Pb Tunnel Junctions Research Articles

Properties of quantum interference devices with VVOxPb tunnel junctions

The properties of double contact quantum interference devices (SQUIDs) with VVOxPb tunnel juctions are studied experimentally. With decreasing temperature an increasing hysteresis in the (I-U) characteristics is observed. At lower temperatures there are multiple critical currents. At sufficiently low inductivities of the SQUID loop and small critical currents single valued critical currents can be reached. The dependences of the quantum interferences from magnetic field and from the temperature pointed out that small tunnel areas and a symmetric construction of the SQUID loop are advantageous. Die Eigenschaften von Zwei-Kontakt-Quanteninterferometern (SQUIDs) mit VVOxPb Tunnelverbindungen werden experimentell untersucht. Mit fallender Temperatur wird eine zunehmende Hysterese in den I-U-Kennlinien beobachtet. Bei tieferen Temperaturen treten Vieldeutigkeiten des kritischen Stromes auf. Bei genugend geringen Induktivitaten der SQUID-Schleifen und bei kleinen kritischen Stromen konnen eindeutige kritische Strome erreicht werden. Die Abhangigkeiten der Quanteninterferenzen vom Magnetfeld und von der Temperatur zeigen. das kleine Tunnelflachen und ein symmetrischer Aufbau der SQUID-Schleife vorteilhaft sind.

Conductance of clean and doped tunnel junctions

The conductance of a clean AlIPb tunnel junction can be understood in terms of an asymmetric oxide barrier but for a doped junction an oxide-organic bilayer must be considered. We report here an organic barrier 8.80 V high and 2.53 Å thick.

The nature of the oxide barrier in inelastic electron tunneling spectroscopy

We have investigated the insulating barrier of A1-oxidized A1Pb tunnel junctions. This study was effected by measuring the inelastic electron tunneling (IET) spectra of various junctions in which the insulator was formed by oxidizing the aluminum either in an O 2 H 2O or D 2O plasma discharge at various temperatures between room temperature and 250°C. We found that those insulators formed at all temperatures in 0 2 and those formed at high temperatures (≳150°C) in H 2O and D 2O are aluminum oxide with no evidence for any appreciable bulk hydroxide. The surfaces of these insulators do have adsorbed hydroxyl groups present, however. The insulators made at lower temperatures (<100°C) are also primarily aluminum oxide, but there is evidence for some bulk hydroxide. These “oxides” have a high concentration of surface hydroxy1 groups with indications of two types of surface hydroxyls.

Explanation of asymmetry in tunnel spectra

The molecular vibrational spectrum from an AlIPb tunnel junction doped with m-cresol has been studied. It is shown that the asymmetry in signal strength with d.c. bias polarity arises from an asymmetry in the junction conductance.

Studies on Al—insulator—Pb tunnel junctions with barriers prepared in glow discharge of CO2 and CO

en

Sample heating and simultaneous temperature measurement in inelastic electron tunneling spectroscopy

A method is described whereby the oxide surface in inelastic electron tunneling spectroscopy (IETS) experiments may be heated resistively with its temperature measured simultaneously via a four-point probe scheme. The utility of the method insofar as surface chemistry experimentation is concerned is demonstrated by following the dehydroxylation of the Al2O3 barrier in an Al–Al2O3–Pb tunnel junction as the preparation temperature is increased from 20 to 250 °C.

A reliable DC squid made with tunnel junctions

D.C. SQUIDS of cylindrical geometry have been fabricated using Nb-NbO x -Pb tunnel junctions. The SQUIDs can be thermally cycled repeatedly and cooled in the earth's magnetic field without significant loss of performance. Flux resolution of 3 \times 10^{-5} \phi_{o}/\sqrt{Hz} above a frequency of 1 Hz has been achieved through optimization of electronic circuitry and careful shielding. Fluctuations and drift in the temperature of the helium bath are found to be the major source of long-term SQUID noise. With a simple temperature stabilizer, drift has been reduced to less than 8 \times 10^{-5} \phi_{o} /hr over a three hour period.

