of Si atoms connected to the outside through long leads. By solving a three-dimensional quantum scattering problem we have computed the conductance for several atomic wires with up to eight Si atoms. The Si atomic wires are found to be metallic and we observed quantized conductance in units of ${2e}^{2}/h.$ A conductance dip is found to develop near the onset of the second quantized plateau as the number of atoms increases, and this can be explained by the existence of a gap in the density of states when the atomic chain is infinitely long.

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TL;DR: In this article, room temperature conductance transients in the SiNx:H/Si interface were reported and explained in terms of a disorder-induced gap-state continuum model for the interfacial defects.
Abstract: Room temperature conductance transients in the SiNx:H/Si interface are reported. Silicon nitride thin films were directly deposited on silicon by the low temperature electron-cyclotron-resonance plasma method. The shape of the conductance transients varies with the frequency at which they are obtained. This behavior is explained in terms of a disorder-induced gap-state continuum model for the interfacial defects. A perfect agreement between experiment and theory is obtained proving the validity of the model.