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 

Call for nominations: Innovation Award on Synchrotron Radiation 2018

The Society of Friends of Helmholtz-Zentrum Berlin (HZB) announces the bestowal of the Innovation Award on Synchrotron Radiation*.

The award was established in 2001 for an excellent achievement which has contributed significantly to the further development of techniques, methods or uses of synchrotron radiation. Scientists and engineers from research institutions, universities, and industry within Europe are addressed. The Innovation Award includes a monetary prize of 3000 Euro and will be bestowed at the Users’ Meeting of HZB (BESSY II) in December 2018.

All nominations should be submitted to the Chair of the Society by September 30, 2018. Suggestions of candidates have to be addressed electronically and must include a concise, verifiable description in English of the scientific-technological achievement. The curriculum vitae, the publication list of the candidate(s) and at five most relevant publications have to be submitted. Two references should be named.

Please address nominations to:

Prof. Dr. Mathias Richter
Chair of the Society of Friends of Helmholtz-Zentrum Berlin
Head of Department Radiometry with Synchrotron Radiation, Physikalisch-Technische Bundesanstalt
Faculty of Mathematics and Natural Sciences, Technische Universität Berlin
Email: mathias.richter@ptb.de

*sponsored by SPECS GmbH and BESTEC GmbH, Berlin.

>Read more about the Friends of Helmholtz-Zentrum Berlin e.V. on the HZB website

Picture: Bessy II at Helmholtz-Zentrum Berlin.

Helmholtz International Fellow Award for N. Mårtensson

The Helmholtz Association has presented the Swedish physicist Nils Mårtensson with a Helmholtz International Fellow Award. 

The synchrotron expert of the University of Uppsala, who heads the nobel comitee for physics, cooperates closely with the HZB-Institute Methods and Instrumentation for Synchrotron Radiation Research. Nils Mårtensson is a professor at Uppsala University. He directed the development of the Swedish synchrotron radiation source Max IV and received a grant from the European Research Council (ERC) in 2013. Mårtensson is a member of the Swedish Academy of Sciences and chairman of the Nobel Committee for Physics. At HZB, he cooperates with Alexander Föhlisch’s team at HZB-Institute Methods and Instrumentation for Synchrotron Radiation Research. Together they run the Uppsala Berlin Joint Laboratory (UBjL) to further develop methods and instruments.

Image: Nils Mårtensson, University of Uppsala, cooperates closely with HZB.

Helmholtz Association supports ATHENA

ATHENA (“Accelerator Technology HElmholtz iNfrAstructure”) is a new research and development platform focusing on accelerator technologies and drawing on the resources of all six Helmholtz accelerator institutions (DESY, Jülich Research Centre, Helmholtz Centre Berlin, Helmholtz Centre Dresden-Rossendorf HZDR, KIT and GSI with the Helmholtz Institute of Jena). The Helmholtz Association has now decided to pay almost 30 million euros towards ATHENA as a strategic development project. “This decision demonstrates the Helmholtz Association’s strong commitment to developing and supplying ground-breaking new accelerator technologies for solving the future challenges faced by society,” says Helmut Dosch, who is the Chairman of DESY’s Board of Directors and also the spokesperson for the Helmholtz Association’s research division Matter.

Together, these centres want to set up two German flagship projects in accelerator research based on innovative plasma-based particle accelerators and ultramodern laser technology: an electron accelerator at DESY in Hamburg and a hadron accelerator at HZDR. At both facilities, a range of different fields of application are to be developed, ranging from a compact free-electron laser, through novel medical uses to new applications in nuclear and particle physics. As soon as they have reached the necessary level of maturity to be put to practical use in a particular area, new compact devices could be built for use in other Helmholtz centres, as well as in universities and hospitals.

>Read more on the Bessy II at HZB website or the DESY website

Support for HZB’s future and call for rapid planning of Bessy III

The Helmholtz-Zentrum Berlin (HZB) has received an evaluation of “excellent” in a review of science programmes undertaken at all Helmholtz Research Centres.

