Rich electronic features of a kagome superconductor

The recently discovered layered kagome metals AV3Sb5 (A=K, Rb, Cs) exhibit diverse correlated phenomena, thought to be linked to so-called Van Hove singularities (VHSs) and flat bands in the material. Using a combination of polarization-dependent angle-resolved photoemission spectroscopy (ARPES) and density-functional theory, researchers led by Professor Ming Shi at the Paul Scherrer Institute directly revealed the sublattice properties of 3d-orbital VHSs and flat bands, as well as topologically non-trivial surface states in CsV3Sb5. The research reveals important insights into the material’s electronic structure and provides a basis for understanding correlation phenomena in the metals.

So-called kagome metals, named after the Japanese woven bamboo pattern their structure resembles, feature symmetrical patterns of interlaced, corner-sharing triangles. This unusual lattice geometry and its inherent features lead, in turn, to curious quantum phenomena such as unconventional, or high-temperature, superconductivity.

The potential for devices that might transport electricity without dissipation at room temperature—as well as a thirst for fundamental theoretical understanding—have led researchers to investigate this new class of quantum materials and try to figure out how electrons interact with the kagome lattice to generate such remarkable features.

A recently discovered class of AV3Sb5 kagome metals, where A can be =K, Rb or Cs, was shown, for instance, to feature bulk superconductivity in single crystals at a maximum Tc of 2.5 at ambient pressure. Researchers suspect that this is a case of unconventional superconductivity, driven by some mechanism other than the phonon exchange that characterizes bonding in the electron-phonon coupled superconducting electron-pairs of conventional superconductivity.

This, as well as other exotic properties observed in the metal, are thought to be connected to its multiple “Van Hove singularities” (VHSs) near the Fermi level. VHSs, associated with the density of states (DOS), or set of different states that electrons may occupy at a particular energy level, can enhance correlation effects when a material is close to or reaches this energy level. If the Fermi level lies in the vicinity of a Van Hove point, the singular DOS determines the physical behavior due to the large number of available low-energy states. In particular, interaction effects get amplified not only in the particle-particle, but also in the particle-hole channels, leading to the notion of competing orders.

Read more on the PSI website

Image: Yong Hu, first author, and Nicholas Clark Plumb, who made the experimental station, at the Surface/Interface Spectroscopy (SIS) beamline of the Swiss Light Source (SLS) (L to R)

Credit: Paul Scherrer Institut / Mahir Dzambegovic

Mobile excitons as neutral information carriers

Excited about excitons? You should be. As charge neutral and thus efficient data transmitters, these quasiparticles could revolutionise electronics – but only if they can move. Now, for the first time, an international collaboration led by PSI have created and detected dispersing excitons in a metal using angle-resolved photoemission spectroscopy. They publish their observations in the journal Nature Materials.

Excitons are temporary bound states between electrons and positively charged holes, created when an electron absorbs a photon and moves to an excited state, leaving behind a hole in the valence band. Mobile excitons, due to their charge neutrality, offer great promise as a means for transmitting information without losses resulting from interactions with other charges en route. In contrast, the numerous interactions of electrons lead to resistance, heating and limitations in computational efficiency. Yet, the phenomenon of mobile excitons in metals has until now remained elusive, with traditional optical experiments only creating and detecting excitons with negligible momentum.  Now, researchers at the Swiss Light Source have observed dispersing excitons with large momentum for the first time in the transition metal trichalcogenide, TaSe3.

Read more on the PSI website

Image: Using ARPES, researchers could create and observe excitons diffusing along the chains of the quasi-1D metal, TaSe3. These mobile excitons come with various internal structures: interchain (red light), intrachain (pink light), or trions, formed from two electrons and a hole (blue light)

Credit: Junzhang Ma

How to get chloride ions into the cell

For the first time, a molecular movie has captured in detail the process of an anion transported across the cell membrane by a light-fueled protein pump. Publishing in Science, the researchers utilized the unique synergy of a Free Electron Laser (SwissFEL) and synchrotron light source (SLS) offered by PSI to unravel the mystery of how light energy initiates the pumping process − and how nature made sure there is no anion leakage back outside.

