Broken particle-hole symmetry at atomically flat a-axis YBa2Cu3O7-d   interfaces
  We have studied quasiparticle tunneling into atomically flat a-axis films of YBa2Cu3O7-d and DyBa2Cu3O7-d through epitaxial CaTiO3 barriers. The junction heterostructures were grown by oxide molecular beam epitaxy and were carefully optimized using in-situ monitoring techniques, resulting in unprecedented crystalline perfection of the superconductor/insulator interface. Below Tc, the tunneling conductance shows the evolution of a large unexpected asymmetrical feature near zero bias. This is evidence that superconducting YBCO crystals, atomically truncated along the lobe direction with a titanate layer, have intrinsically broken particle-hole symmetry over macroscopically large areas. 
Bruce A. Davidson, Revaz Ramazashvili, Simon Kos, and James N.   Eckstein