A break junction is an 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 controlled 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
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.
Use
Break junctions are used to make electrical contacts to and study single molecules.