Séminaire de l'IQ : Arezoo Afshar

Arezoo Afshar Département de Physique Université de Sherbrooke Séminaire de maîtrise Titre: Fermi-surface reconstruction by charge-density-wave order in the cuprate superconductor LSCO Résumé: Since the discovery of cuprate superconductors in 1986, the key enduring question is: why is the superconducting transition temperature Tc so high ? An answer to this question requires that we understand the link between the superconducting phase and two other phases of cuprates: a phase of charge-density-wave (CDW) order and the mysterious pseudogap phase. We also need to understand the link between the latter two phases. For my MSc project, my goal was to delineate the region of CDW order in the doping phase diagram of the cuprate La2-xSrxCuO4 (LSCO), in the T = 0 limit in the absence of superconductivity. For this purpose, I performed measurements of the Hall coefficient RH and the Seebeck coefficient S at low temperature and high magnetic field, on several samples of LSCO in the doping range from x = 0.07 to x = 0.15. Because the magnetic field needed to suppress superconductivity at some of these dopings exceeds 20 T, some measurements had to be done at national high magnetic field labs in France and USA. Transport measurements can detect the presence of CDW order via the impact it has on the Fermi surface, namely a reconstruction that produces a small electron-like Fermi pocket, detected as a drop in RH(T) and S/T to negative values at low temperature, as previously established for the cuprate YBCO [1-3]. In LSCO, we observe a similar drop in RH(T) and S/T to negative values at x = 0.11, 0.12 and 0.13, the three dopings where CDW order has been observed in LSCO by x-ray diffraction [4]. Extending to lower and higher dopings, we find that CDW-induced Fermi-surface reconstruction is confined to 0.085 < p < 0.15. The fact that the CDW phase ends at pCDW = 0.15, distinctly below the end point of the pseudogap phase at p* = 0.18, implies that the two phases are distinct. One can therefore treat them separately in their impact on superconductivity. [1] LeBoeuf et al., Nature 450, 533 (2007). [2] Laliberté et al., Nature Communications 2, 432 (2011). [3] Hücker et al., Physical Review B 90, 054514 (2014); Blanco-Canosa et al., Physical Review B 90, 054513 (2014). [4] Croft et al., Physical Review B 89, 224513 (2014).