Very Narrow Intersubband Excitations in High Mobility 2DESs

We report the observation of very narrow collective intersubband excitations (ISBE) of 2D electron systems (2DESs) with ultra-high mobility ($\mu \ge $15x10$^{\mathrm{6}}$ cm$^{\mathrm{2}}$/Vs) in high quality GaAs quantum structures. These findings from resonant inelastic light scattering (RILS) experiments are used as tools for exploration of links between transport mobility and collective electron behavior in 2DES of high perfection. We find that the line-widths of collective ISB modes can be as low as 80$\mu $eV. Comparison of ISBE measurements from several samples exhibits a variation in line-width of more than a factor of two. There is, however, a surprising lack of direct correlation between ISBE line-width with mobility in the range 15$\ge \mu \ge $24x10$^{\mathrm{6}}$ cm$^{\mathrm{2}}$/Vs. Measurements of ISBE by RILS will be evaluated as a method to explore the interplay of quality (as indicated by mobility) and fundamental interactions in the fractional quantum Hall effect.