Vortex-glass transition and vortex pinning behavior in three-dimensional NbTiN epitaxial films

In the present paper, the vortex dynamics in two NbTiN epitaxial films have been explored via analyzing the current-voltage (I-V) curves measured at different temperatures and fields. The thicknesses of the two films are similar to 50 and similar to 100 nm, respectively, and they are three-dimensional (3D) with respect to superconductivity. The films transform from normal state to superconducting state with decreasing temperature at zero field, and the values of superconducting transition temperature T c are 8.73 and 12.03 K for the similar to 50- and similar to 100-nm-thick films, respectively. When a magnetic field with magnitude 0.5 T less than or similar to mu 0 H less than or similar to 7 T and perpendicular to the film plane is applied, the isothermal I-V data show a 3D vortex-glass (VG) scaling collapse at each field, suggesting that a 3D vortex-liquid phase to VG phase transition occurs in the NbTiN films. By analyzing the critical current density, we found that core pinning is the main pinning mechanism in the films. In addition, it is found that the normal point-like pinning dominates the pinning behavior at low field regime. While in the high field regime, besides the normal point-like pinning, other pinning mechanisms, such as Delta kappa pinning, also exist in the films. Our results not only provide strong experimental support for the occurrence of VG phase transition in conventional superconductors, but also provide fundamental information for the application of NbTiN films.