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Weak coupling of pseudoacoustic phonons and magnon dynamics in the incommensurate spin-ladder compound Sr14Cu24O41

Xi Chen, Dipanshu Bansal, Sean Sullivan, Douglas L. Abernathy, Adam A. Aczel, Jianshi Zhou, Olivier Delaire, and Li Shi
Phys. Rev. B 94, 134309 – Published 21 October 2016

Abstract

Intriguing lattice dynamics have been predicted for aperiodic crystals that contain incommensurate substructures. Here we report inelastic neutron scattering measurements of phonon and magnon dispersions in Sr14Cu24O41, which contains incommensurate one-dimensional (1D) chain and two-dimensional (2D) ladder substructures. Two distinct pseudoacoustic phonon modes, corresponding to the sliding motion of one sublattice against the other, are observed for atomic motions polarized along the incommensurate axis. In the long wavelength limit, it is found that the sliding mode shows a remarkably small energy gap of 1.7–1.9 meV, indicating very weak interactions between the two incommensurate sublattices. The measurements also reveal a gapped and steep linear magnon dispersion of the ladder sublattice. The high group velocity of this magnon branch and weak coupling with acoustic and pseudoacoustic phonons can explain the large magnon thermal conductivity in Sr14Cu24O41 crystals. In addition, the magnon specific heat is determined from the measured total specific heat and phonon density of states and exhibits a Schottky anomaly due to gapped magnon modes of the spin chains. These findings offer new insights into the phonon and magnon dynamics and thermal transport properties of incommensurate magnetic crystals that contain low-dimensional substructures.

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  • Received 6 May 2016
  • Revised 26 August 2016

DOI:https://doi.org/10.1103/PhysRevB.94.134309

©2016 American Physical Society

Physics Subject Headings (PhySH)

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Xi Chen1, Dipanshu Bansal2, Sean Sullivan1, Douglas L. Abernathy3, Adam A. Aczel3, Jianshi Zhou1,4, Olivier Delaire5,2, and Li Shi1,4,*

  • 1Materials Science and Engineering Program, Texas Materials Institute, The University of Texas at Austin, Austin, Texas 78712, USA
  • 2Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
  • 3Quantum Condensed Matter Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
  • 4Department of Mechanical Engineering, The University of Texas at Austin, Austin, Texas 78712, USA
  • 5Department of Mechanical Engineering and Materials Science, Duke University, Durham, North Carolina 27708, USA

  • *lishi@mail.utexas.edu

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Issue

Vol. 94, Iss. 13 — 1 October 2016

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