Abstract

Spontaneous symmetry-breaking quantum phase transitions play an essential role in condensed-matter physics1,2,3. The collective excitations in the broken-symmetry phase near the quantum critical point can be characterized by fluctuations of phase and amplitude of the order parameter. The phase oscillations correspond to the massless Nambu–Goldstone modes whereas the massive amplitude mode, analogous to the Higgs boson in particle physics4,5, is prone to decay into a pair of low-energy Nambu–Goldstone modes in low dimensions2,6,7. Especially, observation of a Higgs amplitude mode in two dimensions is an outstanding experimental challenge. Here, using inelastic neutron scattering and applying the bond-operator theory, we directly and unambiguously identify the Higgs amplitude mode in a two-dimensional S  =  1/2 quantum antiferromagnet C9H18N2CuBr4 near a quantum critical point in two dimensions. Owing to an anisotropic energy gap, it kinematically prevents such decay and the Higgs amplitude mode acquires an infinite lifetime.

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Acknowledgements

T.H. thanks C. D. Batista for the insightful discussion, Q. Ye for the initial neutron polarization set-up and R. Erwin for the development of 3He efficiency correction software. T.H. also thanks D. L. Q. Castro, Z. L. Lu and Z. Hüsges for the assistance during the experiment. One of the authors (M.M.) is supported by JSPS KAKENHI Grant Number 26400332. A portion of this research used resources at the High Flux Isotope Reactor, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory. Access to MACS was provided by the Center for High Resolution Neutron Scattering, a partnership between NIST and NSF under Agreement No. DMR-1508249.

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Affiliations

  1. Quantum Condensed Matter Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA

    • Tao Hong
    • , Sachith E. Dissanayake
    •  & David A. Tennant
  2. Department of Physics, Shizuoka University, Shizuoka 422-8529, Japan

    • Masashige Matsumoto
  3. National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA

    • Yiming Qiu
    • , Wangchun Chen
    • , Thomas R. Gentile
    •  & Shannon Watson
  4. Department of Materials Science and Engineering, University of Maryland, College Park, Maryland 20742, USA

    • Wangchun Chen
  5. Department of Chemistry, The University of Jordan, Amman 11942, Jordan

    • Firas F. Awwadi
  6. Carlson School of Chemistry and Biochemistry, Clark University, Worcester, Massachusetts 01610, USA

    • Mark M. Turnbull
  7. Instrument and Source Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA

    • Harish Agrawal
  8. Helmholtz-Zentrum Berlin für Materialien und Energie, D-14109 Berlin, Germany

    • Rasmus Toft-Petersen
    •  & Bastian Klemke
  9. Lehrstuhl für Theoretische Physik I, TU Dortmund, D-44221 Dortmund, Germany

    • Kris Coester
  10. Lehrstuhl für Theoretische Physik I, Staudtstrasse 7, Universität Erlangen-Nürnberg, D-91058, Germany

    • Kai P. Schmidt

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Contributions

T.H. conceived the project. F.F.A. and M.M.T. prepared the samples. The polarization apparatus and corrections were provided by W.C., T.R.G. and S.W. T.H., Y.Q., H.A., R.T.-P. and B.K. performed the neutron-scattering measurements. T.H., M.M., D.A.T., S.E.D., K.C. and K.P.S. analysed the data. All authors contributed to writing of the manuscript.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Tao Hong.

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https://doi.org/10.1038/nphys4182

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