Over the past several years, rare-earth nickelates (RNiO3) have been the subject of intense investigation because of their rich phase diagram that comprises a sharp temperature-driven metal-to-insulator transition and an unusual antiferromagnetic ground state. Recently, we used high-resolution resonant inelastic x-ray scattering (RIXS) at the Ni L3 edge to study simultaneously the bond and magnetic orders that develop in these materials and that can be controlled via heterostructuring. Following our initial measurement of magnetic excitations in bulk-like films [1,2] we have now observed striking variations of the spin dynamics in different nickelate heterostructures and were able to explain these data in terms of a microscopic model [3]. The results show that RIXS gives valuable insights into the interplay of different collective ordering phenomena in this prototypical transition-metal oxide.

[1] Y. Lu et al., Phys. Rev. X 8, 031014 (2018)
[2] https://www.esrf.eu/home/UsersAndScience/Publications/Highlights/esrf-highlights-2018.html
[3] K. Fürsich et al., Phys. Rev. B 99, 165124 (2019) - Editors' Suggestion more

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)


Magnetoelastic coupling of pseudospins

Spin-orbit entangled pseudospins give rise to exotic types of magnetism not found in conventional magnets. For example, the pseudospin pairs on different bonds of a honeycomb lattice tend to align along mutually orthogonal directions, leading to strong frustration and Kitaev quantum spin liquid state. This peculiar behavior of pseudospins stems from the way they interact with each other, but how pseudospins interact with the lattice remains largely unexplored. In pair of papers, we present a new theoretical description of the pseudospin-lattice coupling as well as experimental evidence that this coupling plays a decisive role in determining the macroscopic magnetization and low-energy dynamics of Sr2IrO4, an archetypal spin-orbit Mott insulator.

"Pseudo-Jahn-Teller Effect and Magnetoelastic Coupling in Spin-Orbit Mott Insulators"
H. Liu and G. Khaliullin, Phys. Rev. Lett. 122, 057203 (2019)

"Pseudospin-lattice coupling in the spin-orbit Mott insulator Sr2IrO4"
J. Porras et al., Phys. Rev. B 99, 085125 (2019) more

Cuprate superconductors are known to harbor charge order in part of their phase diagram. The order has a two-dimensional (2D) character at zero magnetic field, whereas a 3D order appears at high fields. In x-ray diffraction measurements under high uniaxial pressure, we have discovered Bragg reflections characteristic of 3D charge order even at zero magnetic field. We also demonstrated a soft phonon mode that is associated with the formation of 3D order. The results provide insights into the normal-state properties of cuprates and illustrate the potential of uniaxial-pressure control of competing orders in quantum materials.


H.H. Kim, S.M. Souliou et al., Science 362, 1040 (2018)


Two new papers report important progress towards the goal of designing new magnetic states in oxide heterostructures. First, we used high-resolution inelastic x-ray scattering to directly determine the magnon dispersion relations and exchange interactions in an epitaxial film of NdNiO3 [1]. We then established an elementary model system – NdNiO3 slabs embedded in a non-magnetic NdGaO3 matrix – to show that both collinear and non-collinear magnetic structures can be realized, depending on the slab thickness. [2] The crossover between both spin structures can be qualitatively understood in a low-energy spin model with the competing exchange interactions determined by RIXS. Magnetic slabs thus provide new perspectives for research on complex magnetism, in analogy to two-dimensional materials created by exfoliation. [1] Y. Lu et al., Phys. Rev. X 8, 031014 (2018)

[2] M. Hepting et al., Nature Physics (2018)


Three-dimensional charge density wave in cuprate films.

Over the past several years, charge density waves have turned out to be the major competitor of high-temperature superconductivity in copper oxides. In resonant x-ray scattering experiments on YBCO thin films  grown epitaxially on SrTiO3, we made the surprising discovery that three-dimensional charge order is present in the absence of magnetic fields, and persists to much higher temperatures than the two-dimensional order investigated in prior work. The results offer new insight into the unusual stability of charge density waves in YBCO, as well as fresh perspectives for experiments elucidating the influence of charge order on the electronic properties of cuprates without the need to apply high magnetic fields.

