Alternative material investigated for superconducting radio-frequency cavity resonators

In modern synchrotron sources and free-electron lasers, superconducting radio-frequency cavity resonators are able to supply electron bunches with extremely high energy. These resonators are currently constructed of pure niobium. Now an international collaboration has investigated the potential advantages a niobium-tin coating might offer in comparison to pure niobium.
At present, niobium is the material of choice for constructing superconducting radio-frequency cavity resonators. These will be used in projects at the HZBsuch as bERLinPro and BESSY-VSR, but also for free-electron lasers such as the XFEL and LCLS-II. However, a coating of niobium-tin (Nb3Sn) could lead to considerable improvements.

Coatings may save money and energy

Superconducting radio-frequency cavity resonators made of niobium must be operated at 2 Kelvin (-271 degrees Celsius), which requires expensive and complicated cryogenic engineering. In contrast, a coating of Nb3Sn might make it possible to operate resonators at 4 Kelvin instead of 2 Kelvin and possibly withstand higher electromagnetic fields without the superconductivity collapsing. In the future, this could save millions of euros in construction and electricity costs for large accelerators, as the cost of cooling would be substantially lower.

> Read more on the HZB website

Image: The photomontage shows a sample of solid, pure niobium before coating (left), and coated with a thin layer of Nb3Sn (right). Copyright: HZB

Helmholtz-Zentrum Berlin has new scientific management

As of 1 June 2019, Prof. Dr. Bernd Rech and Prof. Dr. Jan Lüning are the new scientific directors of Helmholtz-Zentrum Berlin für Materialien und Energie. Bernd Rech is responsible for the “Energy and Information” department and Jan Lüning heads the “Matter” department. Thus the HZB Supervisory Board has appointed two internationally recognised experts at the top of HZB.
For the first two years, Bernd Rech will be the spokesman of the scientific direction of HZB. Then he will hand the role of spokesman over to Jan Lüning. The two scientific directors have been appointed for a period of five years. In November 2018, the HZB Supervisory Board made Bernd Rech and Jan Lüning the provisional scientific directors until final contracts could be negotiated. These have now been finalised, making their appointment permanent as of 1 June 2019.

>Read more on the HZB website

Image: Prof. Dr. Jan Lüning (l.) and Prof. Dr. Bernd Rech (r.) have been appointed as scientific directors of HZB since June 1, 2019.
Credit: HZB/P. Dera

“Molecular scissors” for plastic waste

A research team from the University of Greifswald and Helmholtz-Zentrum-Berlin (HZB) has solved the molecular structure of the important enzyme MHETase at BESSY II.

MHETase was discovered in bacteria and together with a second enzyme – PETase – is able to break down the widely used plastic PET into its basic building blocks. This 3D structure already allowed the researchers to produce a MHETase variant with optimized activity in order to use it, together with PETase, for a sustainable recycling of PET. The results have been published in the research journal Nature Communications.

Plastics are excellent materials: extremely versatile and almost eternally durable. But this is also exactly the problem, because after only about 100 years of producing plastics, plastic particles are now found everywhere – in groundwater, in the oceans, in the air, and in the food chain. Around 50 million tonnes of the industrially important polymer PET are produced every year. Just a tiny fraction of plastics is currently recycled at all by expensive and energy-consuming processes which yield either downgraded products or depend in turn on adding ‘fresh’ crude oil.

>Read more on the BESSY II at HZB website

Image: At the MX-Beamlines at BESSY II, Gottfried Palm, Gert Weber and Manfred Weiss could solve the 3D structure of MHETase.
Credit: F. K./HZB

Godehard Wüstefeld receives the Horst Klein Research Prize

The physicist Dr. Godehard Wüstefeld was awarded the Horst Klein Research Prize at the annual conference of the German Physical Society.

