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Collective modes in 4d-metal compounds and heterostructures

Periodic Reporting for period 4 - Com4Com (Collective modes in 4d-metal compounds and heterostructures)

Reporting period: 2020-07-01 to 2021-09-30

Compounds of transition metals with 4d valence electrons (“4d metals”) play eminent roles in many areas of condensed matter physics ranging from unconventional superconductivity to oxide electronics, but fundamental questions about the interplay between the spin-orbit coupling and electronic correlations at the atomic scale remain unanswered. Momentum-resolved spectroscopies of collective electronic excitations yield detailed insight into the magnitude and spatial range of the electronic correlations, and have thus decisively shaped the conceptual understanding of quantum many-body phenomena in 3d-electron systems. We have devised and built a novel resonant inelastic x-ray scattering (RIXS) instrument capable of determining the dispersion relations of electronic collective modes in 4d-metal compounds with full momentum-space coverage, high energy resolution, and monolayer sensitivity.

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 the first half of the project period, we have invested much effort in building a synchrotron beamline that supplies a high flux of photons with energies intermediate between the conventional soft x-ray (photon energy < 1 keV) and hard x-ray (> 5 keV) spectral ranges. These photons are required for RIXS experiments at the dipole-active L-absorption edges of 4d-metals. We then designed and built a spectrometer for intermediate-energy resonant inelastic x-ray scattering ("IRIXS") with high momentum resolution, full momentum-space coverage, and sensitivity sufficient to probe microcrystals and atomically thin films (see the picture). This capability is currently unique worldwide. The current energy energy resolution is about 100 meV, which proved to be sufficient for experiments on dispersive spin waves, spin-orbit excitations, and spin-state transitions in model compounds including the honeycomb-lattice antiferromagnet SrRu2O6, the square-lattice antiferromagnet Ca2RuO4, and the spin-liquid candidate RuCl3. The achievement of this key goal of the ERC project has been announced in invited presentations at several conferences and workshops. The first set of experiments on different model compounds is described in a series of papers that is about to be submitted for publication.

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.
In the next phase of spectrometer development, we are targeting an energy resolution of about 30 meV, comparable to the magnon bandwidths of a large variety of 4d-metal compounds. The initial studies on ruthenium-based materials will be extended to other important 4d-metal systems including molybdates and rhodates. We will also explore various strategies for in-situ modification of the electronic ground state and excitations, including external dc currents (which were recently shown to induce an insulator-to-metal transition in Ca2RuO4), uniaxial strain, gate-fields in field-effect device structures, and in-situ deposition of dopant layers. In parallel, we will refine and extend the theoretical description of the interplay between spin-orbit coupling, crystalline electric fields, and exchange interactions in 4d-metal compounds.
(a) Pictorial representation and (b) dispersions of magnon and Higgs modes in Ca2RuO4
Photograph of the PETRA-III beamline endstation with the IRIXS spectrometer