  We identify carrier scattering at densities below which it has previously
been observed in semiconductor quantum wells. These effects are evident in the
peakshapes of 2D double-quantum spectra, which change as a function of
excitation density. At high excitation densities ($\geq 10^{9}$
carriers/,cm$^{-2}$) we observe untilted peaks similar to those reported in
previous experiments. At low excitation densities (<$10^{8}$ carriers
cm$^{-2}$) we observe narrower, tilted peaks. Using a simple simulation, we
show that tilted peak-shapes are expected in double-quantum spectra when
inhomogeneous broadening is much larger than homogeneous broadening, and that
fast pure-decoherence of the double-quantum coherence can obscure this peak
tilt. These results show that carrier interactions are important at lower
densities than previously expected, and that the `natural' double-quantum
peakshapes are hidden by carrier interactions at the excitation densities
typically used. Furthermore, these results demonstrate that analysis of 2D
peak-shapes in double-quantum spectroscopy provides an incisive tool for
identifying interactions at low excitation density.

Optics;Mesoscale and Nanoscale Physics