Benedetta Casu’s #My1stLight

Synchrotron: Destiny


When I was a physics student, the Physics Department of my University in the capital city of Sardinia organized a journey to Berlin for the senior master students to visit the most important labs. Among them, there was BESSY I. What an incredible experience, everything looked so fantastic, exciting, and complicated.


After that, for sake of curiosity, I attended the Italian synchrotron School that at the time was organized in Sardinia. I attended the school because I wanted to know more about synchrotron light, but I was sure that it would stay a “cultural opportunity” and nothing more.


A few years later I was offered a Ph.D. position at the University of Potsdam. The plan was that I would have been in charge of photocurrent investigations. BUT, the Ph.D. student that was in charge of the beamtime at Synchrotron in the same research group was never back from his vacation preferring to stay in sunny Spain. My supervisor decided that I would take over the Synchrotron beamtimes.


My very first beamtime was with the last photon at BESSY I.

Since then, I had the opportunity to perform wonderful experiments using Synchrotron facilities all over Europe, from working with the world record laterally resolved PEEM-LEEM at BESSY II to measuring XMCD at 150 mK at Petra III. I am also one of the German national delegates of the European Synchrotron and FEL User Organisation (ESUO).


Synchrotron was certainly my destiny

Image: Benedetta Casu during beamtime at BESSY II

Credit: Benedetta Casu

Green hydrogen: Nanostructured nickel silicide shines as a catalyst

Electrical energy from wind or sun can be stored as chemical energy in hydrogen, an excellent fuel and energy carrier. The prerequisite for this, however, is efficient electrolysis of water with inexpensive catalysts. For the oxygen evolution reaction at the anode, nanostructured nickel silicide now promises a significant increase in efficiency. This was demonstrated by a group from the HZB, Technical University of Berlin and the Freie Universität Berlin as part of the CatLab research platform with measurements among others at BESSY II.

Electrolysis might be a familiar concept from chemistry lessons in school: Two electrodes are immersed in water and put under voltage. This voltage causes water molecules to break down into their components, and gas bubbles rise at the electrodes: Oxygen gas forms at the anode, while hydrogen bubbles form at the cathode. Electrolysis could produce hydrogen in a CO2-neutral way – as long as the required electricity is generated by fossil free energy forms such as sun or wind.

The only problem is that these reactions are not very efficient and extremely slow. To speed up the reactions, catalysts are used, based on precious and rare metals such as platinum, ruthenium or iridium. For large-scale use, however, such catalysts must consist of widely available and very cheap elements.

Read more on the HZB website

Image: Crystalline nickel silicide (left) is chemically transformed into nanostructured material with excellent catalytic properties for the electrolytic splitting of water and the production of valuable nitrile compounds. 

Credit: © P. Menezes /HZB/TU Berlin

Buckyballs on gold are less exotic than graphene

C60 molecules on a gold substrate appear more complex than their graphene counterparts, but have much more ordinary electronic properties. This is now shown by measurements with ARPES at BESSY II and detailed calculations.

Graphene consists of carbon atoms that crosslink in a plane to form a flat honeycomb structure. In addition to surprisingly high mechanical stability, the material has exciting electronic properties: The electrons behave like massless particles, which can be clearly demonstrated in spectrometric experiments. Measurements reveal a linear dependence of energy on momentum, namely the so-called Dirac cones – two lines that cross without a band gap – i.e. an energy difference between electrons in the conduction band and those in the valence bands.

Variants in graphene architecture

Artificial variants of graphene architecture are a hot topic in materials research right now. Instead of carbon atoms, quantum dots of silicon have been placed, ultracold atoms have been trapped in the honeycomb lattice with strong laser fields, or carbon monoxide molecules have been pushed into place on a copper surface piece by piece with a scanning tunneling microscope, where they could impart the characteristic graphene properties to the electrons of the copper. 

Artificial graphene with buckyballs?

A recent study suggested that it is infinitely easier to make artificial graphene using C60 molecules called buckyballs. Only a uniform layer of these needs to be vapor-deposited onto gold for the gold electrons to take on the special graphene properties. Measurements of photoemission spectra appeared to show a kind of Dirac cone.

