Junction Fluctuations Research Articles

Noise properties of ferromagnetic tunnel junctions

We report measurements of voltage fluctuations in magnetic tunnel junctions which exhibit both high and low magnetoresistance (MR). The voltage noise power normalized to the square of the junction bias voltage was 10−14/Hz at a frequency of 1 Hz in a high MR junction. Low MR junctions had significantly higher noise power at 1 Hz and the origin of the noise was not magnetic. In these junctions, random telegraph noise was observed over a wide range of temperatures and junction biases. The results are consistent with a two-channel model of conduction, one of which is spin independent and gives rise to large noise. A noise measuring technique provides evidence for bias-dependent current-path rearrangements. The data support the existence of an inhomogeneous (filamentary-like) current-flow pattern across the tunnel junction associated with the spin-independent channel.

Correlated fluctuations in polymer networks

Polymer micronetworks with Cayley tree and random topologies are compared. The fluctuations of junctions in random networks exhibit significant departures from the mean value corresponding to those in Cayley tree topology. Correlations among the fluctuations are examined for the two types of networks. Correlations are stronger, and cover longer distances along the chain contours in random networks, compared to those observed in Cayley trees. This is a consequence of the non-uniform spatial distribution of connected vertices in a random network, as opposed to Cayley trees in which chains extend between junctions located in successive tiers only. The relaxation spectra of the two types of networks are compared. The cumulative distribution of frequencies obeys a power law dependence on frequency, with exponents of 1.4 and 1.1 for trifunctional Cayley tree and random networks, respectively. These exponents are significantly smaller than the Debye value of 3 for regular three dimensional crystals.

Quantum Chaotic Scattering and Resistance Fluctuations in Mesoscopic Junctions

Quantum Chaotic Scattering and Resistance Fluctuations in Mesoscopic Junctions

Magnetic field and microwave effects on critical current fluctuations in HTS grain-boundary Josephson junctions

The excess noise levels in HTS grain boundary Josephson junctions, when expressed as critical current fluctuations |/spl delta/I/sub c/|, vary linearly with I/sub c/ as the temperature changes. By contrast, when the critical current is suppressed by an external magnetic field, the measured fluctuations |/spl delta/I/sub c/| remain nearly constant while L itself undergoes order-of-magnitude changes. This effect, which does not seem to have been widely recognized, is examined with respect to the spatial modulation of the critical current density within the junction. Calculations based on this model are in good agreement with our experimental measurements. In related studies, we measure the excess noise levels between Shapiro steps in a microwave-irradiated junction, and find them to agree with a modified version of the Likharev-Semenov equation for thermal noise.

The effects of solution concentration on network properties of thin films of polyorganosiloxanes crosslinked in the solid state

The stress-strain properties of polyorganosiloxane networks crosslinked in the solid state but cast from trichloroethylene solutions with polymer volume fractions in the range 0.018 to 0.158 were investigated. The polyorganosiloxane studied was polydimethylsiloxane (PDMS) containing crosslinkable SiCHCH 2 and SiH functional groups. The moduli were found to increase with increasing concentration of the solution from which the network was cast, the same trend that has been previously observed for networks actually crosslinked in solution. These results were explained in terms of the theories of rubber elasticity. The PDMS networks formed from concentrated solution had a relatively higher effective interconnectivity of the network chains and a higher degree of constraints of fluctuations of the network junctions than did the networks formed from more dilute solutions. Since the chemical crosslink density was the same for all the networks, those formed from the more concentrated solutions had a higher number of physical entanglements. These results suggest that the time scale for conformational rearrangement of the rubber is longer than that for solvent evaporation, so that the solution conformation is maintained in the solid state.

Rubber elasticity: A contact-probability model with harmonic entanglement constraints

This paper is devoted to the theoretical analysis of nonideality effects in stretched rubber samples. The joint effect of packing forces and topological constraints is accounted for by adopting harmonic potentials between all pairs of network atoms, in addition to fixing a suitable set of junctions at the macroscopic sample surface according to the James–Guth theory. The potential minima are set at the average, affinely deformed interatomic distances. The force constant of each interaction is proportional to the probability of interatomic contact in the undeformed state and is inversely proportional to the square strain ratio along any space direction, thus accounting for the variation of the entanglement concentration with sample stretching. The proportionality factor of the pair potential is an adjustable parameter of the theory. A periodic coarse-grained model is used and the sample free energy is evaluated through normal-mode self-consistent analysis. Both the Mooney effect and the observed radius of gyration of the chain strands projected along different directions are properly accounted for. The results are similar to those of the Ronca–Allegra theory, which is based on direct application of constraints to the junction fluctuations. However, the present approach also embodies features of the theories which adopt the tube model. Finally, the variation of the Mooney constant C2 with sample swelling is accounted for in a semiquantitative way.

