  In a number of widely-studied materials, such as Si, AlAs, Bi, graphene,
MoS2, and many transition metal dichalcogenide monolayers, electrons acquire an
additional, spin-like degree of freedom at the degenerate conduction band
minima, also known as "valleys". External symmetry breaking fields such as
mechanical strain, or electric or magnetic fields, can tune the
valley-polarization of these materials making them suitable candidates for
"valleytronics". Here we study a quantum well of AlAs, where the
two-dimensional electrons reside in two energetically degenerate valleys. By
fabricating a strain-inducing grating on the sample surface, we engineer a
spatial modulation of the electron population in different valleys, i.e., a
"valley superlattice" in the quantum well plane. Our results establish a novel
manipulation technique of the valley degree of freedom, paving the way to
realizing a valley-selective layered structure in multi-valley materials, with
potential application in valleytronics.
