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Berry-Phase Tuning in Heavy f-Electron Metals

English title Berry-Phase Tuning in Heavy f-Electron Metals
Applicant Janoschek Marc
Number 200650
Funding scheme Project funding
Research institution Paul Scherrer Institut
Institution of higher education Paul Scherrer Institute - PSI
Main discipline Condensed Matter Physics
Start/End 01.02.2022 - 31.01.2026
Approved amount 700'990.00
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Keywords (6)

Strongly correlated electron systems; Magnetic Frustration; Topology; Berry curvature; f-electron materials; Quantum Materials

Lay Summary (German)

Für adiabatische Änderungen auf einem geschlossenen Weg durch einen Parameterraum kehrt ein quantenmechanisches System im Allgemeinen in seinen Ausgangszustand zurück. Falls der Parameterraum jedoch nicht-definierte Singularitäten aufweist, und sich deswegen durch eine nicht-triviale Topologie auszeichnet, ergibt sich eine zusätzliche Phase in der Wellenfunktion des Systems, die dessen Zustand beschreibt. Diese nicht-triviale Phase wird Berry Phase genannt. Topologische Materialien sind dadurch gekennzeichnet, dass die quantenmechanische Wellenfunktion der Elektronen bei deren Bewegung durch das Material eine solche Berry Phase aufsammelt. Interessanterweise hat diese zusätzliche Phase tiefgreifende Effekte auf das Verhalten der Elektronen, was zu neuartigen Materialeigenschaften mit Potential für zukünftige elektronischen Bausteine führt.
Lay summary

Inhalt und Ziel des Forschungsprojekts

Das Ziel dieses Projekts ist es sogenannte f-Elektronen Materialien experimentell zu untersuchen, in welchen eine Berry Phase dann entsteht, wenn Elektronen sich durch topologisch nicht-triviale magnetische Strukturen bewegen. Da in f-Elektronen Materialien magnetische Strukturen oft durch externe Kontrollparameter beeinflusst werden kann, bietet dies ideale Voraussetzungen, um die Berry Phase zu „tunen“, und damit Materialeigenschaften zu kontrollieren. Ziel des Projekts ist es einen experimentellen Zusammenhang zwischen Berry Phasen und topologisch nicht-trivialen magnetischen Strukturen herzustellen, und zu testen, ob Materialeigenschaften so kontrolliert werden können. Wir verwenden hierzu Neutronenstreuung, eine Methode, die besonders empfindlich auf magnetische Strukturen ist.

Wissenschaftlicher und gesellschaftlicher Kontext des Forschungsprojekts

Bereits nach heutigem Kenntnisstand ist es klar, dass topologischen Materialien großes Potential für das Design von neuartigen Elektronikbausteinen haben. Das von diesem Projekt angestrebte Verständnis, wie topologische Materialeigenschaft „maßgeschneidert“ werden können, stellt einen wichtigen Schritt auf dem Weg zu solchen Anwendungen dar.

Direct link to Lay Summary Last update: 29.05.2021

Responsible applicant and co-applicants



In metals containing lanthanides and actinides the hybridization of localized f-electrons with itinerant conduction electrons is well-known to result in a strongly renormalized electronic band structure and heavy electronic quasiparticles. These inherently narrow electronic bands, characteristic of strong electronic correlations, exhibit a band width comparable to energy scales of other degrees of freedom. Because lattice, charge, spin and orbital degrees of freedom in f-electron metals are all driven by energy scales of virtually identical magnitude, these materials are highly-tunable with experimentally accessible (uniaxial) pressures and magnetic fields. Decades of extensive research have established firmly that the underlying strongly correlated electronic state supports a zoo of nearly-degenerate quantum ground states such as unconventional superconductivity, complex magnetism, electronic nematic and charge order and hidden orders. In addition to these topologically-trivial quantum states that are classified via the notion of symmetry breaking, the large spin-orbit coupling presents in heavy fermion metals also enables strongly correlated quantum states that are topologically non-trivial. However, we are only at the beginning of discovering these novel states. Kondo insulators in which the hybridization of itinerant conduction and localized f-electrons induces a band gap are one promising route. Here we propose to pursue an alternative route to explore topology that is based on the frequent emergence of complex spin structures in f-electron materials. Notably, when conduction electrons propagate through a non-coplanar spin texture, they may exhibit spontaneous topological Hall effect in the absence of any externally applied magnetic field. Due to the exchange interaction, the underlying local moment texture aligns the spins of the conduction electrons inducing a topological real-space Berry phase in the electron wavefunction, which is indistinguishable from a ‘real’ magnetic flux. Similarly, it has been shown that non-collinear spin configurations on frustrated lattices in combination with strong spin-orbit coupling may also result in a large anomalous Hall effect. In the latter case, the associated Berry phase is, however, created by the electronic structure in momentum space. Taken together, with their large spin-orbit coupling, their tendency for complex spin configurations and their high tunability via external parameters make heavy fermion metals are an ideal playground to explore and understand topological phases of matter. Notably, theory has demonstrated that heavy fermion metals generally support the required non-collinear spin textures. Several recent experiments show via a combination of scattering techniques and Hall measurements that topological spin textures such as skyrmions exist in this class of materials and result in giant topological Hall signals characteristic of finite Berry curvature.Here we propose to build on this early promise by investigating a selected set of heavy electron metals, which exhibit giant topological Hall signals, as recently shown by us and our collaborators. Notably, in addition to establishing the correspondence between their complex spin configurations and the anomalous Hall effect via neutron diffraction experiments, we will employ advanced neutron spectroscopy together with state-of-the-art modelling to establish a quantitative link between the underlying microscopic Hamiltonian and the observed giant Berry phases. Finally, we will explore if tuning of these materials via external control parameters can be used to systematically control the Berry phase and hence, the resulting topological and functional properties.