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Electron-Phonon Interactions in Rare-Earth Nickelates and Superconducting Cuprates

English title Electron-Phonon Interactions in Rare-Earth Nickelates and Superconducting Cuprates
Applicant Schmitt Thorsten
Number 178867
Funding scheme Project funding (Div. I-III)
Research institution Paul Scherrer Institut
Institution of higher education Paul Scherrer Institute - PSI
Main discipline Condensed Matter Physics
Start/End 01.10.2018 - 30.09.2021
Approved amount 355'240.00
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Keywords (6)

high temperature cuprate superconductors; low-energy excitations; metal insulator transitions; Resonant Inelastic X-Ray Scattering (RIXS); rare-earth nickelates; electron-phonon coupling

Lay Summary (German)

Lead
Übergangsmetalloxide besitzen eine starke Kopplung zwischen Ladungs-, Orbital-, Spin- und Gitterfreiheitsgraden, wodurch eine Vielzahl konkurrierender Ordnungen entsteht. Viele dieser Materialien zeichnen sich durch große Elektron-Elektron-Korrelationen aus, die die elektronischen und magnetischen Eigenschaften solcher stark korrelierter Materialien bestimmen. Darüber hinaus führt das Wechselspiel mit dem Freiheitsgrad des Gitters zu Elektron-Phonon-Wechselwirkungen, die neuartige Phänomene wie Metall-Isolator-Übergänge (MIÜ), Ladungsdichtewellenordnung oder Supraleitung auslösen und eine hohe Relevanz für neue Technologien versprechen.
Lay summary

Inhalt und Ziel des Forschungsprojektes

Das Hauptziel des vorliegenden Forschungsvorhabens ist die Untersuchung des Zusammenspiels der Freiheitsgrade und der besonderen Rolle der Elektron-Phonon-Kopplung in den MIÜ der seltenen Erdnickelate (RNiO3) und den Hochtemperaturkupratsupraleitern. Die meisten RNiO3 durchlaufen MIÜ, begleitet von einer alternierenden Verzerrung der Position der Sauerstoffatome. Dotierung der antiferromagnetischen und isolierenden Kupratoxide mit Löchern oder Elektronen induziert Supraleitung, von der allgemein angenommen wird, dass sie durch Spinfluktuationen vermittelt wird. Dennoch ist die Rolle von Phononen in diesem Cooper-Paarungsmechanismus, die die Supraleitung unterdrücken oder verstärken könnten, immer noch umstritten. Deshalb wollen wir in diesem Forschungsvorhaben die Rolle der Elektron-Phonon-Kopplung in den MIÜ von RNiO3 und der Cooper-Paarung in Kupratsupraleitern mit resonanter unelastischer Röntgenstreuung (engl. RIXS) untersuchen.

 

Wissenschaftlicher und gesellschaftlicher Kontext des Forschungsprojektes

In einer breiteren Perspektive werden solche Studien dazu beitragen, die Physik des MIÜ in Übergangsmetalloxiden aufzuklären und zum verbesserten Verständnis der Kupratmaterialien and insbesondere der Hochtemperatursupraleitung beitragen. Weiterhin wird die RIXS Methode an diesem Forschungsthema in idealer Weise weiterentwickelt werden können. Dies ist eine wichtige Vorraussetzung für die spätere Anwendung der RIXS Technik in Studien an funktionellen Materialien mit direktem Anwendungsbezug.

Direct link to Lay Summary Last update: 06.08.2018

Responsible applicant and co-applicants

Employees

Associated projects

Number Title Start Funding scheme
170760 High-field THz source for pump-probe experiments at SwissFEL 01.11.2017 R'EQUIP
141828 NCCR MARVEL: Materials’ Revolution: Computational Design and Discovery of Novel Materials (phase I) 01.05.2014 National Centres of Competence in Research (NCCRs)
160765 Mott Physics Beyond the Heisenberg Model in Iridates and Related Materials 01.01.2016 Sinergia
141325 Coupled spin, charge and orbital dynamics of low-dimensional cuprates 01.09.2013 Project funding (Div. I-III)
141962 Mott Physics Beyond the Heisenberg Model in Iridates and Related Materials 01.01.2013 Sinergia
183300 High-resolution soft-X-ray ARPES facility at Swiss Light Source 01.03.2019 R'EQUIP

Abstract

Transition metal oxides are quantum materials that host strong coupling between charge, orbital, spin and lattice degrees of freedom, thereby giving rise to a multitude of competing orders. Many of these materials are characterized by large electron-electron correlations that govern the electronic and magnetic properties of such strongly correlated materials. In addition, the interplay with the lattice degree of freedom results in electron-phonon interactions triggering novel phenomena, like metal-insulator transitions (MITs), charge-density wave ordering or superconductivity, promising high relevance for new technologies.The main objective of the present proposal is to study the interplay of the degrees of freedom and the particular role electron-phonon coupling is playing in the MITs of rare-earth nickelates RNiO3 (R=rare-earth ion) and the high-temperature cuprate superconductors. RNiO3 are negative charge transfer materials with Ni 3d8L (L=hole in the oxygen ligand) electronic configuration in the metallic state. Most RNiO3 undergo MITs accompanied with a breathing distortion of the oxygen atoms in the basic structural NiO6 units. This breathing motion leads to bond disproportionation in the insulating phase with expanded NiO6 octahedra and Ni 3d8 sites without holes on oxygen, alternating with collapsed octahedra and Ni 3d8L2 sites with on average two holes in the oxygen environment. This intimate connection between the breathing distortion and the electronic configurations suggests that electron-phonon interactions lie at the heart of the MIT in RNiO3. The parent compounds of the high-temperature cuprate superconductors are charge transfer insulators with a positive charge transfer energy and long-range antiferromagnetic ordering. Doping these antiferromagnetic insulators with holes or electrons induces superconductivity, which is generally believed to be mediated by spin-fluctuations. Nevertheless, the role of phonons in this Cooper-pairing mechanism, which might suppress or enhance superconductivity, is still controversial.The experimental method chosen here is Resonant Inelastic X-ray Scattering (RIXS) that is a powerful probe of excitations from the electronic ground state of correlated materials involving lattice, charge, orbital and spin degrees of freedom, for which the ADRESS beamline of the Swiss Light Source is one of the leading facilities. Coupling of lattice and charge degrees of freedom lends RIXS the ability to quantify the value and momentum dependence of the electron-phonon coupling through analysis of the signatures of multiple phonon excitations present in the spectra.Specifically, we aim to employ RIXS for clarifying the role of electron-phonon coupling in the MIT of RNiO3 and the Cooper-pairing in cuprate superconductors focusing on the bilayer cuprate Bi-2212. In particular, we will focus on O K-edge RIXS to determine the changes in electron-phonon coupling strength across the temperature, rare-earth ion R and thickness-induced MITs in RNiO3. Furthermore, we will assess changes in the relative spectral weight between the different phonon harmonics in cuprate superconductors of different doping across the superconducting dome. Thereby, we will evaluate for Bi-2212 if the electron-phonon coupling has any feedback effect as a function of temperature and doping that points towards suppression or enhancement of superconductivity. On a wider perspective, such studies will help to elucidate the physics of MITs in transition metal oxides and to further the understanding of the mechanism of unconventional superconducting pairing.
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