Copper oxide superconductors (called cuprates) are fascinating in more than one way, they not only have the highest critical temperatures Tc known to date at ambient pressure but also present in their so-called normal phase above Tc a mysterious phase called the pseudogap phase which hosts several electronic orders which are interconnected and which influence the superconducting phase below Tc. These orders in the pseudogap phase can break translational invariance like charge order, quarter turn rotation invariance like nematic order or time reversal invariance like current loops between copper and oxygen atoms at the nanometric scale.
In order to understanding this mysterious phase, we propose to focus on the model lanthanum-based cuprate La1.6−xNd0.4SrxCuO4 (Nd-LSCO) which has a sufficiently low critical temperature Tc (of the order of ten kelvins) to study the pseudogap phase and all its electronic orders at low temperature without being disturbed by the thermal fluctuations which can scramble the signal of the electronic orders.
To reach this goal, we have developed in our team,( the SQUAP team of MPQ) , an original optical technique called the electronic Raman scattering. The latter allows by spectral measurements at low temperature and a subtle play of polarizations, to probe the different electronic orders that can be encountered in a material and in particular the pseudogap phase.
We thus hope to reveal through this study the true nature of the pseudogap phase, which remains widely debated for the moment. We will have as a partner for the crystal growth of the materials : Professor Bruce Gaulin from the University of Mc Master in Hamilton in Canada and Professor Louis Taillefer from the University of Sherbrooke who has already studied this system by transport measurements. Transport and optical measurements are complementary. We will also benefit from the expertise of the theoretical group of the Collège de France led by Antoine Georges. This internship subject will naturally lead to a doctoral thesis in our team and and travel between Paris-Cité and Canadian universities is being considered.
[1] La supraconductivité dans les oxydes de cuivre : où en est-on ? , Les reflets de la Physique 70, october 2021.
[2] Exploration of the Hg-based cuprate superconductors by Raman spectroscopy under hydrostatic pressure, N. Auvray, et al., Phys. Rev. B 103, 195130 (2021).
[3] Intimate link between Charge Density Wave, Pseudogap and Superconducting Energy Scales in Cuprates, B. Loret, et al., Nat. Phys. 15, 771 (2019).