The award recognizes his outstanding scientific achievements in accelerator physics in the development of BESSY II and BESSY VSR.
Over the last thirty years, Dr. Godehard Wüstefeld has made decisive contributions to the further development of storage-ring-based synchrotron radiation sources. Thanks to its innovative concepts, the performance and application areas of storage rings have been consistently expanded. Wüstefeld participated in the development of BESSY II and the Metrology Light Source and implemented several innovations there.

>Read more on the BESSY II at Helmholtz-Zentrum Berlin website

Image: Dr. Godehard Wüstefeld was awarded the Horst Klein Research Prize.
Credit: DPG

Water is more homogeneous than expected

In order to explain the known anomalies in water, some researchers assume that water consists of a mixture of two phases even under ambient conditions.

However, new X-ray spectroscopic analyses at BESSY II, ESRF and Swiss Light Source show that this is not the case. At room temperature and normal pressure, the water molecules form a fluctuating network with an average of 1.74 ± 2.1% donor and acceptor hydrogen bridge bonds per molecule each, allowing tetrahedral coordination between close neighbours.
Water at ambient conditions is the matrix of life and chemistry and behaves anomalously in many of its properties. Since Wilhelm Conrad Röntgen, two distinct separate phases have been argued to coexist in liquid water, competing with the other view of a single-phase liquid in a fluctuating hydrogen bonding network – the continuous distribution model. Over time, X-ray spectroscopic methods have repeatedly been interpreted in support of Röntgen’s postulate.

>Read more on the BESSY II at HZB website

Image: Water molecules are excited with X-ray light (blue). From the emitted light (purple) information on H-bonds can be obtained.
Credit: T. Splettstoesser/HZB

Superferromagnetism with electric-field induced strain

Data storage in today’s magnetic media is very energy consuming. Combination of novel materials and the coupling between their properties could reduce the energy needed to control magnetic memories thus contributing to a smaller carbon footprint of the IT sector. Now an international team led by HZB has observed at the HZB lightsource BESSY II a new phenomenon in iron nanograins: whereas normally the magnetic moments of the iron grains are disordered with respect each other at room temperature, this can be changed by applying an electric field: This field induces locally a strain on the system leading to the formation of a so-called superferromagnetic ordered state.
Switching magnetic domains in magnetic memories requires normally magnetic fields which are generated by electrical currents, hence requiring large amounts of electrical power. Now, teams from France, Spain and Germany have demonstrated the feasibility of another approach at the nanoscale: “We can induce magnetic order on a small region of our sample by employing a small electric field instead of using magnetic fields”, Dr. Sergio Valencia, HZB, points out.

>Read more on the Bessy II at HZB website

Image: The cones represents the magnetization of the nanoparticles. In the absence of electric field (strain-free state) the size and separation between particles leads to a random orientation of their magnetization, known as superparamagnetism
Credit: HZB

Photocathodes with high quantum efficiency at bERLinPro

A team at the HZB has improved the manufacturing process of photocathodes and can now provide photocathodes with high quantum efficiency for bERLinPro.

Teams from the accelerator physics and the SRF groups at HZB are developing a superconducting linear accelerator featuring energy recovery (Energy Recovery Linac) as part of the bERLinPro project. It accelerates an intense electron beam that can then be used for various applications – such as generating brilliant synchrotron radiation. After use, the electron bunches are directed back to the superconducting linear accelerator, where they release almost all their remaining energy. This energy is then available for accelerating new electron bunches.

Electron source: photocathode

A crucial component of the design is the electron source. Electrons are generated by illuminating a photocathode with a green laser beam. The quantum efficiency, as it is referred to, indicates how many electrons the photocathode material emits when illuminated at a certain laser wavelength and power. Bialkali antimonides exhibit particularly high quantum efficiency in the region of visible light. However, thin films of these materials are highly reactive and therefore very sensitive, so they only work at ultra-high vacuum.

