  We explore the possibility of synchro-curvature maser in the magnetosphere of
neutron stars (NSs). Unlike previous studies, we employ relativistic quantum
mechanics, solving the Dirac equation for an electron in helical magnetic
fields and calculating the radiative transition rates perturbatively. Assuming
that the curvature of magnetic-field lines is much larger than the Larmor
radius, we utilize adiabatic spinor rotations to obtain the wave functions of
an electron. We classify the electron states further either by the spin
operator projected on the magnetic field or by the helicity operator. We then
evaluate numerically the true absorption rates accounting for the induced
emission for some parameter values typical to the outer gaps of different types
of NSs. We show that maser is indeed possible for a range of parameters. We
will also present the dependence on those parameters systematically. We finally
give a crude estimate of the amplification factor in the outer gap of NSs,
which seems to favor millisecond pulsars as the host of maser emissions.
