A study of the Mott transition in Ca₃Ru₂O₇ by tunneling spectroscopy

The extensive d-electron orbitals in Ca₃Ru₂O₇ generate strong coupling between the spin-orbit and the lattice, resulting in an exotic ground state and leading to interesting properties at low temperatures. For instance, this compound undergoes a metal-non-metal transition at 48 K. There have been numerous amounts of theoretical efforts made to understand the nature of the metal-non-metal transition and experimental data are important for a proper understanding of this phenomenon. We have measured the density of states of Ca₃Ru₂O₇ across the transition by tunneling spectroscopy. This is possible because Ca₃Ru₂O₇ remains poorly conducting even in its non-metallic phase at low temperatures. The density of states is very different along the c-axis and a b-plane directions. Gap features develop at temperatures below the transition, which we can designate as the charge gap. We discuss the properties and temperature dependence of this gap feature.