Highlights

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

Resonant neutron reﬂectometry for hydrogen detection

May 02, 2022
New neutron scattering method enables fast and precise determination of the hydrogen content in thin-film structures and electronic device.
Hydrogen as a green fuel will play a central role in future energy management. The production of hydrogen by electrolysis, the efficient storage in solid materials, and the conversion into electrical energy in fuel cells is based on the interaction of hydrogen at the surface of electrodes and storage materials. Understanding and optimizing these technologies requires quantitative information about the hydrogen concentration inside materials on nanometer scales, in particular close to the surface and in thin films.
A second rapidly developing research frontier is taking advantage of hydrogen intercalation to modify the electronic properties of solids and solid-state devices. Prominent examples include targeted modiﬁcation of the lattice architecture and doping level of quantum materials, modulation of the exchange coupling and magnetic anisotropy of magnetic multilayers and devices, and solid state gas sensors. Artificial neural networks, the key component for machine learning algorithms, might gain efficiency by a synapse design with ultra low power consumption. The synapses are programmed by a gating voltage charging or discharging a thin conductive layer with hydrogen to modify its resistivity.
Neutron reflectometry (NR) is a distinguished method for the analysis of hydrogen distributions in thin films. By analogy with the optics of light, NR measures the neutron intensity reflected from the surface of a thin film as a function of the angle of incidence. Experimental reflectivity curves are then modeled to extract the depth dependence of the “neutron optical potential” ρ(z). Injection of hydrogen in the studied sample leads to a modification of ρ(z), which can be traced via the altered reﬂectivity. Hydrogen concentrations of 5 at.% with a depth resolution of one nanometer can be reliably measured by conventional NR, but real-time experiments remain limited by the required exposition times to slow processes on the scale of minutes to hours. more

Heike Kamerlingh Onnes Prize goes to Bernhard Keimer

_{Prize named after discoverer of superconductivity is awarded for scattering experiments on superconductors}
Bernhard Keimer will receive the prize named after Heike Kamerlingh Onnes (https://kamerlingh-onnes-prize.ch), the Dutch physicist who discovered superconductivity in 1911, at the M2S conference in Vancouver in July 2022 (https://www.m2s-2022.com). He will be recognized for "illuminating neutron and X-ray scattering experiments revealing resonant magnetic excitations and ordering phenomena in cuprate superconductors". Two co-awardees will also be recognized for scattering experiments on superconductors: Giacomo Ghiringhelli (Politecnico di Milano, Italy) and Pengcheng Dai (Rice University, USA). Hidenori Takagi, then at the University of Tokyo, received the Kamerlingh Onnes Prize in 2006 for "pioneering and seminal transport experiments which illuminated the unconventional nature of the metallic state of high temperature superconducting cuprates".
The Heike Kamerlingh Onnes Prize was established in 2000 by the organizers of the "International Conference on Materials and Mechanisms of Superconductivity (M2S)" and is sponsored by Elsevier, Publisher of Physica C – Superconductivity and its Applications. The Prize, consisting of 7500 €, is awarded every 3 years and recognizes outstanding experiments which illuminate the nature of superconductivity other than materials.

Optical conductivity and superconductivity in highly overdoped La2‑xCaxCuO4 thin films

Optical conductivity and superconductivity in highly overdoped La_2‑xCa_xCuO₄ thin films

Chemical substitution is widely used to modify the charge carrier concentration (``doping'') in complex quantum materials, but the influence of the associated structural disorder on the electronic phase behavior remains poorly understood. We synthesized thin films of the prototypical high-temperature superconductor La_2-xCa_xCuO₄ with minimal structural disorder and characterized their doping levels through measurements of the optical conductivity. We find that high-temperature superconductivity is stable up to much higher doping levels than previously found for analogous compounds with higher levels of disorder. The results imply that doping-induced disorder is the leading cause of the degradation of superconductivity for large carrier concentration, and they open up a previously inaccessible regime of the phase diagram of high-temperature superconductors to experimental investigation. more

Hidden Charge Order in an Iron Oxide Square-Lattice Compound

The FeO₂ square-lattice compound Sr₃Fe₂O₇ exhibits a charge-ordering transition that had remained "hidden" to standard diffraction probes for more than fifty years. Neutron Larmor Diffraction and Resonant X-ray Scattering have now revealed a surprisingly simple “checkerboard” charge-ordering pattern in the FeO₂ layers. As the checkerboards in adjacent layers are stacked in a nearly random fashion, the corresponding diffraction features are strongly broadened, thus explaining the "invisibility" to standard probes. The solution of the fifty-year-old Sr₃Fe₂O₇ conundrum holds an important lesson for research on other hidden-order materials as well. more

Valentin Zimmermann wins Physics Prize of the University of Stuttgart

Valentin Zimmermann has received a prize of the Faculty for Physics and Mathematics at the University of Stuttgart for his MSc degree in Physics. The prize was bestowed at the Faculty's annual Summer Event.

