Analysis Of Conductance Fluctuations Research Articles

Quantum interference in silicon one-dimensional junctionless nanowire field-effect transistors

We investigate the low-temperature transport in 8-nm-diam Si junctionless nanowire field-effect transistors fabricated by top down techniques with a wraparound gate and two different phosphorus doping concentrations. First we extract the intrinsic gate capacitance of the device geometry from a device that demonstrates Coulomb blockade at 12 mK with over 500 Coulomb peaks across a gate-voltage range of 6 V indicating the formation of an island in the entire 150-nm-long nanowire channel. In two other devices, made from silicon on insulator wafers that were doped to an activated dopant concentration of Si:P $4\ifmmode\times\else\texttimes\fi{}{10}^{19}$ and $2\ifmmode\times\else\texttimes\fi{}{10}^{20}\phantom{\rule{4pt}{0ex}}{\text{cm}}^{\ensuremath{-}3}$, we observe quantum interference and use the extracted gate coupling to determine the mean free paths from the dominant energy scale on the gate-voltage axis. For the higher doped device, the analysis yields a mean free path of $4\ifmmode\pm\else\textpm\fi{}2\phantom{\rule{4pt}{0ex}}\text{nm}$, which is on the order of the average spacing of phosphorus atoms and suggests scattering on unactivated or activated dopants. For the device with an implanted phosphorus density of $4\ifmmode\times\else\texttimes\fi{}{10}^{19}\phantom{\rule{4pt}{0ex}}{\text{cm}}^{\ensuremath{-}3}$, the quantum interference effects suggest a mean free path of $10\ifmmode\pm\else\textpm\fi{}2\phantom{\rule{4pt}{0ex}}\text{nm}$, which is comparable to the nanowire width, and thus allows for coherent formation of transversal modes. The results suggest that the low-temperature mobility is limited by scattering on phosphorus dopants rather than the expected surface roughness scattering for nanowires with diameters larger than or comparable to the Fermi wavelength. A temperature-dependent analysis of universal conductance fluctuations indicates a phase-coherence length greater than the nanowire length for temperatures below 1.9 K, and decoherence from one-dimensional electron-electron interactions dominates transport for higher temperatures. Our measurements, therefore, provide insight into scattering and dephasing mechanisms in technologically relevant silicon device geometries, which will help with future design choices with regard to, e.g., doping density.

Evidence of Gumbel distributions of conductance fluctuations in bacteriorhodopsin thin films

By considering a set of experiments carried out on bacteriorhodopsin in vitro by Casuso et al (2007 Phys. Rev. E 76 041919), we extract the conductance as function of the applied voltage. The microscopic interpretation of experiments shows that charge transfer is ruled by a direct tunneling (DT) mechanism at low bias and by a Fowler–Nordheim (FN) tunneling mechanism at high bias. A nucleation region at the cross-over between the DT and FN regimes can be identified. A theoretical analysis of conductance fluctuations is performed by calculating the corresponding variance and the probability density functions (PDFs): these constitute a powerful indicator in order to understand the internal dynamics of the system. Conductance fluctuations are non-Gaussian and follow well the standard generalized Gumbel distributions G(a). In particular, at low bias, the PDFs are bimodal and can be resolved in at least a couple of G(a) functions with different values of the shape parameter a. The nucleation region is characterized by a single Gumbel distribution, G(1). At increasing bias, the G(1) distribution turns in a bimodal distribution. We discuss possible correlations between the voltage dependence of the G(a) and the microscopic mechanisms that determine the electrical response of the system.

Quantum interference corrections to magnetoconductivity in graphene

We studied weak localization (WL) and antilocalization (WAL) in graphene at temperatures between 0.3 K and 15 K. At low carrier density, we observed a transition from WL to WAL driven by the increasing of the magnetic field while at high carrier density, WAL was suppressed as a consequence of trigonal warping of the conical energy bands. We analyzed the magnetic-field-driven WL-WAL transition, evaluating the relative strengths of the various elastic-scattering mechanisms and estimating the decoherence lengths (${l}_{\ensuremath{\varphi}}$) and rates (${\ensuremath{\tau}}_{\ensuremath{\varphi}}^{\ensuremath{-}1}$) as a function of temperature, using an alternative method with respect to previously reported studies. We relate the small values of ${l}_{\ensuremath{\varphi}}$ here reported, confirmed by a complementary analysis of universal conductance fluctuations, to an effective breaking of time-reversal symmetry due to the slowly varying disorder of the long-range type.

