  The effective mass at the Fermi level is measured in the strongly interacting
two-dimensional (2D) electron system in ultra-clean SiGe/Si/SiGe quantum wells
in the low-temperature limit in tilted magnetic fields. At low electron
densities, the effective mass is found to be strongly enhanced and independent
of the degree of spin polarization, which indicates that the mass enhancement
is not related to the electrons' spins. The observed effect turns out to be
universal for silicon-based 2D electron systems, regardless of random
potential, and cannot be explained by existing theories.