Inelastic-Tunneling Spectra of Organic Compounds

Inelastic electron tunneling has been studied in Al-${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$-Pb tunnel junctions doped with a wide range of organic compounds. These include simple straight-chain hydrocarbons, amino acids, aromatic ring compounds, pyrimidine bases, cyclic saturated hydrocarbons, and other nonaromatic ring compounds. The observed inelastic-tunneling spectra have been analyzed in terms of the expected vibrational modes of these molecules and results show that the method has sufficient sensitivity to differentiate many of the characteristic modes in these compounds. Comparison with Raman and infrared spectra have been made and a detailed analysis of this comparison is included. In general, the tunneling spectra often show a better over-all agreement with Raman spectra than with infrared spectra, but important differences exist between the different methods in a number of cases. In most cases the identification of specific modes in the tunneling spectrum is in quite good agreement with both the Raman and infrared spectra. The tunneling spectra have been recorded at 4.2 K in most cases, but improved resolution can be obtained near 1 K and preliminary examples are included. The doping has been accomplished using a combination of vacuum chambers which allows doping of successive junctions with different organic compounds without cross contamination. The experiments demonstrate that the inelastic-tunneling technique can be applied to a complete range of organic compounds and may prove to be highly useful technique for the analysis of very small quantities of material.

The growth of thin PbO layers on lead films

Abstract The growth of ∼ 10 to 100 A thick, orthorhombic PbO layers on lead was investigated by ellipsometry, electron tunneling and X-ray diffraction. Oxides were grown on 2000 to 15000 A thick, vacuum-deposited lead films, using: oxygen pressures from 10 −2 to 10 +3 Torr; temperatures from −90 to + 150°C; and, times up to several weeks. Apparatus is described for monitoring ellipsometric parameters during oxidation and fabricating PbPbOPb tunnel junction devices in situ . The results indicate that lead recrystallization processes can accelerate the oxidation, so that the rates depend complexly on temperature, lead thickness and microsctructure. The refractive indices provide an indication of the extent of lead recrystallization effects on the oxidation process as well as the accuracy of the oxide thickness determination. X-ray findings indicate that the PbO has the orthorhombic PbO structure, is highly strained while being epitaxially related to the lead, and has a thickness close to that obtained ellipsometrically. Electron tunneling data provide a measure of oxide thickness fluctuations from the average or ellipsometrically-derived values.

Study of effective phonon spectrum of superconducting Nb 3Sn alloy by electron tunneling

Investigations of Nb/sub 3/Sn/Al/sub 2/O/sub 3//Nb/sub 3/Sn and Nb/sub 3/ Sn/I /Pb tunnel junctions (I is Al/sub 2/O/sub 3/ or an oxide of Nb/sub 3/Sn) are reported in th e voltage region up to 34 mV, at 4.2 deg K. Both the differential conductance and the second derivative of tunnel current against voltage were measured for the tunnel junctions. In the measurements ten samples of Nb/sub 3/ Sn (0.5 to O.7 mu thickness) with the critical temperature T = 17.6 to 18.1 deg K were used. (MHR)

The influence of the accumulation layer on the conductance characteristics of MIS tunnel junctions

The inflex structure found by Tsui in conductance characteristics of InAsInAs oxidePb tunnel junctions at the bias equal to the bulk Fermi degeneracy is explained as a consequence of the modification of the semiconductor band wave functions by the accumulation layer.

Zero bias anomalies in GaAsPb junctions doped with Cu 2+ ions at the interface

By introducing a layer of Cu 2+ ions at the interface of GaAsPb tunnel junctions, we have observed zero bias conductance peak comparable to that observed in TaIAl junctions. Its magnetic field dependence has been studied at 1°K with field strengths extending to 80kG.

On the 2Δp/n structure of PbPbOPb tunnel junctions

The fine-structure of the tunneling characteristics observed [1] in PbPbOPb superconducting tunnel junctions has been interpreted as 2 Δ p/ n structure due to the breaking of p electron pairs and the tunneling of n electrons in the same processes.