微小 Josephson 接合での電荷/位相のスクィージング

微小 Josephson 接合での電荷/位相のスクィージング

Correlation of critical current and resistance fluctuations in bicrystal grain boundary Josephson junctions

We have performed a detailed analysis of the low frequency 1/f noise in YBa2Cu3O7−δ and Bi2Sr2CaCu2O8+x grain boundary Josephson junctions (GBJs) fabricated on SrTiO3 bicrystal substrates. The normalized fluctuation of the critical current, δIc/Ic, and the normal resistance, δRn/Rn were found to be almost independent of temperature and the misorientation angle. Furthermore, the magnitude of the fluctuations is very similar for both high-Tc cuprates. Correlation experiments showed that the fluctuations of Ic and Rn are anti-correlated. Our analysis strongly suggests that the source of 1/f noise in high-Tc bicrystal GBJs are localized defect states in an insulating grain boundary barrier with fluctuating electron occupation. The effective charge trapping time within single traps was found to decay exponentially with increasing bias voltage.

Two-level current fluctuations in ultrasmall double tunnel junctions†

Two level current fluctuations (TLFs) are reported in a very large normal tunnel junction (area about 0-1 mm2) at liquid nitrogen temperature. Due to the presence of small Pt particles inside the tunnel barrier and the non-uniformity of a SiO insulator layer, very narrow current channels are created. TLFs are believed to result from single electrons trapped at defect sites within the insulator barrier of the junction. 1-V, total noise power versus bias and dl/AV characteristics were measured and a signal analysis of the TLFs was also performed.

Josephson-Current Fluctuations in Dirty SNS Junctions

Sample-to-sample fluctuations of the Josephson current in superconductor-dirty normal metal-superconductor junctions are studied numerically using a tight-binding model. Particular attention is given to the cases where the correlation energy of the normal metal is smaller than the superconducting pair potential. It is found that the amplitude of the critical-current fluctuation is of order of the correlation energy. This result is in qualitative agreement with the theory of Al'tshuler and Spivak.

Resonant tunneling and charge fluctuations in mesoscopic tunnel junctions

Quantum fluctuations in mesoscopic tunnel junctions are studied in the presence of high charging energies. The processes of sequential tunneling, electron cotunneling and resonant tunneling are analyzed within a new nonperturbative resummation technique. The current and the average charge are shown to be related to the spectral density describing the charge excitations of the single electron transistor. An energy-dependent finite lifetime is obtained which leads to the theory of inelastic electron cotunneling and gives important corrections to the classical result for strong tunneling and finite temperatures. An additional energy renormalization leads to a logarithmic temperature dependence of the renormalized system parameters. Several formulas for the line shape of the conductance oscillations are presented which show the possibility to observe experimentally quantum fluctuation effects at realistic temperatures.

An approach to fluctuation and elasticity in polymer networks

The fluctuation of strands and junctions in fully crosslinked polymer networks under deformation is investigated along with the deformation free energy and elasticity. Nonergodic, disordered system as it is, statistical properties of a polymer networks need to be described on the basis of average over the partial configuration space accessible to the chains. In this work, we reconsider the ensemble average for the bulk and single-chain properties and devise a response theory to relate the fluctuations of the single strand with an external deformation. In the light of this formulation, we assess the ‘‘cooperative network’’ theory of Ronca and Allegra (RA), which represents a statistical mechanical theory with no adjustable parameters yielding good agreements with experiments. Their formulation of the fluctuation under strains is rather ad hoc, since it is adjusted to meet the empirically expected behaviors in the affine and phantom network limits. We find their free energy can be put into a form much more significant and suggestive, which combines with our approach to yield an alternative theory. An approximation within our approach yields better agreements with the experiments on elasticity.

Enhancement of Conductance Fluctuations in Normal Metals Sandwiched between Superconductors

Conductance fluctuations in superconductor-normal metal-superconductor junctions are studied at zero temperature. It is shown that the amplitude of the fluctuations is no longer characterized by the universal value, of order of e 2 / h , but generally depends on the sample size and the phase relaxation length L φ . In the particular cases where the sample size is much smaller than L φ , we find that an enhancement of the fluctuations greater than the order of e 2 / h can be expected to arise. It is ascribed to a manifestation of the large fluctuation in the level density, which is caused by confinement of quasiparticles in the normal-metal region due to Andreev reflection.

Interpretation of the small-strain moduli of model networks of polydimethylsiloxane

Small-strain moduli reported in the literature for polydimethylsiloxane (PDMS) model networks have been critically re-examined, with account being taken of recent studies which have demonstrated the occurrence of significant side reactions in hydrosilylations. When the effects of such side reactions on network structure parameters are properly taken into account, the elasticity results can be well explained using the recent molecular theory of Flory and Erman. The revised calculations show a transition in the modulus from the affine to the phantom limit of deformation as the degree of chemical crosslinking increases. This is to be expected when the constraints on the fluctuation of junctions vanish (because of the decreased interspersion of chains), and such constraints are expected to vanish even in the small-strain region when the chains are sufficiently short. In lieu of carefully controlled reactions with well-defined stoichiometries, it appears that the procedure best suited for testing the various theories of rubber-like elasticity is a plot of the modulus G (as approximated by the sum 2 C 1 + 2 C 2 of the Mooney-Rivlin constants) against the phantom modulus [f∗] ph (as approximated by 2 C 1). Also of importance is the difference between the ‘effective’ number of network chains v (relevant to theory) and the commonly used ‘active’ number of chains v a.

