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Description
Confinement of a 2D electron gas to form a 1D system allows observation of conductance quantization with values of 2ne2/h where the integer n is 1,2,3,4 and the factor of 2 is spin degeneracy. This formula is based on spatial quantization and ballistic conduction. However, in 1996 a deviation from this simple behaviour occurred when a conductance plateau, or structure, was found near 0.7(2ne2/h) taking the name 0.7 structure. It is often a conductance plateau and can be between 0.8 and 0.6, the initial description of the effect was attributed to spin polarization arising from a ferromagnetic coupling, which resulted in only one spin direction being transmitted in a longer sample, with partial transmission of the other spin in a shorter sample. It is found that the conductance below the 0.7 is spin polarised. Both thermal and noise measurements indicate that the spins split with only one fully transmitted, application of a magnetic field which lifts the spin degeneracy of the plateaus also reduces the 0.7 to 0.5, ie a complete spin polarisation. It has been suggested that the spins are polarised but the polarisation axis slowly rotates in time which is consistent with the experiments.
When the confinement is weakened the 0.7 disappears, as do the first integer plateaus, and can be replaced by a new quantization with fractional values such as 1/6, ½, 1/5 and 2/5 in units of e2/h. This Non-Magnetic Fractional Quantization may be thought to have certain similarities to the Fractional Quantum Hall Effect except that there is no magnetic field, the fractions can be even as well as odd and there is no filling factor to determine the fractional value. This effect has been found in a range of semiconductors such as electrons in GaAs, InGaAs, InAs and holes in GaAs. These effects will be discussed along with possible theoretical explanations.