40-year controversy in solid-state physics resolved

An international team at BESSY II headed by Prof. Oliver Rader has shown that the puzzling properties of samarium hexaboride do not stem from the material being a topological insulator, as it had been proposed to be.

Theoretical and initial experimental work had previously indicated that this material, which becomes a Kondo insulator at very low temperatures, also possessed the properties of a topological insulator. The team has now published a compelling alternative explanation in Nature Communications, however.

Samarium hexaboride is a dark solid with metallic properties at room temperature. It hosts Samarium, an element having several electrons confined to localized f orbitals in which they interact strongly with one another. The lower the temperature, the more apparent these interactions become. SmB6 becomes what is known as a Kondo insulator, named after Jun Kondo who was first able to explain this quantum effect.

In spite of Kondo-Effect: some conductivity remains

About forty years ago, physicists observed that SmB6 still retained remnant conductivity at temperatures below 4 kelvin, the cause of which had remained unclear until today. After the discovery of the topological-insulator class of materials around 12 years ago, hypotheses grew insistent that SmB6 could be a topological insulator as well as being Kondo insulator, which might explain the conductivity anomaly at a very fundamental level, since this causes particular conductive states at the surface. Initial experiments actually pointed toward this.

>Read more on the Bessy II website

Image: Electrons with differing energies are emitted along various crystal axes in the interior of the sample as well as from the surface. These can be measured with the angular-resolved photoemission station (ARPES) at BESSY II. Left image shows the sample temperature at 25 K, right at only 1 K. The energy distribution of the conducting and valence band electrons can be derived from these data. The surface remains conductive at very low temperature (1 K).
Credit: Helmholtz Zentrum Berlin

Complex tessellations, extraordinary materials

Simple organic molecules form complex materials through self-organisation

An international team of researchers lead by the Technical University of Munich (TUM) has discovered a reaction path that produces exotic layers with semiregular structures. These kinds of materials are interesting because they frequently possess extraordinary properties. In the process, simple organic molecules are converted to larger units which form the complex, semiregular patterns. With experiments at BESSY II at Helmholtz-Zentrum Berlin this could be observed in detail.

Only a few basic geometric shapes lend themselves to covering a surface without overlaps or gaps using uniformly shaped tiles: triangles, rectangles and hexagons. Considerably more and significantly more complex regular patterns are possible with two or more tile shapes. These are so-called Archimedean tessellations or tilings.

Materials can also exhibit tiling characteristics. These structures are often associated with very special properties, for example unusual electrical conductivity, special light reflectivity or extreme mechanical strength. But, producing such materials is difficult. It requires large molecular building blocks that are not compatible with traditional manufacturing processes.

 

>Read more on the Bessy II website

Image: The new building block (left, red outline) comprises two modified starting molecules connected to each other by a silver atom (blue). This leads to complex, semiregular tessellations (right, microscope image).
Credit: Klappenberger and Zhang / TUM

Perovskite solar cells: perfection not required!

Experiments at BESSY II reveal why even inhomogeneous perovskite films are highly functional

Metal-organic perovskite layers for solar cells are frequently fabricated using the spin coating technique. If you follow the simplest synthesis pathway and use industry-relevant compact substrates, the perovskite layers laid down by spin coating generally exhibit numerous holes, yet attain astonishingly high levels of efficiency. The reason that these holes do not lead to significant short circuits between the front and back contact and thus high-rate charge carrier recombination has now been discovered by a HZB team headed by Dr.-Ing. Marcus Bär in cooperation with the group headed by Prof. Henry Snaith (Oxford Univ.) at BESSY II.

>Read more on the HZB website.

BER II and BESSY II User Meeting at HZB

More than 600 scientists registered for the 9th annual BER II and BESSY II User Meeting in December.

The Friends of Helmholtz-Zentrum Berlin recognised outstanding work in the field of synchrotron radiation with an Innovation Award, and the best doctoral dissertation with the Ernst-Eckard-Koch prize.

Research with synchrotron radiation was the focus on Wednesday. On Thursday, the „Science Day“, a broad overview on user research was given. The public talk entitled “Interfacing with the brain using organic electronics” found great appeal with the audience: George Malliaras from the University of Cambridge spoke about the potential of organic electronics as brain implants for treating neurological diseases such as epilepsy. New materials and their characterisation at synchrotron sources will play a crucial role in this development. A vendor exhibition by 56 companies on new technical and optical instruments for research was well visited by the scientists.

>Read more on the HZB website.

Image: The Innovation Award of Freundeskreis HZB was given to a team of DESY, Hamburg.
Credit: HZB

Antiferromagnetic dysprosium reveals magnetic switching with less energy

HZB scientists have identified a mechanism with which it may be possible to develop a form of magnetic storage that is faster and more energy-efficient.

They compared how different forms of magnetic ordering in the rare-earth metal named dysprosium react to a short laser pulse. They discovered that the magnetic orientation can be altered much faster and with considerably less energy if the magnetic moments of the individual atoms do not all point in the same direction (ferromagnetism), but instead point are rotated against each other (anti-ferromagnetism). The study was published in Physical Review letters on 6. November 2017 and on the cover of the print edition.

Dysprosium is not only the atomic element with the strongest magnetic moments, but it also possesses another interesting property: its magnetic moments point either all the same direction (ferromagnetism) or are tilted against each other, depending on the temperature. This makes it possible to investigate in the very same sample how differently oriented magnetic moments behave when they are excited by an external energy pulse.