Analysis of band structures at BESSY II

“That would be really quite amazing,” says Dr. Andrei Varykhalov, of HZB, who heads a photoemission and scanning tunneling microscopy group. “Because the C60 molecule is absolutely nonpolar, it was hard for us to imagine how such molecules would exert a strong influence on the electrons in the gold.” So Varykhalov and his team launched a series of measurements to test this hypothesis.

In tricky and detailed analyses, the Berlin team was able to study C60 layers on gold over a much larger energy range and for different measurement parameters. They used angle-resolved ARPES spectroscopy at BESSY II, which enables particularly precise measurements, and also analysed electron spin for some measurements.

Read more on the HZB website

Image: Using density functional theory and measurement data from spin-resolved photoemission, the team investigated the origin of the repeating Au(111) bands and resolved them as deep surface resonances. These resonances lead to an onion-like Fermi surface of Au(111).

Credit: © HZB

Third-highest oxidation state secures rhodium a place on the podium

Oxidation states of transition metals describe how many electrons of an element are already engaged in bonding, and how many are still available for further reactions. Scientists from Berlin and Freiburg have now discovered the highest oxidation state of rhodium, indicating that rhodium can involve more of its valence electrons in chemical bonding than previously thought. This finding might be relevant for the understanding of catalytic reactions involving highly-oxidized rhodium. The result was recognized as a „very important paper“ in Angewandte Chemie.

Transition metals in high or unusual oxidation states might play an important role as catalysts or reaction intermediates in chemical reactions. Because transition metals are already well characterized in most cases, the discovery of a new oxidation state of rhodium came as a real surprise. The identification of rhodium(VII) was made possible by PhD student Mayara da Silva Santos and co-workers, who were able to isolate the species from any reactant in a low-temperature ion trap, and perform x-ray absorption spectroscopy for its characterization. 

BESSY II was essential for the discovery

These kinds of experiments are highly demanding, and can, at present, only be carried out at BESSY II. „The combination of advanced sample preparation, low-temperature ion trapping, and x-ray spectroscopy is unique. Because these essential tools can even be applied to more complex systems, we anticipate further insight into exotic transition metal oxides“, says Vicente Zamudio-Bayer, head of the ion trap group at beamline UE52-PGM, who develops and operates the ion trap endstation at BESSY II. „What was important for us was that our surprising experimental findings could be substantiated by Sebastian Riedel‘s group at FU Berlin, who performed state-of-the-art calculations on the species in question“, explains Zamudio-Bayer. “Even rhodium in oxidation state +6 is very rare, so we had to be extremely careful about +7. New oxidation states are not discovered every day”, says Mayara da Silva Santos.

Read more on the HZB website

Image: For the first time, a team has detected rhodium in the +7 oxidation state, the third highest oxidation state experimentally among all elements in the periodic table. © https://doi.org/10.1002/anie.202207688

Giant Rashba semiconductors show unconventional dynamics with potential applications

Germanium telluride is a strong candidate for use in functional spintronic devices due to its giant Rashba-effect. Now, scientists at HZB have discovered another intriguing phenomenon in GeTe by studying the electronic response to thermal excitation of the samples. To their surprise, the subsequent relaxation proceeded fundamentally different to that of conventional semimetals. By delicately controlling the fine details of the underlying electronic structure, new functionalities of this class of materials could be conceived. 

In recent decades, the complexity and functionality of silicon-based technologies has increased exponentially, commensurate with the ever-growing demand for smaller, more capable devices. However, the silicon age is coming to an end.  With increasing miniaturisation, undesirable quantum effects and thermal losses are becoming an ever-greater obstacle. Further progress requires new materials that harness quantum effects rather than avoid them. Spintronic devices, which use spins of electrons rather than their charge, promise more energy efficient devices with significantly enhanced switching times and with entirely new functionalities.

