Magnetic-force-microscope study of interlayer "kinks" in individual   vortices in underdoped cuprate YBa$_2$Cu$_3$O$_{6+x}$ superconductor
  We use magnetic force microscopy to both image and manipulate individual vortex lines threading single crystalline YBa$_2$Cu$_3$O$_{6.4}$, a layered superconductor. We find that when we pull the top of a pinned vortex, it may not tilt smoothly. Sometimes, we observe a vortex to break into discrete segments that can be described as short stacks of pancake vortices, similar to the "kinked" structure proposed by Benkraouda and Clem. Quantitative analysis gives an estimate of the pinning force and the coupling between the stacks. Our measurements highlight the discrete nature of stacks of pancake vortices in layered superconductors. 
Lan Luan, Ophir M. Auslaender, Douglas A. Bonn, Ruixing Liang, Walter   N. Hardy, Kathryn A. Moler