Magnetic Field Effects on the Superconducting and Quantum Critical Properties of Layered Systems with Dirac Electrons

We study the effects of an external magnetic field on the superconducting properties of a quasi-two-dimensional system of Dirac electrons at an arbitrary temperature. An explicit expression for the superconducting gap is obtained as a function of temperature, magnetic field and coupling parameter ($\lambda_{\rm R}$). From this, we extract the $B \times \lambda_{\rm R}$, $T\times \lambda_{\rm R}$ and $B \times T$ phase diagrams. The last one shows a linear decay of the critical field for small values thereof, which is similar to the behavior observed experimentally in the copper doped dichalcogenide $Cu_xTiSe_2$ and also in intercalated graphite. The second one, presents a coupling dependent critical temperature $T_c$ that resembles the one observed in high-$T_c$ cuprates in the underdoped region and also in $Cu_xTiSe_2$. The first one, exhibits 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. This should be observed in planar materials containing Dirac electrons, such as $Cu_xTiSe_2$.