Willem Rischau, Universite de Genève

jeudi 17 octobre 2019 à 14 h 00 Amphi Holweck, bât C, 1ème étage

Superconductivity and ferroelectricity in strontium titanate

The large band-gap semiconductor strontium titanate SrTiO3 becomes a metal with a superconducting instability after removal of an extremely small number of oxygen atoms, making it one of the most dilute superconductors known today. On the other hand, a long-range ferroelectric order can be induced by either substituting part of strontium with calcium or by an isotopic 18O-substitution above a quantum critical point (QCP) at 0.33 at% 18O. Although the superconductivity in this material has been discovered half a century ago, the pairing mechanism leading to the superconducting dome is still under debate. The superconducting and ferroelectric orders in this material may be accidental neighbors or intimately connected, as in a recently proposed quantum critical scenario where pairing is mediated by the ferroelectric soft mode [1,2].
We find that in Sr1-xCaxTiO3-δ ferroelectricity and metallicity coexist in a narrow window of doping making this system a ferroelectric metal [3,4]. As the carrier concentration is increased, the ferroelectric-like order is eventually destroyed at a critical doping level at which the Friedel oscillations generated by neighboring dipoles interfere destructively. In both Sr1-xCaxTiO3-δ and SrTi(18Ox16O1-x)3-δ close to the QCP, we find that the superconducting critical temperature is enhanced [3,5]. This strongly supports the role played by the ferroelectric soft mode in the emergence of superconductivity, restricting possible theoretical scenarios for pairing.

[1] J.M. Edge et al., Phys. Rev. Lett. 115, 247002 (2015).
[2] D. van der Marel, F. Barantani and C.W. Rischau, Phys. Rev. Research 1, 013003 (2019).
[3] C.W. Rischau et al., Nat. Phys. 13, 643 (2017).
[4] J. Wang, L. Yang, C.W. Rischau, Z. Xu, Z. Ren, T. Lorenz, J. Hemberger, X. Lin and K. Behnia. arXiv 1909.04278 (2019).
[5] A. Stucky et al., Scientic Reports 6, 37582 (2016).

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