Resistively detected NMR in a two-dimensional electron system near nu=1: Clues to the origin of the dispersive lineshape

Resistively detected nuclear magnetic resonance (NMR) measurements on 2D electron systems near the nu=1 quantum Hall state are reported. In contrast to recent results of Gervais {[}Phys. Rev. Lett. 94, 196803 (2005)], a dispersive line shape is found at all radio-frequency powers studied and Korringa-type nuclear spin-lattice relaxation is observed. The shape of the unexplained dispersive line shape is found to invert when the temperature derivative of the longitudinal resistance changes sign. This suggests that both Zeeman and thermal effects are important to resistively detected NMR in this regime.