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Humboldt-Universität zu Berlin - Faculty of Mathematics and Natural Sciences - Department of Physics

Research Highlights of the Department of Physics

Sharing is Caring! Discovering the materials of the future with FAIR data management and AI

Panel discussion with Prof Claudia Draxl (HU) and Prof Matthias Scheffler (HU and Fritz Haber Institute of the Max Planck Society), moderated by Prof Peter Frensch (HU, Vice President for Research) at the Berlin Science Week 2020

Many products owe their function to novel materials, but how can we find materials that perform particularly well and enable new technologies? By making smarter use of our knowledge!

Huge amounts of scientific data are generated worldwide, but much of it is considered waste because this data does not serve the specific research project. However, it could contain valuable information for other research approaches. This calls for a rethink: Recycle the Waste!

At this year's Berlin Science Week, Claudia Draxl and Matthias Scheffler demonstrated that this approach can only be realized with Artificial Intelligence and a FAIR data infrastructure. (FAIR stands for Findable, Accessible, Interoperable, and Re-purposable.) Such an infrastructure enables the productive handling of scientific data and is thus essential for the development of future technologies.

Draxl and Scheffler have already established such an infrastructure for computational materials science: The NOMAD Lab is the world's largest database of material properties, which now contains 100 million calculations. Draxl and Scheffler are even going one step further. As part of the NOMAD Center of Excellence, they are developing computational materials science into new applications, preparing it for the coming generation of high-performance computers (exascale computers).

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Video of the panel discussion in German


Solid-State Theory Group

The Novel Materials Discovery (NOMAD) Laboratory

The NOMAD Center of Excellence


Density fluctuations in amorphous silicon discovered

For the first time, it has been possible to identify atomic substructures in amorphous silicon with a resolution of 0.8 nanometres using X-ray and neutron scattering at BESSY II and BER II. Such a-Si:H thin films have been used for decades in solar cells, TFT displays, and detectors. The results show that three different phases form within the amorphous matrix, which dramatically influences the quality and lifetime of the semiconductor layer. The structural elucidation was achieved in a collaboration between the Department of Physics of the HU, the Helmholtz-Zentrum Berlin and the Technical Universities of Eindhoven and Delft.

Silicon does not have to be crystalline, but can also be produced as an amorphous thin film. If additional hydrogen is incorporated during the production of these thin layers, so-called a-Si:H layers are formed. Such a-Si:H thin films have been used for decades for various applications, for example as contact layers in highly efficient tandem solar cells made of perovskite and silicon. In this study, it was shown that the a-Si:H network is by no means a homogeneously amorphous material. The amorphous matrix is interspersed with nanometre-sized areas of varying local density, from cavities to areas of extremely high order. It was possible to experimentally observe and quantitatively measure these inhomogeneities in differently produced a-Si:H thin films. To do this, the results of complementary analytical methods were combined to form an overall picture.

At the nanometer scale, voids corresponding to slightly more than 10 missing atoms were discovered. These voids arrange themselves with a distance of about 1.6 nanometres. In addition, nanometre-sized regions were discovered in which silicon atoms are better ordered compared to the surrounding material. These densely ordered domains (DOD) contain hardly any hydrogen. The DODs form aggregates of up to 15 nanometres in diameter. The DOD regions are able to reduce mechanical stress in the material and thus contribute to the stability of the a-Si:H thin film. The voids on the other hand, can promote electronic degradation of the semiconductor layers

Targeted optimization of manufacturing processes with regard to the substructures now discovered could enable new applications such as optical waveguides for programmable photonic systems or a future silicon battery technology.

Structural model of highly porous a-Si:H, which was deposited very quickly, calculated based on measurement data. Densely ordered domains (DOD) are drawn in blue and cavities in red. The grey layer represents the disordered a-Si:H matrix. The round sections show the nanostructures enlarged to atomic resolution (below, Si atoms: grey, Si atoms on the surfaces of the voids: red; H: white) Figure: Eike Gericke/HZB


Quantification of nanoscale density fluctuations in hydrogenated amorphous silicon

Eike Gericke, Jimmy Melskens, Robert Wendt, Markus Wollgarten, Armin Hoell, Klaus Lips

Publication 29 October 2020 Phys. Rev. Lett. 125 (2020) 18, 185501

Print Version of the Cover (pdf)


Prof. Simone Raoux, Institute for Nanospectroscopy, HZB


Quantum microscopy reveals invisible bio-features

Researchers from Humboldt-Universität zu Berlin and the Experimental and Clinical Research Center in Berlin show how quantum light can help the field of bioimaging. In their new experiment, featured on the cover of "Science Advances", the team uses entangled photons to image a bio-sample probed by "invisible" light without ever looking at that light. This avoids the usually severe problems that stem from poor performance and high price of broadband mid-IR light sources and cameras. Instead, the researchers only use a normal laser and commercial CMOS camera. This makes their mid-IR microscopy technique not only robust, fast and low noise, but also cost-effective - making it highly promising for real-world applications. They show this by taking microscopic images of a tissue sample from a mouse heart.

