D.R. Leadley1, R.J. Nicholas2, D.K. Maude3, J.C. Portal3, J.J. Harris4 and C.T. Foxon5
The Quantum Hall state at filling factor \nu=1, with all spin up states in the lowest Landau level occupied and all spin down states empty, may be regarded as an itinerant ferromagnet. While distinct from a spontaneous magnetisation in zero field, this is a ferromagnet as the single particle description must be replaced by many body correlations. The quantum Hall ferromagnet is particularly interesting because the collective excitations, known as Skyrmions, carry both spin and charge.
We report transport measurements in GaAs/GaAlAs heterojunctions that investigate the energy gap for excitation out of the ferromagnetic ground state. In a single particle picture this is just the Zeeman splitting g\muBB. However it has long been known that the gap at \nu=1 is much greater, so an enhanced g- factor was proposed to account for the many body effects. We have used hydrostatic pressure to reduce the magnitude of the g-factor, which actually becomes zero at ~16 kbar. There are two principle results. (i) The gap measured over a range pressures, which reduce the g-factor by an order of magnitude, depends only on the electron density. So it is certainly not proportional to the bare g-factor but is really an exchange interaction. (ii) The width of the \nu=1 minimum decreases with increasing pressure, i.e. the range of filling factor for which the conductivity vanishes is less. This means that at high pressure more of the localised states become extended. We consider what the transport measurements tell us about Skyrmions.
Presented at: Institute of Physics, Condensed Matter and Materials Physics Conference, York 1996