Previous Research : Skyrmions and Composite Fermions in GaAs/(Ga,Al)As Single Heterojunctions

Introduction:

At high fields and low temperatures electrons form a new type of interacting electron liquid, which has new types of excitations or particles associated with it. Under these conditions we observe the Integer and Fractional Quantum Hall Effects. The electrons re-arrange themselves into new particles which have flux vortices attached to them (Composite Fermions) or have a chiral structure (Skyrmions). This behaviour can be enhanced when the g-factor of the electrons is made equal to zero by the application of hydrostatic pressure.

1. nu = 1 and Skyrmion formation.

Activation studies of the energy gap at nu = 1 indicate that the dominant contribution is due to exchange energy, and recent studies have suggested that this may be due to the excitation of Skyrmion-antiSkyrmion pairs. As the Zeeman energy approaches zero there is a dramatic increase in the size of Skyrmion excitations. The width of the resistivity minimum decreases but by less than the decrease in Zeeman energy, indicating that localisation of the Skyrmions changes.

2. The influence of Zeeman energy on Composite Fermions.

We have found that the = 1/3 state may be viewed as a Composite Fermion ferromagnet. As the Zeeman energy falls we find that the energy gap falls until around 17kbar the 1/3 state is destroyed. By contrast unpolarized states such as 2/5 and 2/3, which consist of doubly degenerate levels remain strong and can be enhanced in strength. At high pressures we can consider the excitations around 1/3 to be Composite Skyrmions.

For very high values of hydrostatic pressure where the Zeeeman energy approaches zero, the Fractional Quantum Hall Effect doubles its degeneracy for all fractions, with only even numerator fractions remaining, such as 2/3 and 4/3, and odd states such as 5/3 disappearing.

Other previous research topics:
	Antimonides - Transport and Optics MOVPE growth Bulk GaInP and GaInP/GaAlInP multiple quantum wells Wide gap Nitride and II-VI Semimagnetic Superlattices