This provides the foundation for future financing of HZB.
Two committees of leading international scientists visited the HZB for a week each at the beginning of this year. They evaluated the HZB’s contributions to the Helmholtz programmes in the research areas of “Matter” and “Energy”. Now the written evaluations are available. The team spirit of all employees involved in the HZB was particularly emphasised.

The report states that the HZB and the Helmholtz Association have made decisions characterised by vision. The right course had been set both in terms of infrastructure and in recruiting people. The HZB can rely on highly competent, committed employees at all levels.

All research programmes of the HZB have received an evaluation of “excellent”. The HZB contributions to the MML programme (From Matter to Materials and Life) focussing on the use of photons are considered to be leading, with all of its participating groups receiving the top marks of “Outstanding” or “Excellent”. The Renewable Energies (RE) and the Future Information Technologies (FIT) programmes, the instrumentation at the BESSY II synchrotron (some of which is unique), and the BER II research reactor were likewise evaluated highly.

>Read more on the Bessy II at HZB website

Image: The review panel of the research field “Matter” visited the HZB on 11th January 2018.
Credit: HZB/J. Bierbaum

 

 

Writing and deleting magnets with lasers

Scientists * have found a way to write and delete magnets in an alloy using a laser beam – a surprising effect.

* at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) together with colleagues from the Helmholtz-Zentrum Berlin (HZB) and the University of Virginia in Charlottesville, USA

The reversibility of the process opens up new possibilities in the fields of material processing, optical technology, and data storage.
Researchers of the HZDR, an independent German research laboratory, studied an alloy of iron and aluminum. It is interesting as a prototype material because subtle changes to its atomic arrangement can completely transform its magnetic behavior. “The alloy possesses a highly ordered structure, with layers of iron atoms that are separated by aluminum atomic layers. When a laser beam destroys this order, the iron atoms are brought closer together and begin to behave like magnets,” says HZDR physicist Rantej Bali.

Bali and his team prepared a thin film of the alloy on top of transparent magnesia through which a laser beam was shone on the film. When they, together with researchers of the HZB, directed a well-focused laser beam with a pulse of 100 femtoseconds (a femtosecond is a millionth of a billionth of a second) at the alloy, a ferromagnetic area was formed. Shooting laser pulses at the same area again – this time at reduced laser intensity – was then used to delete the magnet.

>Read more on the Bessy II at HZB website

Image: Laser light for writing and erasing information – a strong laser pulse disrupts the arrangement of atoms in an alloy and creates magnetic structures (left). A second, weaker, laser pulse allows the atoms to return to their original lattice sites (right). (Find the entire image here)
Credit: Sander Münster / HZDR

Twin Orbit operation successfully tested at BESSY II

The first “Twin Orbit User Test week” at BESSY II in February 2018 was a big success and can be considered as an important step towards real user operation.

Physicists at Helmholtz-Zentrum Berlin have been able to store two separate electron beams in one storage ring. The twin orbit operation mode can serve users with different needs of the time structure of the photon pulses simultaneously and offers elegant options regarding the future project BESSY VSR.

The Twin Orbit operation mode makes use of non-linear beam dynamics and provides two stable well separated orbits for storing two electron beams in one storage ring. The bunch fill patterns of both orbits can be chosen, to some extent, independently, which allows for fulfilling normally incompatible user needs, simultaneously. For example, one orbit can be used to store a homogenous multi bunch fill to deliver high average brilliance for photon hungry experiments, whereas only one single bunch is stored on the other orbit for timing experiments, providing a much lower pulse repetition rate.

>Read more on the Bessy II at HZB website

Image: A synchrotron source point image of a bending magnet of the Twin Orbit modus. The second orbit closes after three revolution and is winding around the standard orbit at the center. (full image here)
Credit: HZB