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

Hun-ho Kim wins Springer Thesis Award

Hun-ho Kim's doctoral dissertation "Uniaxial pressure study of charge density waves in a high-Tc cuprate superconductor" was selected for the Springer Thesis Award, which implies publication in book form in the Springer Theses series. Hun-ho Kim had graduated with distinction from the University of Stuttgart in 2020.In this thesis work, Hun-ho Kim performed an elegant series of resonant and non-resonant x-ray scattering experiments on the high-temperature superconductor YBa₂Cu₃O_(6.67) under highly homogeneous uniaxial pressure and demonstrated the power of this method to modulate the interplay between superconducting and charge-density-wave (CDW) phases without introducing disorder and inhomogeneity. Specifically, he showed that uniaxial pressure enhances short-range-ordered CDWs in the copper-oxide planes and confirmed that the underlying order parameter is uniaxial, thus conclusively resolving a long-standing debate (H.H. Kim et al., Phys. Rev. Lett. 126, 037002 (2021) more

Uniaxial strain control of charge density waves in cuprates

Our team at the Max Planck Institute for Solid State Research, in collaboration with the Max Planck Institute for Chemical Physics of Solids in Dresden, the Karlsruhe Institute of Technology, and the European Synchrotron Radiation Facility in Grenoble, France, applied a new approach to the investigation of charge density waves (CDW) in high-T_c superconducting cuprates. We combined uniaxial strain with resonant x-ray scattering (RXS) to explore a segment of the phase space of correlated electrons in the cuprates that had previously been inaccessible. Our findings clarify the nature of the CDW while demonstrating that the combination of uniaxial strain and RXS can become a powerful tool to shine light onto the complex interplay of electronic correlations in quantum materials. more

Magnetization reversal by longitudinal lattice shaking

An international research team with scientists from the Max Planck Institute for Solid State Research in Stuttgart, together with collaborators from FELIX Laboratory and Radboud University in Nijmegen, the University of Bialystok, and Delft University of Technology, has demonstrated that shaking atoms in a crystal for a short time can lead to a permanent switching of magnetization. Ultrafast modification of the crystal field environment has the potential to become the most universal way to manipulate magnetization. It could be the starting point for innovative IT concepts such as ultrafast opto-spintronic devices. more

Bond order ans spin excitations in nickelate thin-film structures

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

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

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

Uniaxial pressure control of competing orders in a high-temperature superconductor

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) more

Measurement and control of exchange interactions in nickelates.

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)

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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.

Higgs mode in a two-dimensional antiferromagnet

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.

Long-range proximity effect of charge density waves in cuprates

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.

Full Professorship for Matthieu Le Tacon

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.

Mesoscopic domains in a cuprate antiferromagnet

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.

d-Wave energy gap iridates

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.

ERC Advanced Grant

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

Bond-directional magnetic interactions

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.

Review of high-temperature superconductivity

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.

Editor's choice: Tunable spin and charge order

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.

Ultrafast structural modifications in a high-temperature superconductor

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.

Highly Cited Researcher

Bernhard Keimer has been named "Highly Cited Researcher" by the Institute for Scientific Information (Thomson-Reuters) based on citation data from 2002-2012.

Editor's choice: Spiral spin correlations at the metal-insulator transition

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.

Ferromagnetic correlations in a heavy-fermion antiferromagne

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.

Optically enhanced coherent transport at room temperature in an underdoped cuprate

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.

Fano resonances in iridates

Infrared ellipsometry reveals pronounced Fano resonances indicating unusual electron-phonon interactions in a geometrically frustrated iridium-oxide compound.

Superconducting cuprates

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.

Spiral spin density wave in nickel oxide superlattices

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.

Zigzag magnetic order in iridates

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.

Antiferromagnetic magnon lifetimes

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.

Three-phase competition in underdoped YBa2Cu3O6+x

Three-phase competition in underdoped YBa₂Cu₃O_6+x

Resonant x-ray scattering experiments on highly ordered 'Ortho-II' YBa₂Cu₃O_6.55 and Zn-substituted YBa₂Cu₃O_6.6 revealed a three-phase competition between spin-modulated,charge-modulated, and superconducting states in underdoped cuprate superconductors.

A density-wave order that competes with superconductivity