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Evidence of quantum dimer excitations in Sr3Ir2O7

M. Moretti Sala, V. Schnells, S. Boseggia, L. Simonelli, A. Al-Zein, J. G. Vale, L. Paolasini, E. C. Hunter, R. S. Perry, D. Prabhakaran, A. T. Boothroyd, M. Krisch, G. Monaco, H. M. Rønnow, D. F. McMorrow, and F. Mila
Phys. Rev. B 92, 024405 – Published 6 July 2015

Abstract

The magnetic excitation spectrum in the bilayer iridate Sr3Ir2O7 has been investigated using high-resolution resonant inelastic x-ray scattering (RIXS) performed at the iridium L3 edge and theoretical techniques. A study of the systematic dependence of the RIXS spectrum on the orientation of the wave-vector transfer Q, with respect to the iridium-oxide bilayer, has revealed that the magnon dispersion is comprised of two branches well separated in energy and gapped across the entire Brillouin zone. Our results contrast with those of an earlier study which reported the existence of a single dominant branch. While these earlier results were interpreted as two overlapping modes within a spin-wave model of weakly coupled iridium-oxide planes, our results are more reminiscent of those expected for a system of weakly coupled dimers. In this latter approach, the lower- and higher-energy modes find a natural explanation as those corresponding to transverse and longitudinal fluctuations, respectively. We have therefore developed a bond-operator theory which describes the magnetic dispersion in Sr3Ir2O7 in terms of quantum dimer excitations. In our model, dimerization is produced by the leading Heisenberg exchange Jc, which couples iridium ions in adjacent planes of the bilayer. The Hamiltonian also includes in-plane exchange J, as well as further neighbor couplings and relevant anisotropies. The bond-operator theory provides an excellent account of the dispersion of both modes, while the measured Q dependence of the RIXS intensities is in reasonable qualitative accord with the spin-spin correlation function calculated from the theory. We discuss our results in the context of the quantum criticality of bilayer dimer systems in the presence of anisotropic interactions derived from strong spin-orbit coupling.

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  • Received 25 December 2014
  • Revised 13 June 2015

DOI:https://doi.org/10.1103/PhysRevB.92.024405

©2015 American Physical Society

Authors & Affiliations

M. Moretti Sala1, V. Schnells2, S. Boseggia3,4, L. Simonelli1,5, A. Al-Zein1, J. G. Vale3, L. Paolasini1, E. C. Hunter6, R. S. Perry3, D. Prabhakaran7, A. T. Boothroyd7, M. Krisch1, G. Monaco1,8, H. M. Rønnow9,10, D. F. McMorrow3, and F. Mila11

  • 1European Synchrotron Radiation Facility, BP 220, F-38043 Grenoble Cedex, France
  • 2Institute for Theoretical Physics and Astrophysics, University of Würzburg, Am Hubland, D-97074 Würzburg, Germany
  • 3London Centre for Nanotechnology and Department of Physics and Astronomy, University College London, London WC1E 6BT, United Kingdom
  • 4Diamond Light Source Ltd, Diamond House, Harwell Science and Innovation Campus, Didcot, Oxfordshire OX11 0DE, United Kingdom
  • 5CELLS-ALBA Synchrotron Radiation Facility, Carretera BP 1413, km 3.3 08290 Cerdanyola del Valles, Barcelona, Spain
  • 6Centre for Science at Extreme Conditions, Peter Guthrie Tait Road, King's Buildings, Edinburgh. EH9 3FD, United Kingdom
  • 7Clarendon Laboratory, Department of Physics, University of Oxford, Parks Road, Oxford OX1 3PU, United Kingdom
  • 8Dipartimento di Fisica, Università di Trento, via Sommarive 14, 38123 Povo (TN), Italy
  • 9Laboratory for Quantum Magnetism, École Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland
  • 10Neutron Science Laboratory, Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan
  • 11Institute of Theoretical Physics, École Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland

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Vol. 92, Iss. 2 — 1 July 2015

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