Oxychlorures de cuivre, supraconducteurs à haute température : un modèle 2D pour les cuprates

High-temperature superconducting copper oxychlorides: a 2D model for cuprates

• Auteur / Autrice : Chafic Fawaz
• Direction : Matteo D'astuto
• Type : Projet de thèse
• Discipline(s) : Physique de la Matière Condensée et du Rayonnement
• Date : Inscription en doctorat le 01/10/2021
• Etablissement(s) : Université Grenoble Alpes
• Ecole(s) doctorale(s) : École doctorale physique
• Partenaire(s) de recherche : Laboratoire : Institut Néel
• Partenaire(s) de recherche : Equipe de recherche : MCBT

Mots clés

Mots clés libres

Cuprates

High temperature superconductors

Strongly correlated systems

Resonant Inelastic X-ray scattering

Raman Spectroscopy

Angle-Resolved Photoemission Spectroscopy

Cuprates

Supraconducteur haute température

Systèmes fortement corrélés

Diffusion inélastique des rayons-X résonante

Spectroscopie Raman

Spectroscopie de photoémission résolue en angle

Résumé

Les cuprates supraconducteurs présentent les températures de transition et les champs magnétiques critiques les plus élevés à pression ambiante. Même si le mécanisme microscopique à l'origine de leur supraconductivité (SC) reste, à ce jour, insaisissable, d'importants progrès ont été réalisés notamment sur la compréhension de l'état normal particulier dont émerge cette propriété. Dans le but de comparer précisément les propriétés électroniques déterminées expérimentalement et les calculs incluant des effets de corrélation, nous proposons d'étudier les cuprates oxychlorures comme des systèmes modèles. Ils possèdent une structure simple, stable en fonction du dopage et de la température, et un caractère 2D marqué, récemment mis en évidence par une forte anisotropie des propriétés SC aussi, contrastant fortement avec d'autres cuprates. Ces propriétés singulières offrent la possibilité d'étudier expérimentalement et théoriquement une SC 2D non conventionnelle dans un échantillon massif.

The origin of high temperature superconductivity (HTS) in cuprates remains highly controversial in spite of an unprecedented research effort for almost 35 years. HTS appears in close proximity to other symmetry-broken states of matter such as antiferromagnetic (AF) insulators and a still mysterious pseudogap (PG) phase at moderate doping. AF fluctuations, present in all three of these phases, may play a crucial role for opening the pseudo-gap, and potentially even for mediating the Cooper pairing, leading to superconductivity. However, a lack of quantitative assessments of real systems has limited this idea to idealized models and a better understanding of the influence of AF fluctuations on the properties of doped cuprates and the emergent PG phase has become an urgent question. In this context, the discovery of hole-doped Ca2CuO2Cl2 (CCOC) HTS oxychloride cuprates, such as Ca2-xNaxCuO2Cl2 (Na-CCOC), offers a promising playground to investigate all three phases on common grounds. They are characterized by : a simple I4/mmm 1-layer structure typical of La2-xMxCuO4 (M=Sr, Ba) or “214” cuprates but which is stable at all doping and temperatures; a lack of high Z ions in their composition; and a strong two-dimensional (2D) character due to the substitution of apical oxygen with chlorine. Preliminary work by some of the applicants has revealed a very puzzling pronounced anisotropic character of the superconducting properties of these compounds: Indeed, while cuprate superconductors typically exhibit anisotropic electronic properties rooted in the layered structure, the superconducting state is usually three-dimensional with only some moderate anisotropy of the magnetic field penetration depth, with the exception of the case of La1.875M0.125CuO4 at a specific doping level (1/8 doping) corresponding to a charge-density wave. CCOC exhibits a similarly anisotropic jump in the diamagnetic susceptibility despite the much less correlated charge order, if any at all, which is required to explain the phenomena. The origin of this quite exceptional behavior of oxychlorides could be due to intrinsic properties of the superconducting phase of these materials, which form pair-density-waves in an electronic structure already modulated by charge-density-wave. Alternatively, it could be the result of an intrinsic 2D electronic structure, that would make them an equivalent, in superconductivity, to graphite made of stackings of 2D graphene layers. This anisotropy would be highly unusual allowing to study the phenomenology of 2D superconductivity in a bulk sample, a fascinating possibility. The goal of this project is twofold. On the one hand, we aim at exploring this novel superconducting anisotropy, to completely characterise it and understand its origin and its interplay with other electronic phases (in particular charge order). On the other hand, we will take advantage of this phenomenology allowing us to theoretically describe the system as a stacking of almost uncorrelated CuO2 planes, for which we will perform state-of-the-art many-body electronic structure calculations. The direct comparison of materials-specific electronic structure theory with experimental results on their electronic and thermal excitations can be expected to provide new keys to understanding the intriguing properties of this important family of high-Tc superconductors.