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Direct observation of the pseudogap quantum critical point in cuprate superconductors - Louis Taillefer

Université de Sherbrooke


The pseudogap is a partial gap that opens in the normal state of cuprate superconductors whose origin is a long-standing puzzle. Its connection to the Mott insulator at low doping p remains ambiguous and its relation to the charge order that reconstructs the Fermi surface at intermediate p is still unclear. I will report measurements of the Hall coefficient in magnetic fields up to 88 T, performed at the LNCMI in Toulouse, which reveal that Fermi-surface reconstruction by charge order in YBCO ends sharply at a critical doping p = 0.16, distinctly lower than the pseudogap critical point at p* = 0.19 [1]. This shows that pseudogap and charge order are separate phenomena. I will also report measurements of the Seebeck coefficient in magnetic fields up to 45 T, performed at the LNCMI in Grenoble and the NHMFL in Tallahassee, which reveal a similar separation in LSCO, a very different cuprate [2]. In YBCO, we find that the Hall number changes from nH = 1 + p at high p to nH = p at low p, starting right at p* [1] – a new signature of the pseudogap quantum critical point. I will present a detailed study, performed at the HFML in Nijmegen, of resistivity and Hall effect in Nd-LSCO, a cuprate whose pseudogap critical point is precisely known [3,4]. We observe the same drop in Hall number as in YBCO, from nH = 1 + p to nH = p at p*, accompanied by a major increase in resistivity, showing that the change is due to a drop in carrier density n, with n nH [5]. The excellent agreement between our Hall data [1,5] and recent calculations of the Hall coefficient in cuprates [6] suggest that the underlying mechanism is a reconstruction of the Fermi surface by the onset of a new periodicity at p*, with wavevector Q = (π, π), akin to the antiferromagnetic order in electron-doped cuprates.
[1] Badoux et al., Nature (22 Feb. 2016) ; arXiv:1511.08162. [2] Badoux et al., arXiv:1512.00292 (2015).
[3] Daou et al., Nature Physics 5, 31 (2009).
[4] Matt et al., Physical Review B 92, 134524 (2015). [5] Collignon et al., to be published.
[6] Storey, Europhysics Letters 113, 27003 (2016).

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