Tunneling spectroscopy in Mott insulators, LaNiO$_{3}$ and NdNiO$_{3}$: Pseudo gaps and real gaps

To explore the low lying excitations of prototypical charge transfer Mott insulators, we fabricated 4-terminal tunnel junctions and measured the temperature dependent tunnel conductance, along with the sheet resistivity, for LaNiO$_{3}$ a metal, and NdNiO$_{3}$ which underwent a metal insulator transition at 125 K. Films were deposited by rf magnetron sputtering on LaAlO$_{3}$ substrates. At low temperatures the tunneling conductance in LaNiO$_{3}$ develops a pronounced pseudogap 20 meV wide. NdNiO$_{3}$ exhibits a pseudo gap above the transition temperature. Just below the transition, the tunnel conductance at 0 bias is strongly suppressed but enhanced at larger bias voltages, signaling a redistribution of the quasi-particle density states as the system enters the insulating phase. At the lowest temperatures the tunnel conductance is suppressed by 4-5 orders of magnitude and a well developed gap appears - 25 meV wide. Comparisons are made with extant optical conductivity as well as recent theories based on Fermi surface instabilities.- SungBin Lee, Ru Chen, and Leon Balents, ``Landau Theory of Charge and Spin Ordering in the Nickelates,'' Phys. Rev. Lett. \textbf{106}, 016405 (2011).