Soft phonon mode coupled with antiferromagnetic order in incipient-ferroelectric Mott insulators Sr1xBaxMnO3

H. Sakai, J. Fujioka, T. Fukuda, M. S. Bahramy, D. Okuyama, R. Arita, T. Arima, A. Q. R. Baron, Y. Taguchi, and Y. Tokura
Phys. Rev. B 86, 104407 – Published 5 September 2012

Abstract

Infrared optical and inelastic x-ray scattering spectra have been systematically investigated in combination with first-principles calculations for paraelectric and antiferromagnetic perovskite Sr1xBaxMnO3 (x=0–0.3) single crystals, which are close to a ferroelectric transition arising from off-center displacement of magnetic Mn4+ ions. All the phonon dispersions measured for the parent compound of x=0 agree well with the results of the first-principles calculation. As the Ba concentration increases, one optical phonon rapidly softens toward zero frequency at room temperature, while the other phonons are almost unchanged. This soft-mode behavior is also reproduced by the first-principles calculations, from which we have predicted the vibration mode of all the optical phonons. The results of the infrared measurements at various temperatures indicate that only the soft phonon mode shows marked temperature variation relevant to the antiferromagnetic transition for all x, whereas other optical modes are almost independent of temperature. The conventional evolution of a soft phonon with decreasing temperature is prevented by the onset of the magnetic order. Below the antiferromagnetic-transition temperature, the soft mode hardens with decreasing temperature and then resoftens toward the lowest temperature. A similar temperature dependence was observed in the nonzero-momentum region by means of the inelastic x-ray scattering measurements, although its magnitude decreases as the momentum is increased from zero. Such a nonmonotonic temperature profile of the soft-mode energy is well explained on the basis of a phenomenological spin-phonon coupling model, which suggests the largest coupling constant yet attained.

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  • Received 6 May 2012

DOI:https://doi.org/10.1103/PhysRevB.86.104407

©2012 American Physical Society

Authors & Affiliations

H. Sakai1,*, J. Fujioka2, T. Fukuda3,4, M. S. Bahramy1, D. Okuyama1, R. Arita1,2, T. Arima5, A. Q. R. Baron4,6, Y. Taguchi1, and Y. Tokura1,2,7

  • 1Cross-Correlated Materials Research Group (CMRG) and Correlated Electron Research Group (CERG), RIKEN Advanced Science Institute, Wako 351-0198, Japan
  • 2Department of Applied Physics, University of Tokyo, Tokyo 113-8656, Japan
  • 3Synchrotron Radiation Research Unit, SPring-8/JAEA, Hyogo 679-5148, Japan
  • 4Materials Dynamics Laboratory, RIKEN SPring-8 Center, Hyogo 679-5148, Japan
  • 5Department of Advanced Materials Science, University of Tokyo, Kashiwa 277-8561, Japan
  • 6Research and Utilization Division, SPring-8/JASRI, Hyogo 679-5198 Japan
  • 7Multiferroics Project, Japan Science and Technology Agency, Tokyo 113-8656, Japan

  • *Present address: School of Physics and Astronomy, University of St. Andrews, North Haugh, St. Andrews KY16 9SS, United Kingdom.

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Vol. 86, Iss. 10 — 1 September 2012

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