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A ferroelectric quantum phase transition inside the superconducting dome of Sr1−xCaxTiO3−δ

Nature Physics volume 13, pages 643–648 (2017)Cite this article

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Abstract

SrTiO3, a quantum paraelectric1, becomes a metal with a superconducting instability after removal of an extremely small number of oxygen atoms2. It turns into a ferroelectric upon substitution of a tiny fraction of strontium atoms with calcium3. The two orders may be accidental neighbours or intimately connected, as in the picture of quantum critical ferroelectricity4. Here, we show that in Sr1−xCaxTiO3−δ (0.002 < x < 0.009, δ < 0.001) the ferroelectric order coexists with dilute metallicity and its superconducting instability in a finite window of doping. At a critical carrier density, which scales with the Ca content, a quantum phase transition destroys the ferroelectric order. We detect an upturn in the normal-state scattering and a significant modification of the superconducting dome in the vicinity of this quantum phase transition. The enhancement of the superconducting transition temperature with calcium substitution documents the role played by ferroelectric vicinity in the precocious emergence of superconductivity in this system, restricting possible theoretical scenarios for pairing.

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Figure 1: Emergence of ferroelectricity and metallicity by atomic substitution in SrTiO3.
Figure 2: Coexistence of ferroelectricity with dilute metallicity.
Figure 3: Evolution of resistivity in Sr1−xCaxTiO3−δ with increasing carrier concentration.
Figure 4: Evolution of the superconducting transition temperature.

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Acknowledgements

This work has been supported by ANR (through the SUPERFIELD and QUANTUM LIMIT projects), by an Ile de France regional grant, by Fonds ESPCI-Paris, by DFG research grant HE-3219/2-1 and by the Institutional Strategy of the University of Cologne within the German Excellence Initiative. X.L. is supported by the Alexander von Humboldt Foundation.

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Authors and Affiliations

  1. Laboratoire Physique et Etude de Matériaux (UMR 8213 CNRS-ESPCI), PSL Research University, 75005 Paris, France

    Carl Willem Rischau, Xiao Lin, Benoît Fauqué & Kamran Behnia

  2. II. Physikalisches Institut, Universität zu Köln, 50937 Köln, Germany

    Xiao Lin, Christoph P. Grams, Dennis Finck, Steffen Harms, Johannes Engelmayer, Thomas Lorenz & Joachim Hemberger

  3. Laboratoire Matériaux et Phénomènes Quantiques (UMR 7162 CNRS-Paris Diderot), 75013 Paris, France

    Yann Gallais

  4. JEIP, USR 3573 CNRS, Collège de France, 75005 Paris, France

    Benoît Fauqué

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  1. Carl Willem Rischau
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Contributions

K.B. conceived the project. C.W.R. and X.L. prepared oxygen-deficient samples and performed resistivity measurements with B.F.; J.H. designed the electric permittivity set-up and carried out the measurements with X.L., C.P.G., D.F. and S.H.; J.H., C.P.G. and D.F. measured sound velocity. T.L. and J.E. performed thermal expansion measurements. Y.G. performed Raman scattering measurements. K.B., C.W.R., X.L. and J.H. wrote the paper. All authors discussed the results and contributed to the manuscript.

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Correspondence to Kamran Behnia.

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The authors declare no competing financial interests.

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Rischau, C., Lin, X., Grams, C. et al. A ferroelectric quantum phase transition inside the superconducting dome of Sr1−xCaxTiO3−δ. Nature Phys 13, 643–648 (2017). https://doi.org/10.1038/nphys4085

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