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Bounding the pseudogap with a line of phase transitions in YBa2Cu3O6+δ


Close to optimal doping, the copper oxide superconductors show ‘strange metal’ behaviour1,2, suggestive of strong fluctuations associated with a quantum critical point3,4,5,6. Such a critical point requires a line of classical phase transitions terminating at zero temperature near optimal doping inside the superconducting ‘dome’. The underdoped region of the temperature–doping phase diagram from which superconductivity emerges is referred to as the ‘pseudogap’7,8,9,10,11,12,13 because evidence exists for partial gapping of the conduction electrons, but so far there is no compelling thermodynamic evidence as to whether the pseudogap is a distinct phase or a continuous evolution of physical properties on cooling. Here we report that the pseudogap in YBa2Cu3O6+δ is a distinct phase, bounded by a line of phase transitions. The doping dependence of this line is such that it terminates at zero temperature inside the superconducting dome. From this we conclude that quantum criticality drives the strange metallic behaviour and therefore superconductivity in the copper oxide superconductors.

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Figure 1: The temperature evolution of resonances in underdoped and overdoped YBCO crystals: superconductivity.
Figure 2: The temperature evolution of resonances across the pseudogap phase boundary.
Figure 3: The phase diagram of YBa2Cu3O6+δ.
Figure 4: The pseudogap boundary inside the superconducting dome.


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We thank E. Abrahams, J. Analytis, P. Bourges, A. Finkel’stein, M. Greven, N. Harrison, K. Modic, C. Varma, I. Vishik and G. Yu for critical reading of the manuscript and informative discussions. Work at Los Alamos National Laboratory (LANL) was supported by National Science Foundation grant DMR-0654118, by the US Department of Energy and by the State of Florida. LANL is operated by LANS LLC. Work at the University of British Columbia was supported by the Canadian Institute for Advanced Research and the Natural Science and Engineering Research Council.

Author information




A.S., J.B.B., S.C.R., R.D.McD. and A.M. designed the experiment. A.S., J.B.B. and A.M. built the electronic circuits and the RUS probe. A.S. and F.F.B. wrote the software and analysed the results. B.J.R., R.L., W.N.H. and D.A.B. prepared the YBCO crystals. A.S., B.J.R., R.D.McD. and A.M. wrote the manuscript.

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Correspondence to Arkady Shekhter.

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The authors declare no competing financial interests.

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Shekhter, A., Ramshaw, B., Liang, R. et al. Bounding the pseudogap with a line of phase transitions in YBa2Cu3O6+δ. Nature 498, 75–77 (2013).

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