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Quantum and classical criticality in a dimerized quantum antiferromagnet

Nature Physics volume 10, pages 373–379 (2014)Cite this article

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Abstract

A quantum critical point (QCP) is a singularity in the phase diagram arising because of quantum mechanical fluctuations. The exotic properties of some of the most enigmatic physical systems, including unconventional metals and superconductors, quantum magnets and ultracold atomic condensates, have been related to the importance of critical quantum and thermal fluctuations near such a point. However, direct and continuous control of these fluctuations has been difficult to realize, and complete thermodynamic and spectroscopic information is required to disentangle the effects of quantum and classical physics around a QCP. Here we achieve this control in a high-pressure, high-resolution neutron scattering experiment on the quantum dimer material TlCuCl3. By measuring the magnetic excitation spectrum across the entire quantum critical phase diagram, we illustrate the similarities between quantum and thermal melting of magnetic order. We prove the critical nature of the unconventional longitudinal (Higgs) mode of the ordered phase by damping it thermally. We demonstrate the development of two types of criticality, quantum and classical, and use their static and dynamic scaling properties to conclude that quantum and thermal fluctuations can behave largely independently near a QCP.

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Figure 1: Pressure–temperature phase diagram of TlCuCl3 extended to finite energies, revealing quantum and thermal critical dynamics.
Figure 2: INS spectra collected at Q = (0 4 0) r.l.u., the wavevector of the minimum energy gap in the quantum disordered phase, where magnetic order is induced with increasing pressure.
Figure 3: Spin dynamics at the quantum and thermal melting transitions.
Figure 4: Quantum and classical criticality.

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Change history

  • 09 April 2014

    In the version of this Article originally published online, in Fig. 2b the data points, the purple shaded region, the green dashed curve and a part of the solid black curve were missing. This has now been corrected in all versions of the Article.

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Acknowledgements

We are grateful to S. Sachdev, A. Sandvik and especially M. Vojta for helpful comments. We thank the sample environment team at the Institut Laue Langevin, where these measurements were performed, for their assistance. This work was supported by the EPSRC, the Royal Society, the Swiss NSF, and the NSF of China under Grant No. 11174365.

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Authors and Affiliations

  1. London Centre for Nanotechnology and Department of Physics and Astronomy, University College London, London WC1E 6BT, UK

    P. Merchant, D. F. McMorrow & Ch. Rüegg

  2. Department of Physics, Renmin University of China, Beijing 100872, China

    B. Normand

  3. Department of Chemistry and Biochemistry, University of Bern, CH–3012 Bern, Switzerland

    K. W. Krämer

  4. Institut Laue Langevin, BP 156, 38042 Grenoble Cedex 9, France

    M. Boehm

  5. Laboratory for Neutron Scattering, Paul Scherrer Institute, CH–5232 Villigen, Switzerland

    Ch. Rüegg

  6. DPMC–MaNEP, University of Geneva, CH–1211 Geneva, Switzerland

    Ch. Rüegg

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Contributions

P.M. and Ch.R. carried out the experiments with the help of instrument scientist M.B. TlCuCl3 single crystals were synthesized by K.W.K. The theoretical and experimental framework was conceived by Ch.R., D.F.M. and B.N. Data refinement and figure preparation were performed by P.M. and Ch.R. The text was written by B.N. and Ch.R.

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Correspondence to Ch. Rüegg.

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Merchant, P., Normand, B., Krämer, K. et al. Quantum and classical criticality in a dimerized quantum antiferromagnet. Nature Phys 10, 373–379 (2014). https://doi.org/10.1038/nphys2902

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