High critical current density and vortex pinning of epitaxial MgB2 thin films

We have studied transport properties of a number of in situ epitaxial MgB2 thin films grown by hybrid physical-chemical vapor deposition. The films show single-crystal-like structure with a high T-c, a narrow DeltaT(c) and a high zero-field j(c). The best j(c)(H=0) of the films at 5 K is comparable to the estimated depairing current density j(0) of MgB2. The zero-field j(c)(T) data of different films can be scaled to j(t)=(1-t(2))(alpha)(1+t(2))(1/2), where j(t)=j(c)(t)/j(c)(0) and t=T/T-c. We have found that alpha=1.7+/-0.1 fits the data measured with the 4-probe method and alphasimilar to2 fits the data derived from the magnetization hysteresis M-H measurements. At low temperature the j(c)(H) of the films show a common feature that j(c)(H) first appears as a plateau when the external field is lower than H-*, where mu(0)H(*)similar to10(-2) T, and then j(c)(H)proportional toH(-beta) when the field is lower than a crossover value mu(0)H(m)similar to1 T, where betasimilar to0.7-1.0 resembling that of epitaxial high-T-c cuprate films. Above H-m, j(c)(H) decreases steeply. These results may be associated with the columnar growth mode of the films, where strong pinning centers are mainly located at the column boundaries or twin boundaries, and within a single-crystal-like column the pinning is weak. At low temperature, nearly temperature independent small peaks and flip-over in M-H loops are observed in the films, consistent with the high thermal activation energy U-0(H,T) values calculated from the resistivity measurement of the films.