Break junction
Encyclopedia
A break junction is a electronic device which consists of two metal wires separated by a very thin gap, on the order of the inter-atomic spacing (less than a nanometer). This can be done by physically pulling the wires apart or through chemical etching or electromigration
. As the wire breaks, the separation between the electrodes can be indirectly controlled by monitoring the electrical resistance of the junction.
After the gap is formed, its width can often be controller by bending the substrate that the metal contacts lie on. The gap can be controlled to a precision of picometers.
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
which is expressed through the electron charge (−e) and Planck's constant 
. 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.
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.
Electromigration
Electromigration is the transport of material caused by the gradual movement of the ions in a conductor due to the momentum transfer between conducting electrons and diffusing metal atoms. The effect is important in applications where high direct current densities are used, such as in...
. As the wire breaks, the separation between the electrodes can be indirectly controlled by monitoring the electrical resistance of the junction.
After the gap is formed, its width can often be controller by bending the substrate that the metal contacts lie on. The gap can be controlled to a precision of picometers.
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
Quantum point contact
A Quantum Point Contact is a narrow constriction between two wide electrically conducting regions, of a width comparable to the electronic wavelength . Quantum point contacts were first reported in 1988 by a Dutch group and, independently, by a British group...
. In this regime conductance decreases in steps equal to the conductance quantum
Conductance quantum
The conductance quantum is the quantized unit of conductance. It is defined as G0 = 2e2/h = ≈ Ω−1. It appears when measuring the conductance of a quantum point contact....
which is expressed through the electron charge (−e) and Planck's constant . 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.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.