Surface dynamics of the layered ruthenate Ca1.9Sr0.1RuO4

Abstract

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 Ca2–xSrxRuO4 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 Sr2RuO4 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, Ca1.9Sr0.1RuO4 exhibits a Mott transition below 154 K in bulk single crystals. However, with HREELS, we observed phonon and conductivity dynamics at the surface of Ca1.9Sr0.1RuO4 crystals below ∼130 K, indicating a lower surface Mott transition temperature than in the bulk. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)