Spintronic devices are coming

Candidates for spintronic devices are semiconductor materials wherein the spins are coupled with the orbital motion of the electrons. This so-called Rashba effect occurs in a number of non-magnetic semiconductors and semi-metallic compounds and allows, among other things, to manipulate the spins in the material by an electric field.

First study in a non equilibrium state

Germanium telluride hosts one of the largest Rashba effects of all semiconducting systems. Until now, however, germanium telluride has only been studied in thermal equilibrium. Now, for the first time, a team led by HZB physicist Jaime-Sanchez-Barriga has specifically accessed a non-equilibrium state in GeTe samples at BESSY II and investigated in detail how equilibrium is restored in the material on ultrafast (<10-12 seconds) timescales. In the process, the physicists encountered a new and unexpected phenomenon.

First, the sample was excited with an infrared pulse and then measured with high time resolution using angle-resolved photoemission spectroscopy (tr-ARPES). “For the first time, we were able to observe and characterise all phases of excitation, thermalisation and relaxation on ultrashort time scales,” says Sánchez-Barriga. The most important result: “The data show that the thermal equilibrium between the system of electrons and the crystal lattice is restored in a highly unconventional and counterintuitive way”, explains one of the lead authors, Oliver Clark.

Read more on the HZB website

Image: Left: Electronic structure of GeTe taken with 11 eV photons at BESSY-II, showing the band dispersions of bulk (BS) and surface Rashba states (SS1, SS2) in equilibrium. Middle: Zoom-in on the region of the Rashba states measured with fs-laser 6 eV photons. Right: Corresponding out-of-equilibrium dispersions following excitation by the pump pulse.

Atomic displacements in High-Entropy Alloys examined

High-entropy alloys of 3d metals have intriguing properties that are interesting for applications in the energy sector. An international team at BESSY II has now investigated the local order on an atomic scale in a so-called high-entropy Cantor alloy of chromium, manganese, iron, cobalt and nickel. The results from combined spectroscopic studies and statistical simulations expand the understanding of this group of materials.

High-entropy alloys are under discussion for very different applications: Some materials from this group are suitable for hydrogen storage, others for noble metal-free electrocatalysis, radiation shielding or as supercapacitors.

The microscopic structure of high-entropy alloys is very diverse and changeable; in particular, the local ordering and the presence of different secondary phases affect significantly the macroscopic properties such as hardness, corrosion resistance and also magnetism. The so-called Cantor alloy, which consists of the elements chromium, manganese, iron, cobalt and nickel mixed in an equimolar proportion, can be considered as a suitable model system for the whole class of these materials.

Local structure studied at BESSY II

Scientists from the Federal Institute for Materials Research (BAM, Berlin), the University of Latvia in Riga, Latvia, the Ruhr University in Bochum and the HZB have now studied the local structure of this model system in detail. Using X-ray absorption spectroscopy (EXAFS) at BESSY II, they were able to precisely track each individual element and their displacements from the ideal lattice positions for this system in the most unbiased manner with the help of statistical calculations and the reverse Monte Carlo method.

Read more on the HZB website

Image: The supercell is randomly filled with the five elements on the fcc-lattice positions; In the starting configuration, all layers are precisely on top of each other. The displacements of all elements in the final configuration have been revealed by a simultaneous fit of the independent experimental spectra with a use of Reverse Monte Carlo simulations.

Credit: © A.Kuzmin / University of Latvia and A. Smekhova / HZ

Calculating the “fingerprints” of molecules with artificial intelligence

With conventional methods, it is extremely time-consuming to calculate the spectral fingerprint of larger molecules. But this is a prerequisite for correctly interpreting experimentally obtained data. Now, a team at HZB has achieved very good results in significantly less time using self-learning graphical neural networks.

“Macromolecules but also quantum dots, which often consist of thousands of atoms, can hardly be calculated in advance using conventional methods such as DFT,” says PD Dr. Annika Bande at HZB. With her team she has now investigated how the computing time can be shortened by using methods from artificial intelligence.

