• Open Access

Observation of a Charge-Neutral Muon-Polaron Complex in Antiferromagnetic Cr2O3

M. H. Dehn, J. K. Shenton, S. Holenstein, Q. N. Meier, D. J. Arseneau, D. L. Cortie, B. Hitti, A. C. Y. Fang, W. A. MacFarlane, R. M. L. McFadden, G. D. Morris, Z. Salman, H. Luetkens, N. A. Spaldin, M. Fechner, and R. F. Kiefl
Phys. Rev. X 10, 011036 – Published 14 February 2020

Abstract

We report a comprehensive muon spin rotation (μSR) study of the prototypical magnetoelectric antiferromagnet Cr2O3. We find the positively charged muon (μ+) occupies several distinct interstitial sites and displays a rich dynamic behavior involving local hopping, thermally activated site transitions, and the formation of a charge-neutral complex composed of a muon and an electron polaron. The discovery of such a complex has implications for the interpretation of μSR spectra in a wide range of magnetic oxides and opens a route to study the dopant characteristics of interstitial hydrogen impurities in such materials. We address implications arising from implanting a μ+ into a linear magnetoelectric and discuss the challenges of observing a local magnetoelectric effect generated by the charge of the muon.

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  • Received 1 September 2019
  • Revised 6 November 2019
  • Accepted 26 November 2019

DOI:https://doi.org/10.1103/PhysRevX.10.011036

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

M. H. Dehn1,2,3,*, J. K. Shenton4, S. Holenstein5,6, Q. N. Meier4, D. J. Arseneau3, D. L. Cortie1,2,7,†, B. Hitti3, A. C. Y. Fang1,‡, W. A. MacFarlane2,3,7, R. M. L. McFadden2,7, G. D. Morris3, Z. Salman6, H. Luetkens6, N. A. Spaldin4, M. Fechner4,8, and R. F. Kiefl1,2,3

  • 1Department of Physics and Astronomy, University of British Columbia, Vancouver, BC V6T 1Z1, Canada
  • 2Stewart Blusson Quantum Matter Institute, University of British Columbia, Vancouver, BC V6T 1Z4, Canada
  • 3TRIUMF, Vancouver, BC V6T 2A3, Canada
  • 4Department of Materials, ETH Zurich, CH-8093 Zürich, Switzerland
  • 5Physik-Institut der Universität Zürich, CH-8057 Zürich, Switzerland
  • 6Laboratory for Muon Spin Spectroscopy, Paul Scherrer Institut, 5232 Villigen PSI, Switzerland
  • 7Department of Chemistry, University of British Columbia, Vancouver, BC V6T 1Z1, Canada
  • 8Max Planck Institute for the Structure and Dynamics of Matter, 22761 Hamburg, Germany

  • *mdehn@triumf.ca
  • Institute for Superconducting and Electronic Materials, Australian Institute for Innovative Materials, University of Wollongong, North Wollongong, NSW 2500, Australia.
  • Department of Physics, Simon Fraser University, Burnaby, BC V5A 1S6, Canada.

Popular Summary

Hydrogen is one of the most ubiquitous impurities in semiconductors and insulators, and has a significant impact on their electronic properties. Understanding how hydrogen alters the behavior of these materials is critical for precise control of charge carriers, upon which modern electronic technology is based. Since isolated hydrogen is extremely hard to study directly, most of the available information comes from the study of muonium, a charge-neutral muon-electron bound state whose electronic structure is virtually identical to hydrogen. In magnetic materials, however, muonium is not observed, and charge-neutral muon states are generally not considered relevant. Here, we report on the discovery of a charge-neutral complex in antiferromagnetic Cr2O3, demonstrating that such states do exist in magnetic materials, thus opening a route to study the electronic structure of hydrogen defects in magnetic oxides.

Combining muon spin rotation experiments with density-functional theory, we establish the existence of a complex composed of a muon and an electron polaron, a quasiparticle composed of an electron and an attendant distortion of the surrounding atomic lattice. Crucially, we find that in a magnetic material, such muon-polaron complexes do not display any signatures conventionally associated with muonium, concealing their presence. This result strongly suggests that similar complexes exist in a wide variety of magnetic insulators and semiconductors, mimicking the behavior of analogous hydrogen impurities.

Our discovery enables the investigation of how hydrogen impurities affect magnetic transition-metal oxides, where they are poorly understood but may significantly influence the electronic properties and ultimately the device functionality of such materials.

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Vol. 10, Iss. 1 — January - March 2020

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