Many bacteria and unicellular algae have light-driven pumps in their cell membranes: proteins that change shape when exposed to photons such that they can transport charged atoms in or out of the cell. Thanks to these pumps, their unicellular owners can adjust to the environment’s pH value or salinity.

One such bacteria is Nonlabens marinus, first discovered in 2012 in the Pacific Ocean. Among others, it possesses a rhodopsin protein in its cell membrane which transports chloride anions from outside the cell to its inside. Just like in the human eye, a retinal molecule bound to the protein isomerizes when exposed to light. This isomerization starts the pumping process. Researchers now gained detailed insight into how the chloride pump in Nonlabens marinus works.

The study was led by Przemyslaw Nogly, once a postdoc at PSI and now an Ambizione Fellow and Group Leader at ETH Zürich, in close collaboration with the ALVRA team at SwissFEL and the MX team at the SLS. It is one of the first studies to fully combine experimental capabilities at these large-scale research facilities, bridging the gap in time resolution to record a full molecular movie of a protein at work. Slower dynamics in the millisecond-range were investigated via time-resolved serial crystallography at SLS while faster, up to picosecond, events were captured at SwissFEL – then both sets of data were put together.

Read more the PSI website

Image: Photoactive chloride pumping through the cell membrane captured by time-resolved serial crystallography: Chloride ions (green spheres) are transported across the cell membrane by the NmHR chloride pump (pink).

Credit: Guillaume Gotthard, Sandra Mous

X-ray microscopy with 1000 tomograms per second

Tomoscopy is an imaging method in which three-dimensional images of the inside of materials are reconstructed in rapid succession. A new world record has now been set at the Swiss Light Source at the Paul Scherrer Institute: with 1000 tomograms per second, it is now possible to non-destructively capture very fast processes and structural changes in materials on the micrometre scale, such as the burning of a sparkler or the foaming of a metal alloy for the production of stable lightweight materials. 

Most people are familiar with computed tomography from medicine: a part of the body is X-rayed from all sides and a three-dimensional image is then calculated, from which any sectional images can be created for diagnosis.

This method is also very useful for material analysis, non-destructive quality testing or in the development of new functional materials. However, to examine such materials with high spatial resolution and in the shortest possible time, the particularly intense X-ray light of a synchrotron light source is required. In the synchrotron light, even rapid changes and processes in material samples can be visualised if it is possible to capture 3-dimensional images in a very short time sequence.

A team led by Francisco García Moreno from the Helmholtz Centre Berlin is working on this, together with researchers from the Swiss Light Source SLS at the Paul Scherrer Institute (PSI). Two years ago, they managed a record 200 tomograms per second, calling the method of fast imaging “tomoscopy”. Now the team has achieved a new world record: with 1000 tomograms per second, they can now record even faster processes in materials or during the manufacturing process. This is achieved without any major compromises in the other parameters: the spatial resolution is still very good at several micrometres, the field of view is several square millimetres and continuous recording periods of up to several minutes are possible.

Read more on the PSI website

Also on the HZB website

Image: Christian Schlepütz at the Tomcat beamline of the Swiss Light Source SLS, where a team of scientists have developed a 3D imaging method capable of recording 1,000 tomograms per second.

Credit: Paul Scherrer Institute/Mahir Dzambegovic

Nanobodies against SARS-CoV-2

Göttingen researchers have developed nanobodies – a type of antibodies – that efficiently block the coronavirus SARS-CoV-2 and its new variants. Those nanobodies, which originate from alpacas inoculated with part of the SARS-CoV-2 virus spike protein – the receptor-binding domain that the virus deploys for invading host cells – could serve as a potent drug against COVID-19. The researchers used the X10SA crystallography beamline at the Swiss Light Source to characterize the interaction between the nanobodies and the coronavirus spikes at the molecular level.

