ERC Advanced Grant

ERC Advanced Grant "Com4Com"

Collective modes in 4d-metal compounds and heterostructures

IRIXS spectrometer

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.

Highlights

Distinct spin and orbital dynamics in Sr2RuO4

Distinct spin and orbital dynamics in Sr₂RuO₄

Conduction electrons in quantum materials form delocalized states that can be quantum-coherent over macroscopic length scales, while being subject to local interactions akin to those in atomic physics. This dichotomy spawns a large variety of collective quantum phenomena and remains one of the major challenges of modern condensed matter physics. The square-lattice compound Sr₂RuO₄ has long served as a model system for the influence of atomic-scale correlations and macroscopic electronic properties – including particularly the unconventional superconducting state that forms at low temperatures. Momentum-space maps acquired with a high-resolution x-ray spectrometer have now revealed a separation of energy scales for spin and orbital correlations, which can be attributed to interactions akin to Hund’s rules in atomic physics. The results serve as a testbed for state-of-the-art many-body theories and yield fresh insight into the origin of superconductivity in Sr₂RuO₄. more

Hakuto Suzuki wins the Bryan S. Coles Prize

Hakuto Suzuki has been awarded the Bryan S. Coles Prize at the Conference on Strongly Correlated Electron Systems (SCES) in Incheon, Korea, for "the experimental development and creative use of a novel RIXS spectrometer in the tender x-ray region, that opens a new route for the SCES community for impactfully investigating a broad range of correlated electron materials and their physics". Hakuto did this work when we was a Humboldt Fellow and postdoctoral researcher in our group. He is currently an assistant profressor at the Tohoku University in Japan. The design and construction of the RIXS spectrometer was supported by the ERC Advanced grant "Collective models in 4d-electron compounds and heterostructures". more

Proximate ferromagnetic state in the Kitaev model material RuCl3

Proximate ferromagnetic state in the Kitaev model material RuCl₃

The exactly soluble Kitaev model embodies key concepts in condensed matter physics such as topological spin-liquid states and emergent Majorana fermions, and RuCl₃ has emerged as an important model system for Kitaev physics in solids. We have used resonant inelastic x-ray scattering with our new IRIXS spectrometer to determine the exchange interactions in RuCl₃, which can now serve as the basis for theoretical work on this system. Since IRIXS only requires micrometer-sized crystals, our approach has the potential to evolve into a powerful screening tool for the rapidly expanding list of spin-liquid candidate materials. more

Spin waves from a microcrystal

An international team of scientists from the Max Planck Institute for Solid State Research, DESY, the University of Stuttgart, Postech, and the University of Tokyo has determined the spectrum of collective magnetic excitations (“spin waves”) in a ruthenium-oxide antiferromagnet, which exhibits an unusually high magnetic ordering temperature. Such measurements are important because they yield insight into the magnetic interactions between spins inside the material, but they usually require large single crystals that are difficult to synthesize. By using the newly developed IRIXS spectrometer at PETRA III, the research team was now able to obtain a complete set of measurements on a microcrystal invisible to the naked eye. The experiment demonstrates the power of the IRIXS method as a novel probe of elementary excitations in a large class of magnetic materials. “Spin waves and spin-state transitions in a ruthenate high-temperature antiferromagnet”
H. Suzuki, H. Gretarsson, H. Ishikawa, K. Ueda, Z. Yang, H. Liu, H. Kim, D. Kukusta, A. Yaresko, M. Minola, J. A. Sears, S. Francoual, H.-C. Wille, J. Nuss, H. Takagi, B. J. Kim, G. Khaliullin, H. Yavaş and B. Keimer, Nature Materials (2019) more

Higgs mode

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)

Ruthenate films

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).