Pressure dependence of phonon modes across the tetragonal to collapsed-tetragonal phase transition in CaFe2As2

R. Mittal, R. Heid, A. Bosak, T. R. Forrest, S. L. Chaplot, D. Lamago, D. Reznik, K.-P. Bohnen, Y. Su, N. Kumar, S. K. Dhar, A. Thamizhavel, Ch. Rüegg, M. Krisch, D. F. McMorrow, Th. Brueckel, and L. Pintschovius
Phys. Rev. B 81, 144502 – Published 6 April 2010

Abstract

The pressure dependence of a large number of phonon modes in CaFe2As2 with energies covering the full range of the phonon spectrum has been studied using inelastic x-ray and neutron scatterings. The pressure range was large enough to cover the first-order phase transition into the so-called collapsed phase where the c-axis contracts by about 6% whereas a and b axes expand by about 1.5%. Our main result is that pressure-induced phonon frequency shifts are well explained by the changes in relevant bond lengths throughout the pressure range, including those associated with the first-order phase transition. Specifically, the frequencies of phonons polarized in the ab plane as well as the Fe-As bond lengths change little across the phase transition. On the other hand, the transverse-acoustic phonons propagating along the c direction stiffen very significantly in response to the large contraction of the bonds along the c axis. Nonmagnetic density-functional calculations describe the frequencies in both the zero pressure and in the collapsed phase in a satisfactory way if based on the respective experimental crystal structures. This suggests that there is no need to invoke changes in magnetic moments on Fe atoms to explain the pressure-induced frequency shifts.

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  • Received 9 November 2009

DOI:https://doi.org/10.1103/PhysRevB.81.144502

©2010 American Physical Society

Authors & Affiliations

R. Mittal1,2, R. Heid3, A. Bosak4, T. R. Forrest5, S. L. Chaplot2, D. Lamago3,6, D. Reznik3,7, K.-P. Bohnen3, Y. Su1, N. Kumar8, S. K. Dhar8, A. Thamizhavel8, Ch. Rüegg5, M. Krisch4, D. F. McMorrow5, Th. Brueckel1,9, and L. Pintschovius3

  • 1Juelich Centre for Neutron Science, IFF, Forschungszentrum Juelich, Outstation at FRM II, Lichtenbergstr. 1, D-85747 Garching, Germany
  • 2Solid State Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India
  • 3Institut für Festkörperphysik, Karlsruher Institut für Technologie (KIT), Postfach 3640, D-76121 Karlsruhe, Germany
  • 4European Synchrotron Radiation Facility, BP 220, F-38043 Grenoble Cedex, France
  • 5London Centre for Nanotechnology, Department of Physics and Astronomy, University College London, London WC1E 6BT, United Kingdom
  • 6Laboratoire Léon Brillouin, CEA-Saclay, F-91191 Gif sur Yvette Cedex, France
  • 7Department of Physics, University of Colorado, Boulder, Colorado 80309, USA
  • 8Department of Condensed Matter Physics and Material Sciences, Tata Institute of Fundamental Research, Homi Bhabha Road, Colaba, Mumbai 400 005, India
  • 9Institut fuer Festkoerperforschung, Forschungszentrum Juelich, D-52425 Juelich, Germany

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Vol. 81, Iss. 14 — 1 April 2010

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