0 O ct 2 00 7 I-V characteristics of the vortex state in MgB 2 thin films

The current-voltage (I-V ) characteristics of various MgB2 films have been studied at different magnetic fields parallel to c-axis. At fields μ0H between 0 and 5 T, vortex liquid-glass transitions were found in the I-V isotherms. Consistently, the I-V curves measured at different temperatures show a scaling behavior in the framework of quasi-two-dimension (quasi-2D) vortex glass theory. However, at μ0H ≥ 5 T, a finite dissipation was observed down to the lowest temperature here, T = 1.7 K, and the I-V isotherms did not scale in terms of any known scaling law, of any dimensionality. We suggest that this may be caused by a mixture of σ band vortices and π band quasiparticles. Interestingly, the I-V curves at zero magnetic field can still be scaled according to the quasi-2D vortex glass formalism, indicating an equivalent effect of self-field due to persistent current and applied magnetic field.