Site-Selective Spectroscopy of Strontium Ruthenate with Soft X-Ray Emission
One of the most burning questions in contemporary solid-state science is the nature of the mechanism driving superconductivity in the high-temperature superconductors (HTSCs), first discovered more than a decade ago. Clues come not only from direct examination of the materials themselves but also from studies of related materials with similar composition and structure.
A multi-institutional group working on Beamline 8.0.1 at the ALS and at the Photon Factory in Japan has demonstrated the ability to selectively examine the electronic structure of bonding electrons associated with oxygen in two different sites in the crystal structures of strontium ruthenate compounds. Although strontium ruthenate is not a HTSC, it has a similar structure to the cuprate HTSCs but with atomic planes containing ruthenium and oxygen atoms rather than copper and oxygen atoms. For one oxygen site, the experimenters were able to demonstrate hybridization (mixing) between electronic states of oxygen and ruthenium. The nature of the hybridization is different from that occurring in HTSCs and results in a different type of chemical bond.
The figure shows how the researchers were able to spectroscopically distinguish oxygen atoms in the ruthenium-oxygen planes, called O(1) oxygens, from oxygen atoms outside the planes, called O(2) or apical oxygens, and how they demonstrated hybridization of ruthenium and oxygen electronic states. The top two curves show the distinctly different spectra for soft x-ray emission from oxygen in the strontium ruthenate compound Sr2RuO4. Excitation at different soft x-ray energies provides the means of distinguishing the oxygen sites. The bottom two curves show ultraviolet photoemission (UPS) and soft x-ray emission from ruthenium. The prominent peak at -6 eV in the ruthenium spectra matches the shoulder in the oxygen O(1) spectrum and is evidence for hybridization of ruthenium and in-plane oxygen electrons, as predicted by theory.
Research conducted by E. Z. Kurmaev, D. A. Zatsepin, and N. Ovechkina (Russian Academy of Sciences, Ural Division, Yekaterinburg); S. Stadler and D. L. Ederer (Tulane University); Y. Harada, S. Shin, M. Kasai, and Y. Tokura (University of Tokyo); M. M. Grush and T. A. Callcott (University of Tennessee, Knoxville); R. C. C. Perera (Berkeley Lab); T. Takahashi (Tokuku University, Japan); and K. Chandrasekaran, R. Vijayaraghavan, and U. V. Varadaraju (Indian Institute of Technology); using the soft x-ray fluorescence endstation at Beamline 8.0.1. Some measurements were also made at the Photon Factory, KEK Laboratory, Japan.
Funding: Russian State Program on Superconductivity, Russian Science Foundation for Fundamental Research, NATO, National Science Foundation, U. S. Department of Energy, and Louisiana Education Quality Special Fund.
ALSNews article about this science highlight
More ALS Science