  Using computer simulations, we studied the diffusion and structural
relaxation in equilibrium smectic liquid crystal bulk phases of parallel hard
spherocylinders. These systems exhibit a non-Gaussian layer-to-layer diffusion
due to the presence of periodic barriers and transient cages, and show
remarkable similarities with the behavior of out-of-equilibrium supercooled
liquids. We detect a very slow inter-layer relaxation dynamics over the whole
density range of the stable smectic phase which spans a time interval of four
time decades. The intrinsic nature of the layered structure yields a
hopping-type diffusion which becomes more heterogeneous for higher packing
fractions. In contrast, the in-layer dynamics is typical of a dense fluid with
a relatively fast decay. Our results on the dynamic behavior agree well with
that observed in systems of freely rotating hard rods, but differ quantitavely,
as the height of the periodic barriers reduces to zero at the nematic-smectic
transition for aligned rods, while it remains finite for freely rotating rods.
