Physics of strongly correlated electron systems

Optical spectroscopy

Neutron Scattering

Raman Scattering

TRISP spectroscopy

Theory

The department uses neutron and X-ray diffraction and spectroscopy as well as optical spectroscopy and Raman scattering to explore the structure and dynamics of materials with strong electron correlations. We also have a strong effort in the development of new spectroscopic methods. As the close collaboration between experimentalists and theorists is essential for the progress in this field, a small theory group operates within the department.

NEWS

Resonant neutron reflectometry scheme. The neutrons form a standing wave inside the resonator formed by the capping Pt (3nm) and the Al2O3 substrate. The intensity of this resonant wave is enhanced by an order of magnitude compared to the incident beam, and depends on the concentration of hydrogen incorporated in the Nb layers (2x20 nm). A thin ferromagnetic Co layer (3 nm) acts as detector for the neutron wave by flipping the spin of the neutrons inside the resonator. The resonant wave is only exited, if the neutrons impinge on one specific angle on the surface. This angle depends directly on the hydrogen concentration in the layer and thus serves as parameter to determine the concentration.

© Max Planck Institute for Solid State Research, Stuttgart, Germany

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