We employed a scanning Hall probe microscope to detect the hopping of individual vortices between pinning sites along grain boundaries in YBa 2Cu3O6+δ thin films in the presence of an applied current. Detecting the motion of individual vortices allowed us to probe the current-voltage (I-V) characteristics of the grain boundary with voltage sensitivity below a femtovolt. We find a very sharp onset of dissipation with V ∞ In with an unprecedented high exponent of n≈290 that shows essentially no dependence on temperature or grain boundary angle. Our data have no straightforward explanation within the existing grain boundary transport models.
Bibliographical noteFunding Information:
The authors would like to thank Janice Guikema and Clifford Hicks for Hall sensors, H. Karapetyan for help in measurements, and M. Beasley and A. Gurevich for useful discussions. This work is funded by an Air Force Multi-University Research Initiative (MURI), by the Center for Probing the Nanoscale (CPN), an NSF NSEC, NSF Grant no. PHY-0425897, and by the U.S.-Israel Binational Science Foundation (BSF). E.Z. acknowledges the support of EUContract No. EU FP7 ERC-AdG.