  We find efficient spin transport in Si at room temperature in lateral spin
valves (LSVs). When the crystal orientation of the spin-transport channel in
LSVs is changed from $\langle$110$\rangle$, which is a conventional cleavage
direction, to $\langle$100$\rangle$, the maximum magnitude of the spin signals
is markedly enhanced. From the analyses based on the one-dimensional spin
diffusion model, we can understand that the spin injection/detection efficiency
in Si$\langle$100$\rangle$ LSVs is larger than that in Si$\langle$110$\rangle$
ones. We infer that, in Si-based LSVs, the spin detection efficiency of the
pure spin current is related to the crystallographic orientation of the valley
structures of the conduction band in Si.