>Read more on the Bessy II at HZB website

Image: Photocathode after its production in the preparatory system.
Credit: J. Kühn/HZB

HZB builds undulator for SESAME in Jordan

The Helmholtz-Zentrum Berlin is building an APPLE II undulator for the SESAME synchrotron light source in Jordan. The undulator will be used at the Helmholtz SESAME beamline (HESEB) that will be set up there by five Helmholtz Centres. The Helmholtz Association is investing 3.5 million euros in this project coordinated by DESY.
SESAME stands for “Synchrotron Light for Experimental Science and Applications in the Middle East” and provides brilliant X-ray light for research purposes. The third-generation synchrotron radiation source became operational in 2017. Egypt, Iran, Israel, Jordan, Pakistan, the Palestinian Authority, Turkey, and Cyprus are cooperating on this unique project to provide scientists from the Middle East with access to one of the most versatile tools for research.

New beamline for soft x-rays

Thus far, SESAME has four beamlines and will now receive a fifth meant to generate “soft” X-ray light in the energy range between 70 eV and 1800 eV. This X-ray light is particularly suitable for investigating surfaces and interfaces of various materials, for observing certain chemical and electronic processes, and for non-destructive analysis of cultural artefacts. The new beamline will be constructed as the Helmholtz SESAME Beamline (HESEB) by the Helmholtz Centres DESY (coordinating Centre), Forschungszentrum Jülich, Helmholtz-Zentrum Dresden-Rossendorf (HZDR), Helmholtz-Zentrum Berlin (HZB) as well as the Karlsruhe Institute of Technology (KIT).

>Read more on the Bessy II at HZB website

Image: The APPLE II UE56 double undulator generates brilliant light with variable polarization.
Credit: HZB

Transition metal complexes: mixed works better

A team at BESSY II has investigated how various iron-complex compounds process energy from incident light. They were able to show why certain compounds have the potential to convert light into electrical energy. 

The results are important for the development of organic solar cells. The study has now been published in the journal PCCP, and its illustration selected for the cover.
Transition-metal complexes – that is a cumbersome word for a class of molecules with important properties: An element from the group of transition metals sits in the centre. The outer electrons of the transition-metal atom are located in cloverleaf-like extended d-orbitals that can be easily influenced by external excitation. Some transition-metal complexes act as catalysts to accelerate certain chemical reactions, and others can even convert sunlight into electricity. The well-known dye solar cell developed by Michael Graetzel (EPFL) in the 1990s is based on a ruthenium complex.

Why not Iron?
However, it has not yet been possible to replace the rare and expensive transition metal ruthenium with a less expensive element, such as iron. This is astonishing, because the same number of electrons is found on extended outer d-orbitals of iron. However, excitation with light from the visible region does not release long-lived charge carriers in most of the iron complex compounds investigated so far.

>Read more on the Bessy II at HZB website

Image: The illustration shows a molecule with an iron atom at its centre, bound to 4 CN groups and a bipyridine molecule. The highest occupied iron orbital is shown as a green-red cloud. As soon as a cyan group is present, the outer iron orbitals are observed to delocalize so that electrons are also densely present around the nitrogen atoms.
Credit: T. Splettstoesser/HZB

Graphene on the way to superconductivity

Scientists at HZB have found evidence that double layers of graphene have a property that may let them conduct current completely without resistance. They probed the bandstructure at BESSY II with extremely high resolution ARPES and could identify a flat area at a surprising location.

Carbon atoms have diverse possibilities to form bonds. Pure carbon can therefore occur in many forms, as diamond, graphite, as nanotubes, football molecules or as a honeycomb-net with hexagonal meshes, graphene. This exotic, strictly two-dimensional material conducts electricity excellently, but is not a superconductor. But perhaps this can be changed.

A complicated option for superconductivity
In April 2018, a group at MIT, USA, showed that it is possible to generate a form of superconductivity in a system of two layers of graphene under very specific conditions: To do this, the two hexagonal nets must be twisted against each other by exactly the magic angle of 1.1°. Under this condition a flat band forms in the electronic structure. The preparation of samples from two layers of graphene with such an exactly adjusted twist is complex, and not suitable for mass production. Nevertheless, the study has attracted a lot of attention among experts.

