We observe a dramatic reduction in the degree of spin-polarization of a twodimensional electron gas in a magnetic field when the Fermi energy moves off the mid-point of the spin-gap of the lowest Landau level, ν = 1. This rapid decay of spin alignment to an unpolarized state occurs over small changes to both higher and lower magnetic field. The degree of electron spin polarization as a function of ν is measured through the magneto-absorption spectra which distinguish the occupancy of the two electron spin states. The data provide experimental evidence for the presence of Skyrmion excitations where exchange energy dominates Zeeman energy in the integer quantum Hall regime at ν = 1.

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Small ensembles of In 0.55 Al 0.45 As self-assembled quantum dots have been studied at low temperatures using near-field scanning optical microscopy. We observe spectral diffusion in individual quantum dot luminescence lines. The phenomenon increases in magnitude with optical power density, but is not visible at low powers. We believe the spectral diffusion to be caused by long time-scale trapping and untrapping of charges created by photoionization or thermal ionization in the immediate vicinity of the quantum dots.

Plateaus in the electron-hole recombination energy and minima in the peak intensity at integer and fractional filling factors occur in the luminescence from ultrahigh-mobility GaAs single quantum wells. At Landau and spin gaps the regions of plateaus and intensity minima broaden as the temperature is reduced, in consort with the transport Hall resistance. A sharp intensity minimum and peak shift is seen at v~ f, while higher-field fractions are characterized by a splitting in the luminescence. The optical anomalies are directly related to the position of the Fermi energy in localized transport states.