M. Bluschke et al., Nature Comm. 9, Article number: 2978 (2018)

First Light at IRIXS

On March 28, 2017, we saw "first light" at our new IRIXS spectrometer at the PETRA-III synchrotron in Hamburg. IRIXS stands for "Intermediate-energy Resonant Inelastic X-ray Scattering", a method that will allow energy- and momentum-resolved measurements of electronic modes in quantum materials with 4d valence electrons. The construction of the unique instrument is made possible by an Advanced Grant from the European Research Council. The photo shows Hlynur Gretarsson sharing a drink to celebrate this event with Markus Tischer, whose group designed the undulators that supply photons to the spectrometer.
See also the time lapse video of him, Simon Mayer, Hasan Yavas and others assembling the spectrometer.
Neutron scattering experiments on the two-dimensional antiferromagnet Ca2RuO4 revealed a well-defined, dispersive "Higgs" mode that modulates the amplitude of the ordered moment. The mode quickly decays into transverse spin waves at the antiferromagnetic ordering wavevector, and through a complete mapping of the transverse modes in the reciprocal space we were able to obtain a quantitative understanding of the decay process. The results establish a novel condensed matter platform for research on the dynamics of the Higgs mode.
The interplay between charge density waves and high-temperature superconductivity is currently under intense investigation .Resonant x-ray scattering experiments have now shown that interfaces with manganates greatly enhance charge density wave formation in cuprate superconductors, over a remarkably large length scale of tens of nanometers. Long-range proximity effects induced by heterointerfaces thus offer a powerful method manipulate the interplay between different collective phenomena in metal oxides.
After seven years as a group leader in our department, Matthieu Le Tacon was appointed Full Professor and Director of the Institute for Solid State Physics at the Karlsruhe Institute of Technology.
The macroscopic properties of materials with strongly correlated electrons are influenced not only by atomic-scale spin and charge correlations, but also by emergent domain structures on sub-micrometer length scales. Neutron Larmor diffraction and dilatometry have yielded new insights into the mechanisms driving the formation of  mesoscopic magnetic and structural domains in the antiferromagnetic parent compound of a high-temperature superconductor.
The electronic structure of iridium oxides has long been known as closely analogous of the one of the cuprate high-temperature superconductors. However, most iridium oxides are insulating, and it has proven difficult to inject charge carriers by doping. Photoemission experiments on an iridium oxide surface that was doped by proximity to a metallic monolayer have now uncovered an energy gap of d-wave symmetry, one of the hallmarks of high-temperature superconductivity.
Bernhard Keimer has won an "Advanced Grant" from the European Research Council (ERC) for a project on magnetic excitations in compounds and heterostructures based on 4d electrons. In the framework of this project, a new beamline for resonant inelastic x-ray scattering will be built at the PETRA-III synchrotron in Hamburg. more
Resonant x-ray scattering experiments on  iridium oxides have demonstrated a novel form of exchange coupling between magnetic moments, whose strength depends on the direction of the magnetic bonds. This discovery opens up new perspectives for solid-state realizations of quantum spin liquids.
Recent experimental and theoretical developments have brought fresh excitement to the field of high-temperature superconductivity. The February 12 issue of Nature contains a review accessible to a general audience.
A new technique that allows Raman scattering from 10 nm thin metal-oxide films has shown that spin and charge order in nickelates can be tuned by epitaxial strain. The paper appeared as an Editor's Choice in Physical Review Letters.
X-ray diffraction has revealed structural modifications in high-temperature superconductors induced by  femtosecond pulses of THz radiation, which appear to be responsible for transient superconductivity far above the equilibrium transition temperature. The results were published in Nature.
Bernhard Keimer has been named "Highly Cited Researcher" by the Institute for Scientific Information (Thomson-Reuters) based on citation data from 2002-2012.
Exchange interactions between electrons have been experimentally measured in a spiral magnet close to a metal-insulator transition. The neutron scattering data indicate that spiral magnetic state arises from a competition between short-range ferromagnetic exchange and longer-distance antiferromagnetic exchange, confirming predictions by de Gennes in 1960.
The heavy-fermion metal CeB6 is known to exhibit antiferromagnetic order at low temperatures. Neutron scattering experiments have now shown that the dominant magnetic excitations are actually ferromagnetic, suggesting that the antiferromagnetism is highly unconventional.
Experiments performed in collaboration with Andrea Cavalleri's group at the MPI in Hamburg revealed coherent transport phenomena strikingly similar to superconductivity when underdoped cuprate single crystals are illuminated by pulsed Terahertz light fields at room temperature.
Infrared ellipsometry reveals pronounced Fano resonances indicating unusual electron-phonon interactions in a geometrically frustrated iridium-oxide compound.
Two new papers in Science highlight the ubiquitous interplay between charge density waves and high-temperature superconductivity, and its influence on the well known "Fermi arc" phenomenon in underdoped copper oxide superconductors.
Resonant x-ray scattering experiments reveal a complex magnetic spiral in nickel oxide superlattices. The spiral polarization plane can be controlled by epitaxial strain and spatial confinement of the conduction electrons.
An unusual form of magnetic order is identified as the ground state of the Kitaev-Heidenberg model (which has deep relations to quantum computation), and found to explain experimental observations on iridium oxide compounds.
Neutron spin echo experiments have led to a complete understanding of the mechanisms that limit the lifetime of antiferromagnetic magnons -- a problem that has remained open for more than 40 years.
Resonant x-ray scattering experiments on highly ordered 'Ortho-II' YBa2Cu3O6.55 and Zn-substituted YBa2Cu3O6.6 revealed a three-phase competition between spin-modulated,charge-modulated, and superconducting states in underdoped cuprate superconductors.
Using resonant x-ray scattering we obtained the unambiguous evidence for long-range incommensurate charge-density-wave fluctuations competing with superconductivity in underdoped (Y,Nd)Ba2Cu3O6+x.

Raman spectroscopy reveals a long-range transfer of electron-phonon coupling between YBa2Cu3O7 and La2/3Ca1/3MnO3 layers in oxide superlattices.

Using inelastic neutron scattering, we have found a magnetic exciton mode in the non-superconducting heavy-fermion metal CeB6, resembling resonant magnetic excitations in unconventional superconductors.
A review article in Nature Materials discusses recent advances in understanding novel interface states that arise in oxide heterostructures due to the charge, spin and orbital reconstruction effects.
A resonant magnetic excitation in the Rb-245 iron-selenide superconductor has been discovered at a wave vector, which differs from the ones characterizing magnetic resonant modes in other iron-based superconductors.
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