Intoductory material
Molecular Electronics Group

Quantum dots (often called 'artificial atoms') are small devices in which the movement of the electrons are confined in all directions. When the size of the confinement region becomes comparable to the Fermi wavelength, one starts to observe the discreteness of the energy spectrum like in normal atoms.

There is a number of ways to fabricate these structures: Maybe the most common technique to do this is by defining a typically micron-size region by shaping a two-dimensional electron gas using gate electrodes placed on the top of a semiconductor heterostructure (this setup is sketched in the figure) or by etching. In addition to the semiconducting quantum dots, nowadays it is possible to perform measurements on metallic nano-grains and very recently it has become possible to integrate single molecules into circuits.

There are two energy scales which describe an isolated quantum dot: the single particle level spacing and the charging energy (the energy cost to put an extra electron to the quantum dot due to the electron-electron interaction). Typically, the charging energy is bigger than the level spacing, but for very small structures (e.g. in the extreme case of a molecule) these two energy scales can be of the same order of magnitude. Coulomb correlations may become only important if the measurement temperature is less than the charging energy. Clearly, this criterion can be only satisfied with our current cooling technology if the charging energy is in the range of a few Kelvins, i.e. the size of the system is in the micron range or below. The behavior of a quantum dot is also very different for temperatures bigger and smaller than the level spacing: while in the former regime electron-hole excitations on the dot are important, for the latter case these excitations do not play an essential role.

A quantum dot can be attached by single- or multi channel contacts to leads to allow for transport of electrons through the artificial atom. Such a coupling to the outside world introduces a new energy scale, the level broadening. Depending on the relation between the afore mentioned energy scales (charging energy, single particle level spacing, temperature and the coupling strength) the transport properties of a quantum dot shows a number of interesting behaviors. For a review of the transport phenomena, see the lecture notes of Pustilnik and Glazman [cond-mat/0501007]