ERC Advanced Grant

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. The key enabling objective of the ERC project is the development of a synchrotron beamline equipped with a novel spectrometer for intermediate-energy (2.5-5 keV) resonant inelastic x-ray scattering (“IRIXS”) at the dipole-active L-absorption edges of 4d metal compounds and heterostructures. To reach this goal, we have commissioned beamline P01 at the PETRA-III in Hamburg (Germany) with high photon flux in the intermediate photon energy range, and we devised and built a novel resonant inelasticx-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 sensitivity sufficient to probe microcrystals and atomically thin films (see picture).

Data from this instrument will yield 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 will also allow us to determine the microscopic exchange interactions
in complex materials with both 3d and 4d valence electrons, and its high sensitivity will enable
experiments on operational device 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 {1] and
Raman spectroscopy [2]) 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 conventional 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.

[1] A. Jain et al., Nature Physics 13, 633 (2017).
[2] M. Souliou et al., Phys. Rev. Lett. 119, 067201 (2017).

See also article in Quanta Magazine ( https://www.quantamagazine.org/elusivehiggs-mode-created-in-exotic-materials-20180228/),
and Editors‘ Choice” in APS News (https://physics.aps.org/articles/v10/46)

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. [3] 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.

[3] C. Dietl et al., Appl. Phys. Lett. 112, 031902 (2018).