The idea: a computer programme from the group of “graphical neural networks” or GNN receives small molecules as input with the task of determining their spectral responses. In the next step, the GNN programme compares the calculated spectra with the known target spectra (DFT or experimental) and corrects the calculation path accordingly. Round after round, the result becomes better. The GNN programme thus learns on its own how to calculate spectra reliably with the help of known spectra.

Read more on the HZB website

Image: The graphical neural network GNN receives small molecules as input with the task of determining their spectral responses. By matching them with the known spectra, the GNN programme learns to calculate spectra reliably.

Credit: © K. Singh, A. Bande/HZB

New discoveries into how the body stores zinc

Zinc deficiency is a global health problem affecting many people and results in a weak immune system in adults and especially in children. This is a challenge for health systems and is quite evident in the Mexican population, for example. Seeking explanations, researchers in Mexico teamed up with international synchrotron experts and gained new insights from studying Drosophila fruit flies, which are known to be a decent model system for human zinc metabolism.


Thanks to beamtime at BESSY II and at the SLS (PSI), they were able to show that the zinc stores in Drosophila flies depend on the tryptophan content of their diet.

“The first experiments were done on the KMC-3 spectroscopy beamline,” relates DFG Fellow Nils Schuth, who is currently researching in Mexico at the Center for Research and Advanced Studies of the National Polytechnic Institute (Cinvestav). “We took organs from a fruit fly and performed direct measurements of the tissue. We gained very revealing information from the data. That was the first step, which already brought us forward. In a second step, we then compared the biological results with various synthesised chemical complexes.”

The project started in 2019. Then came the pandemic and travel restrictions. The next measurements were therefore performed at the Paul Scherrer Institute (PSI) on the SLS, where the two research institutes were already cooperating. In the spring of 2021, new measurements performed at BESSY II confirmed their discoveries.

Read more on the HZB website

Image: Confocal images of the kidney-like Malpighian tubule from a Drosophila larva at two magnifications. More details in the main article.

Credit: © Erika Garay (Cinvestav)

Unravelling tautomeric mixtures

RIXS at BESSY II allows to see clearly

A team at HZB has developed a method of experimentally unravelling tautomeric mixtures. Based on resonant inelastic X-ray scattering (RIXS) at BESSY II, not only proportions of the tautomers can be deduced, but the properties of each individual tautomer can be studied selectively. This method could yield to detailed information on the properties of molecules and their biological function. In the present study, now advertised on the cover of “The Journal of Physical Chemistry Letters” the technique was applied to the prototypical keto-enol equilibrium.

Many (organic) molecules exist as a mixture of two almost identical molecules, with the same molecular formula but one important difference: A single hydrogen atom sits in a different position. The two isomeric forms transform into each other, creating a delicate equilibrium, a “tautomeric” mixture. Many amino acids are tautomeric mixtures, and since they are building blocks of proteins, they may influence their shape and function and thus their biological functions in organisms.

Until now: Mission impossible

Until now, it has been impossible to selectively investigate the electronic structure of such tautomeric mixtures experimentally: Classical spectroscopic methods “see” only the sum of the signals of each molecular forms – the details of the properties of the two individual tautomers cannot be determined.

Now at BESSY II: it works

A team led by HZB physicist Prof. Alexander Föhlisch has now succeeded in providing a method of experimentally unravelling tautomeric mixtures. Using inelastic X-ray scattering (RIXS) and a data processing/evaluation method newly developed at HZB, the individual proportions of the tautomers can be clearly deduced from the measured data. “We can experimentally separate the signal of each individual molecule in the mixture by X-ray scattering, which leads to a detailed insight into their functionality and chemical properties,” says Dr. Vinicíus Vaz Da Cruz, first author of the paper and postdoc in Föhlisch’s team.

Read more on the HZB website

Image: The illustration visualises the experimental method, here on the prototypical keto-enol equilibrium. It appears on the cover of “The Journal of Physical Chemistry Letters”.

Credit: © Martin Künsting / HZB

Gender equality today for a sustainable tomorrow

The theme for International Women’s Day, 8 March, 2022 (IWD 2022) is, “Gender equality today for a sustainable tomorrow”, recognizing the contribution of women and girls around the world, who are leading the charge on climate change adaptation, mitigation, and response, to build a more sustainable future for all.