Unlike antibodies, nanobodies can be produced on an industrial scale and at a low cost and therefore meet the global demand for COVID-19 therapeutics. The new nanobodies, which can bind and neutralize the virus up to 1000 times better than previously developed antibodies, are currently in preparation for clinical trials.

Read more on the PSI website

Image: The figure shows how two of the newly developed nanobodies (blue and magenta) bind to the receptor-binding domain (green) of the coronavirus spike protein (grey), thus preventing infection with SARS-CoV-2 and its variants.

Credit: Thomas Güttler / Max Planck Institute for Biophysical Chemistry

Understanding the physics in new metals

Researchers from the Paul Scherrer Institute PSI and the Brookhaven National Laboratory (BNL), working in an international team, have developed a new method for complex X-ray studies that will aid in better understanding so-called correlated metals. These materials could prove useful for practical applications in areas such as superconductivity, data processing, and quantum computers. Today the researchers present their work in the journal Physical Review X.

In substances such as silicon or aluminium, the mutual repulsion of electrons hardly affects the material properties. Not so with so-called correlated materials, in which the electrons interact strongly with one another. The movement of one electron in a correlated material leads to a complex and coordinated reaction of the other electrons. It is precisely such coupled processes that make these correlated materials so promising for practical applications, and at the same time so complicated to understand.

Strongly correlated materials are candidates for novel high-temperature superconductors, which can conduct electricity without loss and which are used in medicine, for example, in magnetic resonance imaging. They also could be used to build electronic components, or even quantum computers, with which data can be more efficiently processed and stored.

Read more on the BNL website

Image: Brookhaven Lab Scientist Jonathan Pelliciari now works as a beamline scientist at the National Synchrotron Light Source II (NSLS-II), where he continues to use inelastic resonant x-ray scattering to study quantum materials such as correlated metals.

Credit: Jonathan Pelliciari/BNL

How catalysts age

PSI researchers have developed a new tomography method with which they can measure chemical properties inside catalyst materials in 3-D extremely precisely and faster than before. The application is equally important for science and industry. The researchers published their results today in the journal Science Advances.

The material group of vanadium phosphorus oxides (VPOs) is widely used as a catalyst in the chemical industry. VPOs have been used in the production of maleic anhydride since the 1970s. Maleic anhydride in turn is the starting material for the production of various plastics, increasingly including biodegradable ones. In industry, the catalytic materials are typically used for several years, because they play an important role in the chemical reactions but are not consumed in the process. Nevertheless, a VPO catalyst changes over time as a result of this use.

In a collaborative effort, scientists from two research divisions at the Paul Scherrer Institute PSI – the Photon Science Division and the Energy and Environment Division – together with researchers at ETH Zurich and the Swiss company Clariant AG, have now investigated in detail the ageing process of VPO catalysts. In the course of their research, they also developed a new experimental method.

Read more in the PSI website

Image: Zirui Gao, a researcher at PSI, has developed a new algorithm for experimental studies that significantly shortens the duration of certain imaging measurements that would otherwise take too long. The researchers used it to investigate ageing processes in a much-used catalyst material on the nanoscale.

Credit: Paul Scherrer Institute/Markus Fischer

Magnetic vortices come full circle

The first experimental observation of three-dimensional magnetic ‘vortex rings’ provides fundamental insight into intricate nanoscale structures inside bulk magnets, and offers fresh perspectives for magnetic devices.

Magnets often harbour hidden beauty. Take a simple fridge magnet: Somewhat counterintuitively, it is ‘sticky’ on one side but not the other. The secret lies in the way the magnetisation is arranged in a well-defined pattern within the material. More intricate magnetization textures are at the heart of many modern technologies, such as hard disk drives. Now, an international team of scientists at PSI, ETH Zurich, the University of Cambridge (UK), the Donetsk Institute for Physics and Engineering (Ukraine) and the Institute for Numerical Mathematics RAS in Moscow (Russia) report the discovery of unexpected magnetic structures inside a tiny pillar made of the magnetic material GdCo2. As they write in a paper published today in the journal Nature Physics [1], the researchers observed sub-micrometre loop-shaped configurations, which they identified as magnetic vortex rings. Far beyond their aesthetic appeal, these textures might point the way to further complex three-dimensional structures arising in the bulk of magnets, and could one day form the basis for novel technological applications.

