• Open Access

Spin-Orbital Excitations in Ca2RuO4 Revealed by Resonant Inelastic X-Ray Scattering

L. Das, F. Forte, R. Fittipaldi, C. G. Fatuzzo, V. Granata, O. Ivashko, M. Horio, F. Schindler, M. Dantz, Yi Tseng, D. E. McNally, H. M. Rønnow, W. Wan, N. B. Christensen, J. Pelliciari, P. Olalde-Velasco, N. Kikugawa, T. Neupert, A. Vecchione, T. Schmitt, M. Cuoco, and J. Chang
Phys. Rev. X 8, 011048 – Published 22 March 2018
PDFHTMLExport Citation

Abstract

The strongly correlated insulator Ca2RuO4 is considered as a paradigmatic realization of both spin-orbital physics and a band-Mott insulating phase, characterized by orbitally selective coexistence of a band and a Mott gap. We present a high resolution oxygen K-edge resonant inelastic x-ray scattering study of the antiferromagnetic Mott insulating state of Ca2RuO4. A set of low-energy (about 80 and 400 meV) and high-energy (about 1.3 and 2.2 eV) excitations are reported, which show strong incident light polarization dependence. Our results strongly support a spin-orbit coupled band-Mott scenario and explore in detail the nature of its exotic excitations. Guided by theoretical modeling, we interpret the low-energy excitations as a result of composite spin-orbital excitations. Their nature unveils the intricate interplay of crystal-field splitting and spin-orbit coupling in the band-Mott scenario. The high-energy excitations correspond to intra-atomic singlet-triplet transitions at an energy scale set by Hund’s coupling. Our findings give a unifying picture of the spin and orbital excitations in the band-Mott insulator Ca2RuO4.

  • Figure
  • Figure
  • Figure
  • Figure
  • Figure
  • Received 4 July 2017
  • Revised 28 January 2018

DOI:https://doi.org/10.1103/PhysRevX.8.011048

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

L. Das1, F. Forte2,3, R. Fittipaldi2,3, C. G. Fatuzzo4,‡, V. Granata2,3, O. Ivashko1, M. Horio1, F. Schindler1, M. Dantz5, Yi Tseng5, D. E. McNally5, H. M. Rønnow4, W. Wan6, N. B. Christensen6, J. Pelliciari5,*, P. Olalde-Velasco5,†, N. Kikugawa7,8, T. Neupert1, A. Vecchione2,3, T. Schmitt5, M. Cuoco2,3, and J. Chang1

  • 1Physik-Institut, Universität Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland
  • 2CNR-SPIN, I-84084 Fisciano, Salerno, Italy
  • 3Dipartimento di Fisica “E.R. Caianiello,” Università di Salerno, I-84084 Fisciano, Salerno, Italy
  • 4Institute of Physics, École Polytechnique Fedérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland
  • 5Swiss Light Source, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland
  • 6Department of Physics, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark
  • 7National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, 305-0047 Japan
  • 8National High Magnetic Field Laboratory, Tallahassee, Florida 32310, USA

  • *Present address: Department of Physics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
  • Present address: Instituto de Fisica, Benemerita Universidad Autonoma de Puebla, Apdo. Postal J-48, Puebla, Puebla 72570, Mexico.
  • Present address: Materials Sciences Division, Lawrence Berkeley National Lab, 1 Cyclotron Road, Berkeley, CA 94720, USA.

Popular Summary

Mott insulators are materials that should conduct electricity, according to conventional theories, but become insulating because of strong interactions between electrons. A particularly interesting situation arises in some materials where the spin and orbital momentum of electrons within atoms also interact (a behavior known as spin-orbit coupling). The compound Ca2RuO4 is one such case, where multiple atomic-level interactions give rise to a wealth of exotic phenomena. We used x-ray scattering experiments to investigate Ca2RuO4 in its Mott-insulating state and explore these interactions in detail.

Specifically, we conducted resonant inelastic x-ray scattering (RIXS) experiments on the oxygen K resonance. These experiments provided access to the low-energy sectors of orbital excitations that have a spin-orbit-coupled nature. We identified four different sectors of excitations. These results suggest that one orbital turns into an almost insulating band and hence becomes a “silent” channel. The remaining (now nearly half-filled) orbitals responsible for the Mott transition constitute the spin-orbit-coupled excitations that are unique fingerprints of the band-Mott-insulating ground state.

Comparing theoretical modeling of Ca2RuO4 with our data suggests that the reported excitation sectors have an internal structure that we only partially resolved. Future improvements to the RIXS instrumentation should allow access to the excitation spectrum from both the ruthenium and oxygen site directly, with improved energy resolution. This will uncover another layer of complexity of this intricate Mott-insulating ground state.

Key Image

Article Text

Click to Expand

Supplemental Material

Click to Expand

References

Click to Expand
Issue

Vol. 8, Iss. 1 — January - March 2018

Subject Areas
Reuse & Permissions
Author publication services for translation and copyediting assistance advertisement

Authorization Required


×
×

Images

×

Sign up to receive regular email alerts from Physical Review X

Reuse & Permissions

It is not necessary to obtain permission to reuse this article or its components as it is available under the terms of the Creative Commons Attribution 4.0 International license. This license permits unrestricted use, distribution, and reproduction in any medium, provided attribution to the author(s) and the published article's title, journal citation, and DOI are maintained. Please note that some figures may have been included with permission from other third parties. It is your responsibility to obtain the proper permission from the rights holder directly for these figures.

×

Log In

Cancel
×

Search


Article Lookup

Paste a citation or DOI

Enter a citation
×