>Read more on the BESSY II at HZB website

Image: The data show that In the case of the two-layer graphene, a flat part of bandstructure only 200 milli-electron volts below the Fermi energy. Credit: HZB

Blue phosphorus – mapped and measured for the first time

For the first time an HZB team was able to examine samples of blue phosphorus at BESSY II and confirm via mapping of their electronic band structure that this is actually this exotic phosphorus modification.

Blue phosphorus is an interesting candidate for new optoelectronic devices. The results have been published in Nano Letters.
The element phosphorus can exist in  various allotropes and changes its properties with each new form. So far, red, violet, white and black phosphorus have been known. While some phosphorus compounds are essential for life, white phosphorus is poisonous and inflammable and black phosphorus – on the contrary – particularly robust. Now, another allotrope has been identified: In 2014, a team from Michigan State University, USA, performed model calculations to predict that “blue phosphorus” should be also stable. In this form, the phosphorus atoms arrange in a honeycomb structure similar to graphene, however, not completely flat but regularly “buckled”. Model calculations showed that blue phosphorus is not a narrow gap semiconductor like black phosphorus in the bulk but possesses the properties of a semiconductor with a rather large band gap of 2 electron volts. This large gap, which is seven times larger than in bulk black phosphorus, is important for optoelectronic applications.

>Read more on the BESSY II at HZB website

Image: https://pubs.acs.org/doi/10.1021/acs.nanolett.8b01305

Boosting the efficiency of silicon solar cells

The efficiency of a solar cell is one of its most important parameters.

It indicates what percentage of the solar energy radiated into the cell is converted into electrical energy. The theoretical limit for silicon solar cells is 29.3 percent due to physical material properties. In the journal Materials Horizons, researchers from Helmholtz-Zentrum Berlin (HZB) and international colleagues describe how this limit can be abolished. The trick: they incorporate layers of organic molecules into the solar cell. These layers utilise a quantum mechanical process known as singlet exciton fission to split certain energetic light (green and blue photons) in such a way that the electrical current of the solar cell can double in that energy range.

The principle of a solar cell is simple: per incident light particle (photon) a pair of charge carriers (exciton) consisting of a negative and a positive charge carrier (electron and hole) is generated. These two opposite charges can move freely in the semiconductor. When they reach the charge-selective electrical contacts, one only allows positive charges to pass through, the other only negative charges. A direct electrical current is therefore generated, which can be used by an external consumer.

>Read more on the BESSY II at Helmholtz-Zentrum Berlin website

Picture: Darstellung des Prinzips einer Silizium-Multiplikatorsolarzelle mit organischen Kristallen
Credit: M. Künsting/HZB

Spectacular transport: Undulator moved to the electron storage ring BESSY II

A worldwide unique undulator developed at Helmholtz-Zentrum Berlin (HZB) was installed in the storage ring BESSY II on September 20, 2018.

It supplies the “Energy Materials In-Situ Lab EMIL” with the hard X-ray light from BESSY II. The transport of the six-ton device was spectacular: several cranes were used to transport the undulator just a few hundred meters from the production building to the storage ring.

Undulators are key components to operate electron storage rings. The electrons pass through complex magnetic structures and are forced into an undulating orbit. This generates synchrotron radiation of great brilliance. What is special about the new undulator is that the magnetic structures are located in a vacuum chamber and cooled with liquid nitrogen. This permits significantly stronger magnetic fields to be generated to deflect the electrons.

>Read more on the BESSY II at HZB website

Image: Arrival in the experimental hall. The undulator was lifted into the storage ring with the overhead crane.
Credit: HZB/S. Zerbe

World record: Fastest 3D tomographic images at BESSY II

An HZB team has developed an ingenious precision rotary table at the EDDI beamline at BESSY II and combined it with particularly fast optics.

This enabled them to document the formation of pores in grains of metal during foaming processes at 25 tomographic images per second – a world record.

