PhD Position : Exploring two boson pairing scenario in cuprates

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Testo originale dell'annuncio (in inglese)

General Scope: Since their discovery in 1986, cuprates have consistently exhibited the highest ambient-pressure critical temperature of all high-temperature superconductors (HTSC). Despite prolonged and intensive international research efforts, the scientific community has not been able to agree on the origin of the Cooper pairing that enables the superconducting state in these compounds, providing numerous partially incompatible or contradicting theories. The origin of superconductivity in HTSC cuprates thereby remains one of the major open questions in condensed matter physics. One possible problem is that the debate has focused almost exclusively on models where the pairing is due to a single boson, whether a phonon, magnon, or a more complex quasi-particle, emerging from the highly correlated quantum state of charge carriers in the CuO2 plane. But nature does not necessarily favor simplicity, and a two-boson scenario, although very difficult to address theoretically, could explain many contradictions in the physics of cuprates. PhD Subject: To this end, we will explore several physical phenomena, particularly in cuprate oxychlorides NaxCa2-xCuO2Cl2, a less studied system but one that offers significant simplifications, especially for Hubbard-type models. We will also revisit certain measurements in other cuprate families to compare them and identify aspects that could be compared to two-boson models. In particular we plan to look at the family of doped La2CuO4 and BSCCO (Bi2Sr2Can−1CunO2n+4+x, n=1,2,3, x<1). We will focus on two types of collective thermal excitations: magnons and phonons, and their effects on the electronic structure, especially the superconducting gap and the pseudo-gap. In particular, we plan to explore the effects of isotopic substitution of oxygen on cuprate oxychlorides, which has never been carried out before, and which allows direct access to the effects of phonons on the electronic structure and superconductivity. We also plan to explore Cu-O bond-stretching phonon modes using RIXS at the oxygen K-edge, which is sensitive to the electron-phonon coupling of these vibrational, in collaboration with L. Chaix, (Inst. Néel), and A. Nicolaou, (Synchrotron Soleil). Regarding interaction with magnons, we will study the energy of the bi-magnon as a function of pressure, measured by Raman spectroscopy, allowing to measure the super-exchange energy J as a function of pressure. Finally, we will also study the superconducting gap and pseudo-gap in this compound, specifically to probe its symmetry using Raman spectroscopy. All Raman experiment will be in collaboration with M.-A. Méasson (Inst. Néel). All the results will be compared to ab-initio c-DMFT calculations performed by our collaborators at IMPMC (Sorbonne University, Paris) and IPCMS (University of Strasbourg). Required Skills: • Solid background in electronic structure and properties of materials. • Basic practical in experimental physics. • Basic knowledge in materials synthesis and solid-state chemistry.