Critical magnetic field of the vortex-free state in NbC thin films and prospects for its observation in MgB2

The critical magnetic fields Hc‖ and Hc2 are measured for thin films of the isotropic superconductor NbC. It is revealed that the critical fields exhibit strong anisotropy due to the vortex-free state of the film in a magnetic field aligned parallel to its surface. The Hc‖/Hc2 ratio at 2 K exceeds 6 and increases with increasing temperature. The dependence Hc‖(T) agrees quantitatively with the concepts of microscopic theory on the vortex-free state of a thin film of a clean superconductor in the temperature range below Tc. As the electron mean free path decreases under irradiation of the film with a low dose of He+ ions, the critical field Hc‖ remains unchanged near Tc but increases significantly at lower temperatures. The well-known theoretical models are used to estimate the electronic parameters and thicknesses of MgB2 films for which the specific features associated with the vortex-free state of the two-gap superconductor can manifest themselves in the temperature dependence of the critical magnetic field Hc‖(T).