  Silicon's weak intrinsic spin-orbit coupling and centrosymmetric crystal
structure are a critical bottleneck to the development of Si spintronics,
because they lead to an insignificant spin-Hall effect (spin current
generation) and inverse spin-Hall effect (spin current detection). Here, we
undertake current, magnetic field, crystallography dependent magnetoresistance
and magneto thermal transport measurements to study the spin transport behavior
in freestanding Si thin films. We observe a large spin-Hall magnetoresistance
in both p-Si and n-Si at room temperature and it is an order of magnitude
larger than that of Pt. One explanation of the unexpectedly large and efficient
spin-Hall effect is spin-phonon coupling instead of spin-orbit coupling. The
macroscopic origin of the spin-phonon coupling can be large strain gradients
that can exist in the freestanding Si films. This discovery in a light, earth
abundant and centrosymmetric material opens a new path of strain engineering to
achieve spin dependent properties in technologically highly-developed
materials.
