  Valleytronics is rapidly emerging as an exciting area of basic and applied
research. In two dimensional systems, valley polarisation can dramatically
modify physical properties through electron-electron interactions as
demonstrated by such phenomena as the fractional quantum Hall effect and the
metal-insulator transition. Here, we address the electrons' spin alignment in a
magnetic field in silicon-on-insulator quantum wells under valley polarisation.
In stark contrast to expectations from a non-interacting model, we show
experimentally that less magnetic field can be required to fully spin polarise
a valley-polarised system than a valley-degenerate one. Furthermore, we show
that these observations are quantitatively described by parameter free ab
initio quantum Monte Carlo simulations. We interpret the results as a
manifestation of the greater stability of the spin and valley degenerate system
against ferromagnetic instability and Wigner crystalisation which in turn
suggests the existence of a new strongly correlated electron liquid at low
electron densities.
