In one dimension the most striking effect is the quantisation of the ballistic conductance, this is a single-electron effect where each transmitted quantum-mechanical subband contributes 2e2/h to the conductance (shown in figure 2) and more surprisingly a novel spin effect termed the 0.7 structure (shown in the lower right inset) which arises from the manner in which electrons interact with each other. Specific phenomena which have come out of this work include:
- Demonstration that a combination of Molecular Beam Epitaxy growth and high resolution electron beam lithography can produce high quality nanostructures in which electrons can be confined to a region of variable geometry with an associated size quantisation in both 1 and 0 dimensions.
- In one dimension we find that the perfection of the GaAs-AlGaAs heterostructure is such that we can observe up to 30 quantised levels each producing a quantised conductance.
- Reducing the disorder results in observation of an extra plateau/structure below the first quantisation which cannot be explained by single-electron theory - the 0.7 structure.
- By means of increasing the driving voltage across a 1D ballistic device it is possible to lift the momentum degeneracy and observe electrons drifting in one direction only. This can result in unusual spin effects as the absence of spin scattering allows electrons to form a spin polarised line.
- Increasing the width of a one dimensional sample allows electrons the freedom to relax position to minimise the electrostatic repulsion, the net effect is the formation of different phases characterised by different topology. For example, with increasing width the channel splits into separate lines of electrons each with a quantised conductance and the transition from a one-dimensional line to a two dimensional array can be observed.