The quality of materials often depends on the manufacturing process. In casting and welding, for example, the rate at which melts solidify and the resulting microstructure of the alloy is important. With metallic foams as well, it depends on exactly how the foaming process takes place. To understand these processes fully requires fast sensing capability. The fastest 3D tomographic images to date have now been achieved at the BESSY II X-ray source operated by the Helmholtz-Zentrum Berlin.

Dr. Francisco Garcia-Moreno and his team have designed a turntable that rotates ultra-stably about its axis at a constant rotational speed. This really depends on the highest precision: Any tumbling around the rotation axis or even minimal deviations in the rotation speed would prevent the reliable calculation of the 3D tomography. While commercially available solutions costing several hundred thousand euros allow up to 20 tomographic images per second, the Berlin physicists were able to develop a significantly cheaper solution that is even faster. ”My two doctoral students at the Technische Universität Berlin produced the specimen holders themselves on the lathe”, says Garcia-Moreno, who not only enjoys working out solutions to tricky technical problems, but possesses a lot of craftsman skill himself as well. Additional components were produced in the HZB workshop. In addition, Garcia-Moreno and his colleague Dr. Catalina Jimenez had already developed specialized optics for the fast CMOS camera during the preliminary stages of this work that allows even for simultaneous diffraction. This makes it possible to record approximately 2000 projections per second, from which a total of 25 three-dimensional tomographic images can be created.

>Read more on the BESSY II at Helmholtz-Zentrum Berlin (HZB) website

Image: Experimental setup is composed of a fast-rotation stage, an IR heating lamp (temperature up to 800 °C), a BN crucible transparent to X-rays, a 200-μm thick LuAG:Ce scintillator, a white-beam optical system, and a PCO Dimax CMOS camera. The incident (red) and transmitted (green) X-ray beams as well as the light path from the scintillator to the camera (blue) are shown.
Credit: HZB

Insight into catalysis through novel study of X-ray absorption spectroscopy

An international team has made a breakthrough at BESSY II.

For the first time, they succeeded in investigating electronic states of a transition metal in detail and drawing reliable conclusions on their catalytic effect from the data. These results are helpful for the development of future applications of catalytic transition-metal systems. The work has now been published in Chemical Science, the Open Access journal of the Royal Society of Chemistry.

Many important processes in nature depend on catalysts, which are atoms or molecules that facilitate a reaction, but emerge from it themselves unchanged. One example is photosynthesis in plants, which is only possible with the help of a protein complex comprising four manganese atom sites at its centre. Redox reactions, as they are referred to, often play a pivotal role in these types of processes. The reactants are reduced through uptake of electrons, or oxidized through their release. Catalytic redox processes in nature and industry often only succeed thanks to suitable catalysts, where transition metals supply an important function.

>Read more about on the BESSY II at HZB website

Image: Manganese compounds also play a role as catalysts in photosynthesis.
Credit: HZB

Shutdown BESSY II: work has started

As of 30 July 2018, BESSY II will be down for several weeks.

In the summer shutdown, important components in the storage ring tunnel will be replaced and overhauled. The first conversion work for the BESSY VSR project also begins.  Upgrading BESSY II into a variable-pulse-length storage ring (BESSY-VSR) will provide unique experimental conditions for researchers worldwide. The shutdown lasts until 30 September 2018, and user operation will recommence on 30 October 2018.

While the ring is down, the HZB employees will be completely modifying the multipole wavelength shifter, the EDDI beamline and the radiation protection hutches. This space will be needed for installing the cold supply for the superconducting cavities in the storage ring. These are key components in the creation of BESSY VSR. Keeping them cold, however, requires an elaborate infrastructure, which is to be built up in the experimental hall over the next two years.

>Read more on the BESSY II at HZB website.

You can take a detailed look at everything that will be going on during the shutdown in the HZB Science Blog

Picture: The experimental hall of Bessy II.
Credit: HZB / D.Butenschön