Having graduated "with distinction" from the University of Stuttgart with a thesis on photon scattering from nickelate heterostructures, Matthias Hepting just won the Springer Thesis Award for outstanding Ph.D. theses. He is the fourth Springer Thesis awardee from our group, following Pegor Aynajian, Aliaksei Charnukha and Alex Frano. He will take up a postdoctoral position at Stanford University in February.
Joel Bertinshaw has been awarded a Postdoctoral Fellowship from the Alexander von Humboldt Foundation. He is doing neutron scattering experiments on unusual magnetic excitations in ruthenium oxides.
Alex Frano received the Ernst-Eckhard-Koch Prize of the Helmholtz Zentrum Berlin for his Ph.D. thesis on resonant x-ray scattering from metal-oxide superlattices. He is now a postdoc at the University of California at Berkeley.
Nakheon Sung has received a postdoctoral fellowship from the Alexander von Humboldt Foundation for research on correlated-electron materials with strong spin-orbit coupling.
Humboldt Prize and Humboldt Fellowship
Prof. Ali Yazdani (Princeton University) was awarded a Research Prize by the Alexander von Humboldt Foundation and will be visiting our department in 2015. The Humboldt Foundation also awarded a postdoctoral fellowship to Matteo Minola, now group leader of the inelastic photon scattering group.
Alex Frano graduated with distinction from the Technical University of Berlin, and won the Springer thesis Award for his outstanding research on metal-oxide heterostructures.
Two major prizes for Mathieu Le Tacon
Mathieu will receive the Academy Prize of the Heidelberg Academy of Sciences and the Young Scientist Prize in Low Temperature Physics of the International Union for Pure and Applied Physics for his his innovative photon scattering experiments on high-temperature superconductors.
Friederike Wrobel was elected Secretary General of the PhDnet representing more than 5000 Ph.D. candidates in the Max Planck Society.
Minerva Award 2014
Eva Benckiser is the winner of a Minerva Award of the Max Planck Society, which will allow her to build up an independent group. The award recognizes her outstanding experimental work on orbital physics of transition metal oxides.
Incoming group leader Bum Joon Kim wins the Bryan R. Coles Prize "for seminal contributions to the discovery and experimental study of an unconventional Mott insulating state induced by relativistic spin-orbit coupling in the transition metal oxide Sr2IrO4".
The leader of the neutron spectroscopy group Dr. Dmytro Inosov is this year's laureate of the Wolfram-Prandl-Prize for his outstanding work on iron-based high-temperature superconductors.
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 . 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.  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.
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.
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.
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.
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.
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.
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.
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 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 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.