Time-series analysis approach for the identification of flooding/loading transition in gas–liquid stirred tank reactors

This work provides a simple method for the objective, quantitative and non-intrusive detection of flooding/loading transition in gas/liquid systems in agitated vessels starting from the time-series analysis of conductance fluctuations along a generic cross section of the vessel. The conductance probe is designed in order to introduce minimal perturbations of the flow field within the vessel. A thorough statistical characterization of the time series collected by the probe provides deeper insight into the fluid dynamics of gas–liquid systems in stirred vessels as well as a simple, objective way to detect flooding/loading transitions.

Magnetic-field-controlled electron dynamics in quantum cavities

Although soft confinement of electrons in ballistic cavities gives rise to a power law of classical probability distributions, the behavior is limited to a narrow range of the variable if the potential is not substantially smooth. The presence of a magnetic field normal to the cavity extends the power-law regime by generating a hierarchical phase space structure. The change in dynamics induced by the magnetic field is confirmed experimentally through analysis of conductance fluctuations in quantum cavities defined by electrostatic gates on a high-mobility heterojunction. Nonfractal conductance fluctuations as a function of the gate bias at zero magnetic field are transformed to be fractal when the cyclotron radius is comparable to the cavity size.

Emittance fluctuations in a mesoscopic diffusive conductor

We report a first principles analysis of low frequency dynamic conductance fluctuations for disordered two-dimensional mesoscopic conductors. In the transport regime where dc conductance shows the familiar universal conductance fluctuations, we discovered that the low frequency emittance also fluctuates with an amplitude that is independent of the impurity scattering strength, showing a degree of generic behavior. When the impurity density is increased such that the dynamic response of the conductor changes from inductivelike to capacitivelike, the emittance distribution is found to cross over from Gaussian-like to non-Gaussian-like; the latter is qualitatively consistent with random matrix theory.

Distribution-function analysis of mesoscopic hopping conductance fluctuations.

Variable-range hopping (VRH) conductance fluctuations in the gate-voltage characteristics of mesoscopic gallium arsenide and silicon transistors are analyzed by means of their full distribution functions (DF's). The forms of the DF predicted by the theory of Raikh and Ruzin have been verified under controlled conditions for both the long, narrow wire and the short, wide channel geometries. The variation of the mean square fluctuation size with temperature in wires fabricated from both materials is found to be described quantitatively by Lee's model of VRH along a one-dimensional chain. Armed with this quantitative validation of the VRH model, the DF method is applied to the problem of magnetoconductance in the insulating regime. Here a nonmonotonic variation of the magnetoconductance is observed in silicon metal-oxide-semiconductor field-effect transistors whose sign at low magnetic fields is dependent on the channel geometry. The origin of this effect is discussed within the framework of the interference model of VRH magnetoconductance in terms of a narrowing of the DF in a magnetic field. \textcopyright{} 1996 The American Physical Society.

Low-dimensional transports in [formula omitted] quasi-one-dimensional wires by a correlation field analysis of the phase coherent interferences

Carrier scattering processes in quasi-one-dimensional wires were studied by a correlation field analysis of conductance fluctuations observed in the low-temperature magnetoresistance of GaAs AlGaAs As split gate wires. At low magnetic field, the amplitude and the correlation field ΔB c of universal conductance fluctuations (UCF) were observed to be independent of the field and to retain universality. However, at high fields, ΔB c was found to increase as a power law of the magnetic field. The experimental results were compared with a semi-classical model for electron propagation at high fields. The models led to a different field dependence with the power law exponents expected for diffusive and ballistic transports. It was found that analysis of the power law exponent is an important tool for the characterization of electron wave propagation in quantum wires.