To mark the day and the theme, Lightsources.org brings you a special #LightSourceSelfie montage featuring just a few of the dedicated women who feature in our video campaign.

International research continued at BESSY II despite the pandemic

2021 was not an easy year for international research: owing to lockdowns and travel bans, science was hit hard by the pandemic situation. Nevertheless, experiments continued at a high level at the BESSY II light source in Berlin Adlershof – thanks in part to new remote service offers. Here are the figures at a glance.

“It makes us happy that BESSY II was dependably available to researchers for around 6000 hours despite the difficult conditions,” says Dr. Antje Vollmer, Head of User Coordination at HZB. The light generated at BESSY II is directed through 25 beamlines to 37 experimental stations. Thus, altogether, light was available for nearly 150,000 hours of research at all the beamlines. This light is used for experiments in many fields, including physics, chemistry and the life sciences. 

47 percent of user groups from abroad

As was to be expected, given that travel had to be limited, COVID-19 left a dip in user visits in 2021. “We counted just under 1400 visits from users last year. What surprised us, in view of the tense situation, was that 30 percent of the user groups came from other European countries and 17 percent were from non-European countries,” reports Antje Vollmer. “In total, we had user groups from 34 countries, which is an astonishing number.”

The fact that researchers from abroad conducted their experiments at BESSY II even in the corona year 2021 underlines the attractiveness of the photon source and the experimental stations, some of which are unique worldwide. “It also shows that the users here are very well looked after by dedicated scientists at the experimental stations and are happy to come back.”

Read more on the HZB website

Image: In 2021, our users at BESSY II came from 34 countries

Credit: © HZB

Unveiling the secrets of biofilms

Most bacteria have the ability to form communities, biofilms, that adhere to a wide variety of surfaces and are difficult to remove. This can lead to major problems, for example in hospitals or in the food industry. Now, an international team led by Hebrew University, Jerusalem, and the Technical University Dresden, has studied a model system for biofilms at the synchrotron radiation facilities BESSY II at HZB and the ESRF and found out what role the structures within the biofilm play in the distribution of nutrients and water.

Bacterial biofilms can thrive on almost all types of surfaces: We find them on rocks and plants, on teeth and mucous membranes, but also on contact lenses, medical implants or catheters, in the hoses of the dairy industry or drinking water pipes, where they can pose a serious threat to human health. Some biofilms are also useful, for example, in the production of cheese, where specific types of biofilms not only produce the many tiny holes, but also provide its delicious taste.

Tissue with special structures

“Biofilms are not just a collection of very many bacteria, but a tissue with special structures,” explains Prof. Liraz Chai from the Hebrew University in Jerusalem. Together, the bacteria form a protective layer of carbohydrates and proteins, the so-called extracellular matrix. This matrix protects the from disinfectants, UV radiation or desiccation and ensures that biofilms are really difficult to remove mechanically or eradicate chemically. However, the matrix is not a homogeneous sludge: “It’s a bit like in a leaf of plants, there are specialized structures, for example water channels residing in tiny wrinkles,” says Chai. But what role these structures play and what happens at the molecular level in a biofilm was not known until now. Together with Prof. Yael Politi, TU Dresden, an expert in the characterization of biological materials, Chai therefore applied for measurement time at the synchrotron radiation source BESSY II at HZB.

“The good thing about BESSY II is that we can map quite large areas. By combining X-ray diffraction with fluorescence, not only can we analyze the molecular structures across the biofilm very precisely, but we can also simultaneously track the accumulation of certain metal ions that are transported in the biofilm and learn about some of their biological roles” Yael Politi points out.

Read more on the HZB website

Image: When bacteria join together to form communities, they may build complex structures. The photo shows wild-type Bacillus subtilis biofilms.