Origin of 1/f noise in Y1Ba2Cu3O7−x step-edge dc SQUIDs

We have fabricated Y1Ba2Cu3O7−x step-edge junction dc superconducting quantum interference devices (SQUIDs) and characterized their noise performance. The current-voltage characteristics of our SQUIDs are of resistively shunted junction type with critical current densities jc of about 104 A/cm2 and maximum flux to voltage transfer functions δV/δΦ of 20 μV/Φ0 at 77 K. We compare model predictions for 1/f noise in dc SQUIDs with experimental data and show that the frequency dependent noise in our devices results from critical current fluctuations of the Josephson junctions. A bias current reversing technique is effective in suppressing this noise contribution. A flux noise Φn of 80 μΦ0/Hz1/2 at 1 Hz and 77 K was measured on a dc SQUID with an inductance Ls of 60 pH.

The effect of fluctuating contact resistance in analog telephone subscriber loops

The effect of submillisecond changes in a simulated contact resistance (CR) on the signals transmitted by split-band, full-duplex, two-wire, 1200 b/s voiceband (analog) data modems used with three differently configured telephone loops is studied. A fluctuating, defective electrical junction in a real telephone loop is simulated with an FET-resistor parallel circuit that is optically decoupled from the laboratory instruments that are driving and monitoring the fluctuations. Both the transmitter and receiver can record erroneous data when the simulated junction fluctuations are greater than approximately 5*. The data structure, the duration and amplitude of the resistance fluctuation, and the location of the fluctuating resistance in the transmission line all affect transmission quality. It is shown that practical levels of sealing current are insufficient to eliminate errors in analog data and voice transmission due to fluctuating, degraded junctions.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Structure and elasticity of polyurethane networks based on poly(butadiene) diol, 4,4′-diphenylmethane diisocyanate and poly(oxypropylene) triol

Relations between structure and equilibrium mechanical properties of polyurethane networks based on poly(butadiene) diol, 4,4′-diphenylmethane diisocyanate (MDI) and poly(oxypropylene) triol (POPT) prepared at various initial ratios of reactive groups were studied. An inhomogeneous structure of the networks was revealed by small-angle X-ray scattering. The microdomains consist of POPT molecules crosslinked with MDI and they have a diameter of about 4–5 nm. The characteristic distance of microdomains is about 9 nm. The poor mutual miscibility of the reaction components and the existence of hydroxyl group association in the reaction mixture are the probable reasons for microdomain formation. The strain dependence of the equilibrium elastic force obeyed the Mooney-Rivlin equation. The experimental moduli of dry and equilibrium swollen samples were higher than those calculated for the affine limit of the Flory junction fluctuation model on the assumption of a homogeneous network formation.

Quantum fluctuations in finite size Josephson junctions

An effective Josephson coupling energy for a one-dimensional Josephson junction is renormalized due to quantum fluctuations of the phase difference. In a long junction at T → 0 a Kosterlitz-Thouless phase transition takes place. The state with a logarithmically divergent phase-phase correlation function shows a nontrivial combination of phase disorder on a junction surface with phase order in the bulk. For finite size junctions the renormalized value of the Josephson coupling energy turns out to be strongly suppressed for small Josephson-to-charging energy ratio. The implications of this effect for Bloch oscillations are discussed.

Discrete conductance fluctuations in silicon emitter junctions due to defect clustering and evidence for structural changes by high-energy electron irradiation and annealing

Observations of discrete conductance fluctuations are reported at voltages well below the breakdown voltage in selected reverse-biased p+-n++ base-emitter junctions originating from gate turn-off thyristors. The occurrence of the phenomenon is attributed to the presence of defect clusters at the p-n junctions. The defect clusters introduce field confinements which activate tunneling processes that would not otherwise be present in these nonabrupt p-n junctions. The fluctuating reverse current was only observed in voltage and temperature regions where the total reverse current was influenced by tunneling-related conduction mechanisms. The experimental observations concerning the voltage and temperature dependences of the fluctuation amplitude and rate deviate from earlier reports on decisive points. Both the amplitude and the switching rate of the observed fluctuations were unstable in time and influenced by the measurement procedure itself. This instability is attributed to small structural changes of the defect clusters. Furthermore, the unstable behavior of the defect clusters also influences the static reverse current-voltage characteristic. Distinct changes were found in the static reverse current-voltage characteristics of selected samples due to high-energy electron irradiation and annealing at 200 °C. A clearly increased uniformity of the reverse current-voltage characteristics between the gate-cathode junctions of gate-turn off thyristors was also found as a result of electron irradiation. The changes observed are interpreted as evidence of structural changes of the defect clusters.

Tunnel-junction noise at low temperatures: Tunneling or local "heating"

A model that considers energy transfer from hot electrons to structural defects provides a simpler explanation for the dynamics of discrete resistance fluctuations in tunnel junctions than one involving tunneling at low temperatures. The issue of whether or not change is being trapped at the defect then becomes irrelevant to the fluctuation dynamics.