Negative Refraction of Excitations in the Bose-Hubbard Model
  Ultracold atoms in optical lattices provide a unique opportunity to study Bose- Hubbard physics. In this work we show that by considering a spatially varying onsite interaction it is possible to manipulate the motion of excitations above the Mott phase in a Bose-Hubbard system. Specifically, we show that it is possible to "engineer" regimes where excitations will negatively refract, facilitating the construction of a flat lens. 
R.A. Henry, J.Q. Quach, C-H. Su, A.D. Greentree, A.M. Martin
cond-mat.quant-gas quant-ph