Mesmerising insights

Determining the magnetisation arrangement within a magnet is extraordinarily challenging, in particular for structures at the micro- and nanoscale, for which studies have been typically limited to looking at a shallow layer just below the surface. That changed in 2017 when researchers at PSI and ETH Zurich introduced a novel X‑ray method for the nanotomography of bulk magnets, which they demonstrated in experiments at the Swiss Light Source SLS [2]. That advance opened up a unique window into the inner life of magnets, providing a tool for determining three-dimensional magnetic configurations at the nanoscale within micrometre-sized samples.

Utilizing these capabilities, members of the original team, together with international collaborators, now ventured into new territory. The stunning loop shapes they observed appear in the same GdComicropillar samples in which they had before detected complex magnetic configurations consisting of vortices — the sort of structures seen when water spirals down from a sink — and their topological counterparts, antivortices. That was a first, but the presence of these textures has not been surprising in itself. Unexpectedly, however, the scientists also found loops that consist of pairs of vortices and antivortices. That observation proved to be puzzling initially. With the implementation of novel sophisticated data-analysis techniques they eventually established that these structures are so-called vortex rings — in essence, doughnut-shaped vortices.

Read more on the PSI website

Image: Magnetic beauty within. Reconstructed vortex rings inside a magnetic micropillar.

Credit: Claire Donnelly

Quantifying oriented myelin in mouse and human brain

Myelin “insulates” our neurons enabling fast signal transduction in our brain; myelin levels, integrity, and neuron orientations are important determinants of brain development and disease. However, myelin imaging methods used in clinics or research are non-specific or destructive.

Using small-angle X-ray scattering tensor tomography (SAXS-TT), we exploited myelin’s ~17nm periodicity to non-invasively derive 3D myelin and neuron orientation maps in macroscopic tissue volumes (Figure). We demonstrated the method on a mouse brain (a-d), a mouse spinal cord, a human visual cortex and two human white matter specimens. We validated the readouts with 2D and 3D histology, and correlated the results with MRI contrasts.

read more on the PSI website

Image: Figure. a) SAXS-TT setup. b) SAXS projection of the mouse brain, with myelin signal intensity and 2D fiber orientation color-encoded. c-d) Tomographic reconstruction results in quantitative 3D myelin maps (c) and a tensor representing neuron orientations in each voxel (d). e-f) Distinct myelin periodicities in the central and peripheral nervous system (CNS/PNS) enable multiplexed imaging (e) and reconstruction (f) of CNS and PNS structures. g) Control and dysmyelinated mouse brain signals, showcasing SAXS-TT’s sensitivity in quantifying minute myelin signals (see colorbar), and myelin integrity.

How remdesivir works against the coronavirus

Researchers at Goethe University Frankfurt, in cooperation with the Paul Scherrer Institute PSI, have probably discovered another, previously unknown mechanism of action of the antiviral remdesivir. Using structural analyses, they have discovered that a decomposition product of the virostatic agent remdesivir binds to the viral protein nsP3 of Sars-CoV-2. This protein helps the virus suppress host cell defence mechanisms. The discovery may be important for the development of new drugs to combat Sars-CoV-2 and other RNA viruses.

The virostatic agent remdesivir disrupts an important step in the propagation of RNA viruses, to which Sars-CoV-2 also belongs: the reproduction of the virus’s own genetic material. This provides the blueprint for the production of new virus particles by the host cell and is present as RNA matrices. To accelerate their reproduction, however, RNA viruses cause the RNA matrices to be copied. To do so, they use a specific protein of their own (an RNA polymerase), which is blocked by remdesivir. Strictly speaking, remdesivir does not do this itself, but rather a substance that is synthesized from remdesivir in five steps when the active agent penetrates a cell.

