Periodic Reporting for period 4 - Com4Com (Collective modes in 4d-metal compounds and heterostructures)
Reporting period: 2020-07-01 to 2021-09-30
Data from this instrument have yielded comprehensive information about the interaction parameters specifying the electronic Hamiltonians of 4d-electron materials, unique insight into the spin-orbital composition of their excited-state wavefunctions, and definitive tests of proposals to realize Kitaev models with spin-liquid states that are potentially relevant in topological quantum computation. The element-specificity of RIXS ha sallowed us to determine the microscopic exchange interactions underlying the macroscopic response of magnetic materials, and its high sensitivity enables experiments on thin-film structures comprising only a few monolayers. We will thus be able to tightly integrate momentum-resolved spectroscopy with state-of-the-art, monolayer-by-monolayer deposition methods of 4d metal-oxide films and heterostructures. The results will fuel a feedback loop comprising synthesis, characterization, and modeling, which will greatly advance our ability to design materials and devices whose functionality derives from the collective organization of electrons.
In a parallel effort, we used complementary spectroscopic probes (including neutron scattering and Raman spectroscopy) as well as extensive theoretical modelling to build a conceptual framework for low-energy electronic excitations in ruthenium oxides and related 4d-metal compounds. In particular, we found that the interplay between the intra-atomic spin-orbit coupling and the inter-atomic exchange interaction generates soft longitudinal “Higgs” excitation in the two-dimensional antiferromagnet Ca2RuO4, in addition to the conventinal transverse magnon excitations (see the picture). These results establish a new condensed-matter platform for research on the dynamics of the Higgs mode, which will be further explored in forthcoming IRIXS experiments. The results were highlighted in several invited talks, in a popular science article in Quanta Magazine ( https://www.quantamagazine.org/elusive-higgs-mode-created-in-exotic-materials-20180228/) as an “Editors‘ Choice” in the online Research Newsletter of the American Physical Society (APS) (https://physics.aps.org/articles/v10/46) and on the front page of the “APS News” magazine (http://www.aps.org/publications/apsnews/201705/upload/May-2017.pdf).
The sensitivity of the IRIXS spectrometer will allow us to conduct spectroscopic experiments on thin films, heterostructures, and interfaces of 4d-metal compounds. In an effort to prepare suitable samples, we synthesized a series of epitaxial thin films of the model compound Ca2RuO4 on substrates that impose different strain conditions, and discovered of a strain-induced transition from the antiferromagnetic insulating to a ferromagnetic metallic state. The capability to vastly modify the magnetic and transport properties of ruthenium oxides by modest epitaxial strain opens up various new perspectives for oxide electronics. Ongoing IRIXS experiments are exploring the low-energy electronic excitations associated with the strain-induced phase transitions.