Aspects of Chaos in Nuclear Physics

This article addresses the role of such concepts as chaos and predictability in the context of nuclear physics. The topic of this article is closely linked with such diverse areas as random-matrix theory, chaos in classical dynamical systems, statistical mechanics of small quantum systems, and the theory of disordered solids. We present recent information on nuclear data and on their analysis in terms of random-matrix models, a summary of work done on classical chaotic systems, on their quantum analogues, and on special systems like the hydrogen atom in a strong magnetic field. Also, we discuss how random-matrix models can be used to simulate chaotic behaviour in small quantum systems, the role of symmetries (isospin, parity, and time-reversal) in chaotic quantum (nuclear) systems, and how chaos surfaces in experimental and theoretical investigations in molecular physics, in the physics of small clusters, and the analysis of conductance fluctuations in solids. (AIP)

Conductance fluctuations in small disordered conductors: Thin-lead and isolated geometries.

We extend the analysis of conductance fluctuations in small disordered metallic systems beyond the conventional thick-lead geometry to thin-lead and isolated geometries. We find that, for the thin-lead geometry, the conductance fluctuations are still given by the ``universal'' value ${e}^{2}$/h, independent of the lead width. In the isolated geometry, the conductance fluctuation is enhanced by a factor (${L}_{\mathrm{in}/\mathrm{L}{)}^{2}\mathrm{\ensuremath{\gg}}1}$ over ${e}^{2}$/h. The typical distance between consecutive peaks and valleys in the structure of conductance fluctuations, both as a function of external magnetic field and of chemical potential, is found to be dramatically reduced in both of these restrictive geometries.

Interpretation of 1/ F fluctuations in ion conducting membranes

The main objective of this work is to resolve some uncertainties associated with the analysis of conductance fluctuations that exhibit 1/f spectral density. To this end, we derive mathematical conditions under which a discrete summation of Lorentzian functions best approximates a strictly 1/f density over a given frequency range. The intrinsic errors associated with spectral density estimates are considered and used as a constraint to determine the smallest number of optimally chosen Lorentzians required to fit a 1/f-like spectrum in a statistically acceptable manner. The results provide criteria concerning the extent to which mechanisms generating a strictly 1/f spectra may be distinguished from those generating sums of Lorentzian spectra. It is found, in particular, that 1/f-like fluctuation spectra observed in a variety of biological and model membranes may well arise from the summation of a few Lorentzian components having appropriate amplitudes and corner frequencies. Consideration of physically realistic models of ion conductive channels indicates that 1/f-like conductance fluctuation spectra could originate naturally as a direct consequence of thermodynamic constraints upon the coefficients of Lorentzian components.

Single-shot channel activation accounts for duration of inhibitory postsynaptic potentials in a central neuron.

In the goldfish Mauthner cell, inhibitory postsynaptic currents evoked by intracellular stimulation of presynaptic neurons decay exponentially, with a mean time constant of 6.65 milliseconds. Analysis of membrane conductance fluctuations induced by iontophoresis of glycine and gamma-aminobutyric acid indicates a mean inhibitory channel lifetime of 7.15 milliseconds. The results thus suggest that the relaxation kinetics of activated inhibitory channels are rate-limiting during decay of the inhibitory postsynaptic current.

Alamethicin-induced single channel conductance fluctuations in biological membranes.

Alamethicin, a polypeptide of molecular weight ∼2,000, induces voltage-dependent conductance phenomena in artificial lipid bilayer membranes that are in some ways similar to those found in excitable membranes1. Alamethicin-indueed conductance is probably a consequence of the formation of predominantly cation-selective channels which span the bilayer, and single open channels fluctuate at random between several well defined conductance states2–5. Statistical analysis of single channel conductance fluctuations in planar lipid bilayers of various compositions has revealed that, whereas the conductance values are relatively constant in a variety of membranes, the average lifetime of the individual conductance states depends strongly on the lipid used to form the bilayer. This variability has tentatively been attributed to differences in lipid fluidity6,7. Alamethicin is therefore a probe of some aspects of membrane composition. The study of single alamethicin channels in various natural membranes would allow comparison with the well defined bilayer systems and might yield information on lipid properties of natural membranes that could be important for the functioning of natural membrane channels and receptors. The experiments reported here show that alamethicin channels have similar conductances in the sarcolemmal (surface) membrane of frog and rat muscle and in lipid bilayers formed from synthetic lecithin. The average lifetime of the conductance states is, however, one order of magnitude larger in the biological membranes.