In the second of these five steps, an intermediate is formed from remdesivir, a substance with the somewhat unwieldy name GS-441524 (in scientific terms: a remdesivir metabolite). GS-441524 is a virostatic agent as well. As the scientists in the group headed by Stefan Knapp from the Institute for Pharmaceutical Chemistry at Goethe University Frankfurt have discovered, GS-441524 targets a Sars-CoV-2 protein called nsP3.

Read more on the PSI website

Image: May Sharpe of PSI’s Macromolecules and Bioimaging Laboratory

Credit: Paul Scherrer Institute/Markus Fischer

Cell cytoskeleton as target for new active agents

Through a unique combination of computer simulations and laboratory experiments, researchers at the Paul Scherrer Institute PSI have discovered new binding sites for active agents – against cancer, for example – on a vital protein of the cell cytoskeleton. Eleven of the sites hadn’t been known before. The study is published in the journal Angewandte Chemie International Edition.

The protein tubulin is an essential building block of the so-called cell cytoskeleton. In cells, tubulin molecules arrange themselves into tube-like structures, the microtubule filaments. These give cells their shape, aid in transporting proteins and larger cellular components, and play a crucial role in cell division.

Thus tubulin performs diverse functions in the cell and in doing so interacts with numerous other substances. “Tubulin can bind an astonishing number of different proteins and small molecules, several hundred for sure,” says Tobias Mühlethaler, a doctoral candidate in the PSI Laboratory of Biomolecular Research and first author of the study. The functions of the protein are guided by means of such bonds. Also, many drugs dock on tubulin and take effect, for example, by preventing cell division in tumours.

Read more on the PSI website

Image: The research team in front of the Swiss Light Source (from left): Andrea Prota, Tobias Mühlethaler and Michel Steinmetz

Credit: Paul Scherrer Institute/Mahir Dzambegovic

Looking for photochemistry inside particles

At the Swiss Light Source (SLS), a new photochemical reaction cell was developed for the X-ray microscope at the PolLux beamline. This allowed the researchers to mimic sunlight mediated chemical reactions in airborne particles we normally inhale. Utilizing the new reaction cell, the X-ray microscope was used to image the interior of particles for the chemistry that produced a high concentration of persistent carbon centered radicals (CCR) and reactive oxygen species (ROS), which are harmful compounds when inhaled and can cause damage in the respiratory tract. Two main factors were 1) a very high particle viscosity that effectively locks the CCRs in a glass-like state and 2) oxygen deficiency, or anoxia, to prevent smaller ROS to be formed with a shorter lifetime that easily diffuse out of the particle before inhalation. When relative humidity in air is <60%, particles can become highly viscous or even glass-like, which drastically reduces the mobility of all molecules. Although sunlight induced radical formation is likely to be unhindered, high viscosity would instead inhibit molecular diffusion and block oxygen from accessing the particle interior. This leads to preservation of large amounts of radicals. Amazingly, this may apply to all organic light absorbing atmospheric compounds making radical abundance and persistence an unforeseen issue until now.

Particles composed of citric acid and iron were investigated as a model for iron containing organic particles. About 1 in 20 airborne particles contain iron in urban areas at a significant concentration as identified by previous studies. The oxidation state of iron was mapped across individual particles using X-ray spectromicroscopy to reveal where photochemical reactions, oxidation and molecular diffusion took place inside. Oxidation and formation of ROS took place rapidly, but surprisingly, only near the particle surfaces, i.e. an oxidized reaction front extending only hundreds of nanometers was directly observed. This was entirely due to the rapid depletion of oxygen in the particle due to slow molecular transport and fast reaction cycling. In addition to X-ray microscopy, the researchers used an electrodynamic balance (collaboration with ETHZ) and a coated wall flow tube reactor to study these radical forming particles and constrain the overall reactive cycle and the production and release of radicals to air.

Read more on the PSI website

Image: A chemical scheme and X-ray image showing particles oxidized only near their surface. Light in iron-organic particles start a cycle of oxidizing reactions (purple text) forming carbon centered radicals (yellow text) and reactive oxygen species (red text). We directly imaged oxidation happening only near the particle surfaces indicated by the brighter colour in micrometer and submicrometer viscous particles in the right image.