Credit: © Liraz Chai/HUJI

New 12 T magnet strengthens energy and magnetism research

Electron paramagnetic resonance (THz-EPR) at BESSY II provides important information on the electronic structure of novel magnetic materials and catalysts. In mid-January 2022, the researchers brought a new, superconducting 12-T magnet into operation at this end station, which promises new scientific insights.

At the THz-EPR end station, unique experimental conditions are provided through a combination of coherent THz-light from BESSY II and high magnetic fields. These capabilities have now been extended by a new superconducting 12 T magnet, acquired through funding from the BMBF network project “ERP-on-a-Chip” and HZB.

Read more on the HZB website

Image: Exhausted but happy: f.l.t.r. – K. Holldack (HZB), A. Schnegg (MPI CEC Mülheim, HZB), T. Lohmiller (HZB, HUB), D. Ponwitz (HZB) after the successful commissioning of the new 12T magnet (green).

Korean scientists test the brand-new MYSTIIC

Jongwoo and his team from Seoul are “friendly users”. This name is given to scientists who do their experiments on a pristine machine, before it goes into user operation. Back in Korea we called them to hear more about their special beamtime and what it means for their battery research.

Who are you and how did you discover BESSY II?

I am Jongwoo Lim, assistant professor at the department of chemistry at Seoul National University. My research group “Battery and Energy Research Lab” counts many talented young scientists. In 2018 a colleague from the Max Planck Society invited me to give a talk and, on this occasion, I visited BESSY II. Back in Seoul I wanted my team to discover this amazing science environment.

Getting beamtime at BESSY II, how does this work?

The competition for beamtime is very strong, many scientists want to come to BESSY II! We send in a proposal and were rejected several times. Finally, after 2 years we got the green light for some beamtime at MAXYMUS, the beamline of the Max Planck Institute for Intelligent Systems (more below). And on top of that, beamline scientist Markus asked us if we were interested to use and test MYSTIIC (Microscope for x-raY Scanning Transmission In-situ Imaging of Catalysts). This new microscope will go into operation in Spring 2022.

Read more on the HZB blog science site

Image: Jongwoo’s team from Korea at BESSY II

Great minds think alike!

Marion Flatken from BESSY II & Luisa Napolitano from Elettra give advice to those at the start of their careers

Our #LightSourceSelfies campaign features staff and users from 25 light sources across the world. We invited them all to answer a specific set of questions so we could share their insights and advice via this video campaign. Today’s montage features Marion Flatken from BESSY II, in Germany, and Luisa Napolitano from Elettra, in Italy. Both scientists offered the same advice to those starting out on their scientific journeys: “Be curious and stay curious”. Light source experiments can be very challenging and the tough days can lead to demotivation and self-doubts. In these times, it is good to seek out support from colleagues, all of whom will have experienced days like this. Even if you think you can’t succeed with your research goals, try because it is amazing what can be achieved through hard work, tenacity and collaboration.

Liquid crystals for fast switching devices

An international team has investigated a newly synthesized liquid-crystalline material that promises applications in optoelectronics. Simple rod-shaped molecules with a single center of chirality self-assemble into helical structures at room temperature. Using soft X-ray resonant scattering at BESSY II, the scientists have now been able to determine the pitch of the helical structure with high precision. Their results indicate an extremely short pitch at only about 100 nanometres which would enable applications with particularly fast switching processes.

Liquid crystals are not solid, but some of their physical properties are directional – like in a crystal. This is because their molecules can arrange themselves into certain patterns. The best-known applications include flat screens and digital displays. They are based on pixels of liquid crystals whose optical properties can be switched by electric fields.

Some liquid crystals form the so-called cholesteric phases: the molecules self-assemble into helical structures, which are characterised by pitch and rotate either to the right or to the left. “The pitch of the cholesteric spirals determines how quickly they react to an applied electric field,” explains Dr. Alevtina Smekhova, physicist at HZB and first author of the study, which has now been published in Soft Matter.

Read more on the HZB website

Image: The photo shows the cells on the modified sample holder which was used in the real experiment. This modified sample holder is mounted within the ALICE chamber at BESSY II.

Credit: © A. Smekhova/HZB