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

Abstract

The critical magnetic fields H c‖ and H c2 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 H c‖/H c2 ratio at 2 K exceeds 6 and increases with increasing temperature. The dependence H c‖(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 T c . As the electron mean free path decreases under irradiation of the film with a low dose of He+ ions, the critical field H c‖ remains unchanged near T c 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 H c‖(T).