Coulomb blockade

Coulomb blockade is the disappearance of the electrical conductivity of a current path through a nano-object, because it can not release or accept any electrical charge due to its small electrical capacity to the environment . The effect is named after Charles Augustin de Coulomb (1736–1806), but was only predicted and observed much later.

Electron microscope image of an arrangement for examining the Coulomb blockade when electricity is transported over an elongated metal island

The nano-object can be a small conductive particle, a conductive island on an insulator or a so-called quantum dot . It is arranged between two conductors (sometimes referred to as “source” and “drain” in analogy to the field effect transistor ). The nano-object must be contacted via two sufficiently high electrical resistances , which in practice happens through the quantum mechanical tunnel effect . That is, there is a small gap between the nano-object and the power lines; this can also be filled with an insulator. The electrons overcome the gap by tunneling.

If an electron passes over to the nano-object, the voltage of the object changes by , whereby the elementary charge and the electrical capacitance are between the object and the environment (including the two conductors). With a sufficiently small object, this capacity can be so small that the energy required to increase the voltage would be greater than the electron energy that is available through thermal excitation and the applied voltage. In this case, the electron cannot muster the energy to reach the nano-object and the flow of current is blocked. ${\displaystyle \Delta U=e/C}$ ${\displaystyle e}$ ${\displaystyle C}$

In order to observe the effect, the capacitance between the nano-object and the environment must be very small and the temperature low enough that thermal stimuli are not sufficient for an electron to “charge” the nano-object. For example, a metal island 100 nanometers by 100 nanometers in size, which can easily be produced using today's lithography technology, has a capacitance of 10 ⁻¹⁵ farads on an oxide layer with a thickness of 1 nanometer and a dielectric constant . In order to be able to observe Coulomb blockade for this object, temperatures below 1  Kelvin are necessary; the applied voltage may only be in the microvolt range. With much smaller objects, however, a Coulomb blockade can also be observed at room temperature. ${\displaystyle \varepsilon =10}$

The phenomenon of the Coulomb blockade is the basis for the single electron transistor .