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Fermionic current from topology and boundaries with applications to higher-dimensional models and nanophysics

We investigate combined effects of topology and boundaries on the vacuum expectation value (VEV) of the fermionic current in the space with an arbitrary number of toroidally compactified dimensions. As a geometry of boundaries we consider two parallel plates on which the fermion field obeys bag boundary conditions. Along the compact dimensions, periodicity conditions are imposed with arbitrary phases. In addition, the presence of a constant gauge field is assumed. The nontrivial topology gives rise to an Aharonov-Bohm effect for the fermionic current induced by the gauge field. It is shown that the VEV of the charge density vanishes and the current density has nonzero expectation values for the components along compact dimensions only. The latter are periodic odd functions of the magnetic flux with the period equal to the flux quantum. In the region between the plates, the VEV of the fermionic current is decomposed into pure topological, single plate and interference parts. For a massless field the single plate part vanishes and the interference part is distributed uniformly. The corresponding results are generalized for conformally-flat spacetimes. Applications of the general formulas to finite-length carbon nanotubes are given within the framework of the Dirac model for quasiparticles in graphene. In the absence of the magnetic flux, two sublattices of the honeycomb graphene lattice yield opposite contributions and the fermionic current vanishes. A magnetic flux through the cross section of the nanotube breaks the symmetry allowing the current to flow along the compact dimension.