Two-dimensional vortex behavior in highly underdoped YBa_2Cu_3O_{6+x}   observed by scanning Hall probe microscopy
  We report scanning Hall probe microscopy of highly underdoped superconducting YBa_2Cu_3O_{6+x} with T_c ranging from 5 to 15 K which showed distinct flux bundles with less than one superconducting flux quantum (Phi_0) through the sample surface. The sub-Phi_0 features occurred more frequently for lower T_c, were more mobile than conventional vortices, and occurred more readily when the sample was cooled with an in-plane field component. We show that these features are consistent with kinked stacks of pancake vortices. 
J. W. Guikema, Hendrik Bluhm, D. A. Bonn, Ruixing Liang, W. N. Hardy   and K. A. Moler