Measurements of the density-dependent many-body electron mass in 2D   GaAs/AlGaAs Heterostructures
  We determine the density-dependent electron mass, m*, in two-dimensional (2D) electron systems of GaAs/AlGaAs heterostructures by performing detailed low-temperature Shubnikov deHaas measurements. Using very high quality transistors with tunable electron densities we measure m* in single, high mobility specimens over a wide range of r_s (6 to 0.8). Toward low-densities we observe a rapid increase of m* by as much as 40%. For 2>r_s>0.8 the mass values fall ~10% below the band mass of GaAs. Numerical calculations are in qualitative agreement with our data but differ considerably in detail. 
Y.-W. Tan (1), J. Zhu (2), H. L. Stormer (1,3,4), L. N. Pfeiffer (4),   K. W. Baldwin (4), and K. W. West (4) ((1)Department...