>Read More on the Bessy II (HZB) website

Image: A short laser pulse pertubates magnetic order in dysprosium. This happens much faster if the sample had a antiferromagnetic order (left) compared to ferromagnetic order (right). Credit: HZB

Approved! The EU INFINITE-CELL project

A large EU-sponsored research project on tandem solar cells in which HZB is participating begins in November 2017.

The goal is to combine thin-film semiconductors made of silicon and kesterites into especially cost-effective tandem cells having efficiencies of over 20 per cent. Several large research institutions from Europe, Morocco, the Republic of South Africa, and Belarus will be working on the project, as well as two partners from industry.

“We not only have detailed experience with kesterite thin films, but also a wide spectrum of analytical methods at our disposal to characterise absorber materials very thoroughly”, explains Prof. Susan Schorr. The FUNDACIO INSTITUT DE RECERCA DE L’ENERGIA DE CATALUNYA (IREC), Spain – a long-term collaborating partner of the HZB, is coordinating the entire project. The project begins with a kick-off workshop in Brussels in November 2017.

HZB launches the HI-SCORE international research school in collaboration with Israel

The Helmholtz-Zentrum Berlin is establishing the Helmholtz International Research School HI-SCORE, which will be oriented towards solar energy research.

To accomplish this, HZB is collaborating with the Weizmann Institute in Rehovot, the Israeli Institute of Technology (Technion) in Haifa, and three Israeli universities as well as universities in Berlin and Potsdam. The project is being funded by the Helmholtz Association.

The name “HI-SCORE” stands for “Hybrid Integrated Systems for Conversion of Solar Energy”. The research themes extend from novel solar cells based on metal-organic perovskites, to tandem solar cells, to complex systems of materials for generating solar fuels. These complex materials systems can convert the energy of sunlight to chemical energy so it can be easily stored in the form of fuel.

Joining forces to advance perovskite solar cells

Great Interest in the HySPRINT Industry Day

No fewer than 70 participants attended the first Industry Day of the Helmholtz Innovation Lab HySPRINT devoted to the topic of perovskite solar cells at Helmholtz-Zentrum Berlin (HZB) on 13 October 2017. This far exceeded the expectations of the event hosts. The knowledge shared on Industry Day will serve as the basis for deepening the collaboration even further with strategically important companies in the scope of HySPRINT.

“Seeing the industry partners’ active participation was very gratifying. We could feel in the lively discussions how there is great interest on both sides to collaborate even more closely on technology transfer,” says Dr. Stefan Gall, project manager of the Helmholtz Innovation Lab HySPRINT (“Hybrid Silicon Perovskite Research, Integration & Novel Technologies”). On the Industry Day, eight companies presented those topics that especially interest them. “From this, certain problems emerged that we are now going to work on targetedly with our industrial partners.”

Missing link between new topological phases of matter discovered

HZB-Physicists at BESSY II have investigated a class of materials that exhibit characteristics of topological insulators.

During these studies they discovered a transition between two different topological phases, one of which is ferroelectric, meaning a phase in the material that exhibits spontaneous electric polarisation and can be reversed by an external electric field. This could also lead to new applications such as switching between differing conductivities.

The HZB researchers studied crystalline semiconductor films made of a lead, tin, and selenium alloy (PbSnSe) that were doped additionally with tiny amounts of the element bismuth. These semiconductors belong to the new class of materials called topological insulators, materials that conduct very well at their surfaces while behaving as insulators internally. Doping with 1-2 per cent bismuth has enabled them to observe a new topological phase transition now. The sample changes to a particular topological phase that also possesses the property of ferroelectricity. This means that an external electric field distorts the crystal lattice, whereas conversely, mechanical forces on the lattice can create electric fields.

 

>Read More on the Bessy II (HZB) website

 Image: The Bismut doping is enhanced from 0% (left) to 2.2% (right). Measurements at BESSY II show that this leads to increased bandgaps. Credit: HZB

The miracle material graphene: convex as a chesterfield

Graphene possesses extreme properties and can be utilised in many ways.

Even the spins of graphene can be controlled through use of a trick. This had already been demonstrated by a HZB team some time ago: the physicists applied a layer of graphene onto a nickel substrate and introduced atoms of gold in between (intercalation).

The scientists now show why this has such a dramatic influence on the spins in a paper published in 2D Materials. As a result, graphene can also be considered as a material for future information technologies that are based on processing spins as units of information.

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Solar hydrogen production by artificial leafs

Scientists analysed how a special treatment improves cheap metal oxide photoelectrodes

Metal oxides are promising candidates for cheap and stable photoelectrodes for solar water splitting, producing hydrogen with sunlight. Unfortunately, metal oxides are not highly efficient in this job. A known remedy is a treatment with heat and hydrogen. An international collaboration has now discovered why this treatment works so well, paving the way to more efficient and cheap devices for solar hydrogen production.

The fossil fuel age is bound to end, for several strong reasons. As an alternative to fossil fuels, hydrogen seems very attractive. The gas has a huge energy density, it can be stored or processed further, e. g. to methane, or directly provide clean electricity via a fuel cell. If it is produced using sunlight alone, hydrogen is completely renewable with zero carbon emissions.

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