Louis Taillefer, Université de Sherbrooke et Institut canadien de recherches avancées

Jeudi 20 Novembre 2014, 14h
Amphi Holweck, Esc C, 1ème etage


Louis Taillefer
Université de Sherbrooke, Sherbrooke, Québec, Canada
Institut canadien de recherches avancées

Since the discovery of quantum oscillations in 2007 [1], we know that the Fermi surface of the underdoped cuprate superconductor YBCO undergoes a reconstruction at low temperature. A signature of this reconstruction is the change of sign in the Hall [2] and Seebeck [3] coefficients, which become negative at low temperature. Observed in other hole-doped cuprates [3,4,5], this phenomenon is generic. By analogy with Eu-LSCO, where charge modulations are responsible for the reconstruction, a similar mechanism was inferred for YBCO [5]. Charge modulations in YBCO have since been observed directly by NMR [6] and X-ray diffraction [7,8,9]. Charge order has also been observed in other materials [10] – it represents a central new fact in the physics of cuprates.
I will discuss the impact of Fermi-surface reconstruction and charge order on superconductivity. We have used electrical, thermal and thermo-electric transport measurements in high magnetic fields to investigate this question. The upper critical field Hc2 was mapped out in YBCO as a function of doping across the phase diagram [11]. A dramatic drop in Hc2 below a critical doping p* = 0.18 shows the strong impact of phase competition, demonstrating that competition is what shapes the Tc dome in holedoped cuprates. In electron-doped cuprates, we have studied the nature of superconducting fluctuations via the Nernst effect to show that here also it is phase competition and not the emergence of fluctuations in the phase of the superconducting order parameter that cause Tc to go down on the underdoped side [12].

[1] N. Doiron-Leyraud et al., Nature 447, 565 (2007).
[2] D. LeBoeuf et al., Nature 450, 533 (2007).
[3] J. Chang et al., Physical Review Letters 104, 057005 (2010).
[4] N. Doiron-Leyraud et al., Physical Review X 3, 021019 (2013).
[5] F. Laliberté et al., Nature Communications 2, 432 (2011).
[6] T. Wu et al., Nature 477, 191 (2011).
[7] G. Ghiringhelli et al., Science 337, 821 (2012).
[8] J. Chang et al., Nature Physics 8, 871 (2012).
[9] A. J. Achkar et al., Physical Review Letters 109, 167001 (2012).
[10] R. Comin et al., Science 343, 390 (2014).
[11] G. Grissonnanche et al., Nature Communications 5, 3280 (2014).
[12] F. F. Tafti et al., Physical Review B 90, 024519 (2014).

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