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.