Break junction

A break junction is an electrical junction between two wires formed by pulling the wires apart to produce electrodes separated by a few atomic distances. In this technique a metal wire is bent or pulled, often using a piezoelectric crystal to apply the necessary force. The bending or pulling causes the metal wire to break in a controlled manner since piezoelectric elongation can be controlled to a precision of angstroms or less (such crystals are used for motion control in scanning tunneling microscopy). As the wire breaks, the separation between the electrodes can be indirectly controlled by monitoring the electrical current through the junction.

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A typical conductance versus time trace during the breaking process (conductance is simply current divided by applied voltage bias) shows two regimes. First is a regime where the break junction comprises a quantum point contact. In this regime conductance decreases in steps equal to the conductance quantum G_Q = 2e² / h which is expressed through the electron charge e and Planck's constant h. The conductance quantum has a value of 7.74 × 10^-5 Siemens, corresponding to a resistance increase of roughly 12.9 KΩ. These step decreases are interpreted as the result of a decrease, as the electrodes are pulled apart, in the number of single-atom-wide metal strands bridging between the two electrodes, each strand having a conductance equal to the quantum of conductance. As the wire is pulled, the neck becomes thinner with fewer atomic strands in it. Each time the neck reconfigures, which happens abruptly, a step-like decrease of the conductance can be observed. This picture inferred from the current measurement has been confirmed by "in-situ" TEM imaging of the breaking process combined with current measurement.^[1][2]

In a second regime, when the wire is pulled further apart, the conductance collapses to values less than the quantum of conductance. This is the tunneling regime where electrons tunnel through vacuum between the electrodes.

Digging into the literature on break junctions and quantum point contacts reveals that the above conceptual description is somewhat oversimplified, but the description is a good first approach to understanding the topic.

Use

This method has been developed to study the conductance of few-atom constrictions of varied metals. The conductance of these constrictions has been compared with theoretical predictions for both the stability and the conductance of possible few-atom configurations. More recently it has been used to study molecules which are inserted in the junction in the liquid phase and binds to them (dithiols) or in the gas phase. This method has several advantages. It is clean, since the junctions can be made in a controlled atmosphere (high vacuum). It is fast and thus enables many independent measurements to be done in a few hours. It is then possible to study the statistical occurrence of a particular type of contact, and build conductance histograms. Lately this method has enabled the more accurate determination of the conduction of a single molecule.

The disadvantage of this technique is that it is a two-terminal technique (that is, it uses only two wires and can be considered an electrical diode), whereas complete determination of electronic properties requires using a three-terminal configuration similar to the source, drain and gate of an MOS transistor.

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