J. G. Adler

High Resolution Electron Tunneling Spectroscopy

A system which is capable of resolving changes in a tunneling junction conductance σ of about one part in 105 as well as providing calibrated values of dσ/dV is described. Methods of simultaneous analog and digital recording on magnetic tape suitable for subsequent computer analysis are given. The flow chart for the computer programming is also presented along with a typical result.

Application of minicomputers in high resolution electron tunneling

A minicomputer−based system for obtaining ’’absolute values’’ of tunnel conductance σ to one part in 105 and simultaneous measurement and ’’calibration’’ of its rate of change dσ/dV is described. The analog, digital, and interface hardware along with the necessary software are discussed in detail and typical results are shown for the system used. The system is capable of presenting the calibrated results in only a few minutes after each sweep is completed, yet requires less than 8 k of core memory, provided a mass storage device such as magnetic tape is available.

Observation of the phonon spectra of MgO by inelastic electron tunneling in metal-insulator-metal junctions

Abstract We have observed the phonon spectrum of thin layers MgO by inelastic electron tunneling in M-I-M junctions. The results are shown to agree well with infrared data on bulk MgO.

Electron tunneling in clean Al-insulator-normal metal junction☆

Abstract The metal phonon peaks have been observed in the derivative of the conductance of clean Al-insulator-metal film junctions using different counter electrodes. Most of the clean junctions show a parabolic tunneling conductance with minimum offset from zero bias. A zero bias anomaly was observed for all these aluminum based junctions.

Apparatus for Measuring Characteristics of Superconducting Tunnel Junctions

Much information about the density of electron states in a superconductor may be obtained using electron tunneling techniques. It is of particular interest to measure the normalized dynamic conductance of superconducting tunnel junctions as a function of applied voltage. This quantity is the ratio of the dynamic conductance (di/dv)s when one or both metallic members of the tunnel junction are in the superconducting state, to the dynamic conductance (di/dv)n when both members are normal. A new method has been developed which enables measurements to be made of (di/dv)s/(di/dv)n to a few parts in ten thousand. With this method only a bridge circuit, a galvanometer amplifier, and an oscilloscope are used. The galvanometer amplifier has a passband from dc to a few cps and an input noise voltage of about 3×10−8 rms V. The circuits of the bridge and amplifier are presented and analyzed. The operation of the circuits for measuring the characteristics of low resistance specimens (≲500Ω) is described.

Strong coupling superconductivity of Pb0.7Bi0.3 alloys

Abstract Superconductivity of Pb 0.7 Bi 0.3 has been studied by electron tunneling techniques. This alloy was chosen because of its extremely strong electron-phonon interaction which leads to large deviations from the weak coupling BCS theory. The temperature dependence of the energy gap along with the effective phonon spectrum, Coulomb pseudopotential, complex gap function, pairing self energy and electron renormalization function have been calculated from the data.

Data calibration in electron tunneling spectroscopy

Practical problems which arise in obtaining calibrated electron tunneling spectra using harmonic detection techniques combined with a resistive bridge are examined. The instrumentation uses a computer‐controlled data acquisition system along with the necessary software. The effect of neglecting various calibration details is shown using real tunneling data. This paper shows the steps which are essential to obtaining good calibrated tunneling spectra even where weak structure is involved. Results are shown to demonstrate the effect which harmonic distortion has on the data, and various calibration schemes are examined to determine which give sufficiently good results to reproduce accurate line shapes. Finally numerical differentiation techniques are examined for obtaining tunneling spectra, but these methods are found lacking in the case of weak structure unless complex smoothing algorithms are used to avoid digitizing errors.

The use of barrier parameters for the characterization of electron tunneling conductance curves

Abstract This paper deals with barrier parameters calculated from the coefficients of polynomial fits to electron tunneling conductance curves for a large sample of junctions with various types of barriers. Methods of data handling, along with a comparison of the results from two different parameter calculations are shown. Finally, relationships between the barrier parameters and other junction characteristics — particularly RA , the product of junction resistance with area (a simple and extremely useful parameter) — are discussed.

Asymmetry in inelastic electron tunneling peak intensities

Abstract Experiments show that for an organic material placed on an oxide one sees an asymmetry of inelastic electron tunneling peak intensity for bias voltages of opposite sign. When the barrier is composed only of the organic material with no deliberate oxidation such asymmetry disappears.

The Technology of IETS

In these remarks we shall address ourselves briefly to some of the questions which arose in previous chapters: (1) The necessity of calibrating second harmonic signals if they are to be used for any purpose other than simply spectroscopy (such as peak intensity studies). (2) Comments on peak asymmetry, i.e. the difference in peak magnitude for positive and negative bias direction. (3) We will attempt to answer the question of why inelastic electron tunneling is studied primarily in Al-Pb junctions.