Surface dynamics of the layered ruthenate Ca_1.9Sr _0.1RuO₄

The strong coupling between electronic, lattice, orbital, and spin degrees of freedom in transition metal oxides implies that the symmetry breaking at the surface of these materials should lead to a rich surface phase diagram and interesting new physics. Here we investigate the surface phases of the Ca _2-xSr_xRuO₄ system that (in bulk) can be tuned from a Mott insulator (x = 0) to a p-wave superconductor (x = 2) without changing the electron count. This system therefore displays a "bandwidth controlled" metal-insulator transition. Electronic and structural properties of these surfaces are studied by variable temperature High Resolution Electron Energy Loss Spectroscopy (HREELS) and Low Energy Electron Diffraction (LEED). It is found that broken symmetry on the Sr ₂RuO₄ surface results in the rotation of RuO₄ octahedra relative to bulk positions along the c-axis, changing the relative in-plane Ru-O bond length and shifting the surface phonon energies. The substitution of Ca ions in Sr₂RuO₄ crystals results in a rotation plus a tilt of the RuO₆ octahedra creating a Mott-insulator phase. In particular, Ca_1.9Sr_0.1RuO₄ exhibits a Mott transition below 154 K in bulk single crystals. However, with HREELS, we observed phonon and conductivity dynamics at the surface of Ca _1.9Sr_0.1RuO₄ crystals below ∼130 K, indicating a lower surface Mott transition temperature than in the bulk.