Limit on Lorentz and CPT Violation of the Neutron Using a Two-Species   Noble-Gas Maser
  A search for sidereal variations in the frequency difference between co-located 129-Xe and 3-He Zeeman masers sets the most stringent limit to date on leading-order Lorentz and CPT violation involving the neutron, consistent with no effect at the level of 10^{-31} GeV. 
D. Bear, R.E. Stoner, R.L. Walsworth, V. Alan Kostelecky, Charles D.   Lane