  The authors study the quantum effect of self-interacting fields in the
classical background of conical space, i.e. around a cosmic string with
infinitesimal width. The renormalized value of $\langle\phi^2\rangle$ and
energy-momentum tensor in the presence of cosmic strings are calculated in the
self-interacting scalar field theory. The amount of condensation is also
estimated in the case of the Dirac Lagrangian with the four-fermion
interaction. The physical implications of the above analyses are discussed.
