  We investigate the fermionic condensate and the vacuum expectation value of
the energy-momentum tensor for a massive fermionic field in the geometry of two
parallel plate on the background of Minkowski spacetime with an arbitrary
number of toroidally compactified spatial dimensions, in the presence of a
constant gauge field. Bag boundary conditions are imposed on the plates and
periodicity conditions with arbitrary phases are considered along the compact
dimensions. The boundary induced parts in the fermionic condensate and the
vacuum energy density are negative, with independence of the phases in the
periodicity conditions and of the value of the gauge potential. Interaction
forces between the plates are thus always attractive. However, in physical
situations where the quantum field is confined to the region between the
plates, the pure topological part contributes as well, and then the resulting
force can be either attractive or repulsive, depending on the specific phases
encoded in the periodicity conditions along the compact dimensions, and on the
gauge potential, too. Applications of the general formulas to cylindrical
carbon nanotubes are considered, within the framework of a Dirac-like theory
for the electronic states in graphene. In the absence of a magnetic flux, the
energy density for semiconducting nanotubes is always negative. For metallic
nanotubes the energy density is positive for long tubes and negative for short
ones. The resulting Casimir forces acting on the edges of the nanotube are
attractive for short tubes with independence of the tube chirality. The sign of
the force for long nanotubes can be controlled by tuning the magnetic flux.
This opens the way to the design of efficient actuators driven by the Casimir
force at the nanoscale.
