  Using coherent x-ray scattering, we evidenced atomic step roughness at the
[111] vicinal surface of a silicon monocrystal of 0.05 degree miscut. Close to
the (1/2 1/2 1/2) anti-Bragg position of the reciprocal space which is
particularly sensitive to the [111] surface, the truncation rod exhibits a
contrasted speckle pattern that merges into a single peak closer to the (111)
Bragg peak of the bulk. The elongated shape of the speckles along the[111]
direction confirms the monoatomic step sensibility of the technique. This
experiment opens the way towards studies of step dynamics on crystalline
surfaces.
