  We study the effects of an external magnetic field on thensuperconducting
phase diagram of a quasi-two-dimensional system of Dirac electrons at an
arbitrary temperature. At zero temperature, there is a quantum phase transition
connecting a normal and a superconducting phase, occurring at a critical line
that corresponds to a magnetic field dependent critical coupling parameter,
which should be observed in planar materials containing Dirac electrons, such
as $Cu_xTiSe_2$. Moreover, the superconducting gap is obtained as a function of
temperature, magnetic field and coupling parameter ($\lambda_{\rm R}$). From
this, we extract the critical magnetic field $ B_{ c } $ as a function of the
temperature. For small values of $ B_{ c } $, we obtain a linear decay of the
critical field, which is similar to the behavior observed experimentally in the
copper doped dichalcogenide $Cu_xTiSe_2$ and also in intercalated graphite.
