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.

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 $G\_Q=2e^2/h$ 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.cite journal|author=H. Ohnishi, Y. Kondo and K. Takayanagi|title=Nature|volume=395|pages=780|year=1998] cite journal|author=V. Rodrigues, T. Fuhrer and D. Ugarte|title=Physical Review Letters|volume=85|pages=4124|year=2000]

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.

Although the above conceptual description is somewhat oversimplified, it is a good first approach to understanding the topic of break junctions and quantum point contacts.

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