GABA and glycine may share the same conductance channel on cultured mammalian neurones

ANALYSIS of conductance fluctuations induced during application of putative neurotransmitters to postsynaptic membranes has led to estimates of the unitary conductance and kinetics of the underlying ionic channels. Estimates have been made for a variety of receptor-coupled channels activated on invertebrate and vertebrate membranes (for review see ref. 1). Recently we have applied the technique to mouse spinal neurones grown in tissue culture and have estimated the conductance and duration of the single channel events underlying responses to the putative inhibitory transmitter γ-aminobutyric acid (GABA)2. We have extended our studies to another inhibitory transmitter, glycine, and report here that channels activated by glycine have properties distinctly different from those activated by GABA and that the amino acid receptors may share a conductance mechanism.

GABA-induced conductance fluctuations in cultured spinal neurones.

ANALYSIS of conductance fluctuations induced by putative neurotransmitters has provided a new level of resolution of the molecular events underlying postsynaptic membrane responses1–3. So far, conductance fluctuation (or ‘noise’) analysis has been applied to neuromuscular synapses in vertebrates and invertebrates1–6 and has not yet been applied at synapses in the mammalian central nervous system (CNS), largely because of the inherent complexity and relative inaccessibility of the CNS. Recently, methods for growing dissociated neurones derived from fetal mouse spinal cord in cell culture have become sufficiently established7 to allow long-term stable intracellular recording. Using this preparation we have applied fluctuation analysis to the membrane current flow induced by the putative inhibitory amino acid transmitter γ-aminobutyric acid (GABA) and report here an initial estimate of the conductance and duration of ion channels activated by GABA.

Conductance fluctuations and ionic pores in membranes.

This review describes recent progress in the analysis of statistical fluctuations in membrane conductance. The discussion is limited to cases where noise analysis was performed for the sake of obtaining information about the molecular properties of the membrane constituents and, therefore, does not cover work concerned with systems analysis that uses noise driving. The presentation is divided into two parts. The first part begins with a simplified treatment of the mathematical theory, based on the restrictive assumption that ionic channels open and close independently and can exist in only two states (open and closed). This minimal mathematical theory provides the basis for making certain kinds of inferences about the properties of single channels in artificial biJayers and post junctional synaptic membranes. Specifically, in these relatively simple instances, analysis of conductance fluctuations can give estimates of two important channel characteristics, the mean length of time a channel remains open and the conduc­ tance of its open state. The second part treats the mathematical basis for fluctuation analysis from a more general point of view without the restrictions imposed in the simplified development: channels are permitted to have more than two states. In this part, the analysis is extended to include inferences beyond those involving simply amplitude and duration as discussed in the first part, and we stress the relation between the kinetics of channel gating as revealed by fluctuation analysis and the relaxation kinetics measured in standard voltage clamp experiments. The extended theory provides the background for surveying applications of fluctuation analysis to more complicated situations in the pharmacologically modified postsynaptic mem­ brane and in gating of sodium and potassium channels of axons.

Inferences about Membrane Properties from Electrical Noise Measurements

Four sources of electrical noise in biological membranes, each with a different physical basis, are discussed; the analysis of each type of noise potentially yields a different sort of information about membrane properties. (a) From the thermal noise spectrum, the passive membrane impedance may be obtained, so that thermal noise measurements are essentially equivalent to the type of since wave analysis carried out by Cole and Curtis. (b) If adequately high frequency measurements could be made, the shot noise spectrum should give information about the average motion of a single ion within the membrane. (c) The number of charge carriers and single ion mobilities within the membrane can possibly be inferred from measurements of noise with a 1/f spectrum. Available data indicate, for example, that increases in axon membrane conductance are not achieved by modulations in the mobility of ions within the membrane. (d) Fluctuations arising from the mechanisms normally responsible for membrane conductance changes can produce a type of electrical noise. Analysis of such conductance fluctuations provides a way to assess the validity of various microscopic models for the behavior of individual channels. Two different probabilistic interpretations of the Hodgkin-Huxley equations are investigated here and shown to yield different predictions about the spectrum of conductance fluctuations; thus, appropriate noise measurements may serve to eliminate certain classes of microscopic models for membrane conductance changes. Further, it is shown how the analysis of conductance fluctuations can, in some circumstances, provide an estimate of the conductance of a single channel.