Figure: Quantum microscopy of a mouse heart. Entangled photons allow for the making of a high-resolution mid-IR image, using a visible light (CMOS) camera and ultralow illumination intensities. In the picture, absorption (left) and phase information (right) from a region in a mouse heart. The yellow scale bar corresponds to 0.1 mm which is about the width of a human hair.

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News of the Humboldt-Universität zu Berlin


Microscopy with undetected photons in the mid-infrared

Inna Kviatkovsky, Helen M. Chrzanowski, Ellen G. Avery, Hendrik Bartolomaeus, Sven Ramelow

Publication 14 October 2020, Science Advances 6, Issue 42


Nonlinear Quantum Optics Group


Hidden Symmetries in Massive Quantum Field Theory

Theoretical models with a large amount of symmetry are ubiquitous in physics and often key to developing efficient methods for complex problems. If the number of symmetries surpasses a critical threshold, a system is called integrable with a prime example being the Kepler problem of planetary motion. While integrability typically comes with a rich spectrum of mathematical methods, it is often hard to identify the underlying symmetries. For the first time quantum integrability was now discovered in the context of massive quantum field theories in four spacetime dimensions. Florian Loebbert and Julian Miczajka (both Humboldt University) together with Dennis Müller (NBI Copenhagen) and Hagen Münkler (ETH Zürich) have shown that large classes of mostly unsolved massive Feynman integrals feature an infinite dimensional Yangian symmetry - a hallmark of integrability. This mathematical structure is highly constraining and it allows to completely fix these building blocks of quantum field theory as has been demonstrated for first examples. The observed Yangian symmetry goes hand in hand with an extension of the important structure of conformal symmetry to situations including massive particles. Remarkably, this discovery suggests that similar symmetry features may also be hidden in massive versions of the celebrated holographic duality between gauge theories and gravity. These findings were recently published in Physical Review Letters 125 (2020) 9, 091602.


Massive Conformal Symmetry and Integrability for Feynman Integrals
Florian Loebbert, Julian Miczajka, Dennis Müller, and Hagen Münkler
Publication 25 August 2020, Phys. Rev. Lett. 125 (2020) 9, 091602


Quantum Field and String Theory Group


Jetting into the dark side: a precision search for dark matter

The nature of dark matter remains one of the great unsolved puzzles of fundamental physics. Unexplained by the Standard Model, dark matter has led scientists to probe new physics models to understand its existence. Many such theoretical scenarios postulate that dark matter particles could be produced in the intense high-energy proton–proton collisions of the LHC. While the dark matter would escape the ATLAS detector unseen, it could occasionally be accompanied by a visible jet of particles radiated from the interaction point, thus providing a detectable signal.

The ATLAS Collaboration set out to find just that. Today, at the International Conference in High-Energy Physics (ICHEP 2020), ATLAS presented a new search for novel phenomena in collision events with jets and high missing transverse momentum (MET). The search was designed to uncover events that could indicate the existence of physics processes that lie outside the Standard Model and, in doing so, open a window to the cosmos.

A monojet event recorded by the ATLAS experiment in 2017, with a single jet of 1.9 TeV transverse momentum recoiling against corresponding missing transverse momentum (MET). The green and yellow bars show the energy deposits in the electromagnetic and hadronic calorimeters, respectively. The MET is shown as the red dashed line on the opposite side of the detector. (Image: ATLAS Collaboration/CERN)

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Physics Briefing, By ATLAS Collaboration, 27th July 2020


Groups: Experimental Particle and Astroparticle Physics and High Energy Physics


Direct measurement of quantum efficiency of single-photon emitters in hexagonal boron nitride

Single-photon emitters (SPEs) in two-dimensional materials are promising candidates for the future generation of quantum photonic technologies. In this work, we experimentally determine the quantum efficiency (QE) of SPEs in few-layer hexagonal boron nitride (h-BN). We employ a metal hemisphere that is attached to the tip of an atomic force microscope to directly measure the lifetime variation of the SPEs as the tip approaches the ℎ-BN. This technique enables nondestructive, yet direct and absolute measurement of the QE of SPEs. We find that the emitters exhibit very high QEs approaching (87±7)% at wavelengths of ≈580  nm, which is among the highest QEs recorded for a solid-state SPE

Schematics of an experiment to measure the quantum efficiency of a single photon emitter in a two-dimensional material: A metal sphere very close to the emitter changes the spontaneous emission rate due to a quantum electrodynamic effect thus revealing its quantum efficiency.


Direct measurement of quantum efficiency of single-photon emitters in hexagonal boron nitride
Niko Nikolay, Noah Mendelson, Ersan Özelci, Bernd Sontheimer, Florian Böhm, Günter Kewes, Milos Toth, Igor Aharonovich, and Oliver Benson
Optica 6 (2019), 1084-1088

Print Version of the Cover (pdf)


Nanooptics Group