  Fractional quantum Hall states (FQHSs) at even-denominator Landau level
filling factors ($\nu$) are of prime interest as they are predicted to host
exotic, topological states of matter. We report here the observation of a FQHS
at $\nu=1/2$ in a two-dimensional electron system of exceptionally high
quality, confined to a wide AlAs quantum well, where the electrons can occupy
multiple conduction-band valleys with an anisotropic effective mass. The
anisotropy and multi-valley degree of freedom offer an unprecedented tunability
of the $\nu=1/2$ FQHS as we can control both the valley occupancy via the
application of in-plane strain, and the ratio between the strengths of the
short- and long-range Coulomb interaction by tilting the sample in the magnetic
field to change the electron charge distribution. Thanks to this tunability, we
observe phase transitions from a compressible Fermi liquid to an incompressible
FQHS and then to an insulating phase as a function of tilt angle. We find that
this evolution and the energy gap of the $\nu=1/2$ FQHS depend strongly on
valley occupancy.