Credit: PSI

Microbes and viruses in the spotlight

The world of microbes and viruses is extremely old and extremely diverse. With the help of the large research facilities at PSI, researchers can look deep into this alien cosmos and above all explore the proteins of exotic beings.

Since they emerged as the first life on our planet around 3.5 billion years ago, they have shaped the earth like no other form of life: microorganisms. In this motley group there are such diverse representatives as bacteria, archaebacteria, algae, yeasts, amoebas or parasites like the malaria pathogen. But as diverse as microorganisms may be, they also do not include one biological form of existence: viruses. Because these are a borderline case between the animate and the inanimate. They do not have their own metabolism and therefore always need a host in order to awaken to life and multiply. The vast majority of microorganisms and viruses are harmless or very useful for humans, for example for digestion or to produce food, to purify wastewater or to form humus.

Read more about the ongoing research at Synchrotron Lichtquelle Schweiz (SLS) and SwissFEL on the PSI website

Image: Researchers are studying how a sodium pump works on a marine bacterium. The knowledge could lead to new insights in neurobiology. (Graphic: Christoph Frei)

In search of the lighting material of the future

At the Paul Scherrer Institute PSI, researchers have gained insights into a promising material for organic light-emitting diodes (OLEDs). The substance enables high light yields and would be inexpensive to produce on a large scale – that means it is practically made for use in large-area room lighting. Researchers have been searching for such materials for a long time. The newly generated understanding will facilitate the rapid and cost-efficient development of new lighting appliances in the future. The study appears today in the journal Nature Communications.

The compound is a yellowish solid. If you dissolve it in a liquid or place a thin layer of it on an electrode and then apply an electric current, it gives off an intense green glow. The reason: The molecules absorb the energy supplied to them and gradually emit it again in the form of light. This process is called electroluminescence. Light-emitting diodes are based on this principle.

Read more on the Swiss FEL and Swiss Light Source website

Image: Grigory Smolentsev in front of SwissFEL

Credit: Paul Scherrer Institute/Mahir Dzambegovic

Toward better motors with X-ray light

Making Switzerland’s road traffic fit for the future calls for research, first and foremost. In the large-scale research facilities of PSI, chemists and engineers are investigating how to improve the efficiency of motors and reduce their emissions.

“The overall transportation system of Switzerland in 2040 is efficient in all aspects.” The primary strategic goal of the Federal Department of the Environment, Transport, Energy and Communications (DETEC) sounds good. The subordinate Swiss Federal Office of Energy (SFOE) specifies that vehicular traffic should pollute the environment less and become more energy-efficient and climate-friendly. Switzerland has set an ambitious goal for itself: to be climate-neutral by 2050.
This is a major challenge. According to the most recent “microcensus” on mobility from 2015, every person living in Switzerland travels around 24,850 kilometres per year. A high number, which also includes trips abroad. In everyday life and within Switzerland, the average per person is nearly 37 kilometres per day – and rising.
According to the Federal Office for the Environment (FOEN), cars, trucks, and buses produce three-fourths of the greenhouse gas emissions in the transportation sector. From this it follows: Whether or not the nation achieves its goal depends heavily on the motors used in these modes of transportation. Their CO2 emissions must be radically reduced. This is precisely the starting point for researchers at PSI and other institutions.

> Read more on the Swiss Light Source (PSI) website

Image: Passenger cars powered by hydrogen fuel cells have a greater range than electric cars, but they are less efficient. PSI researchers want to change that.
Credit: Adobe Stock/Graphic: Stefan Schulze-Henrichs

Soft X-ray Laminography: 3D imaging with powerful contrast mechanisms

Soft X-ray 3D imaging has already been realized at synchrotron radiation sources using either scanning transmission X-ray microscopy (STXM) schemes or tomography-based concepts. However, the maximum accessible sample volume is severely limited by the reduced penetration depth of the lower-energy soft X-ray radiation. This becomes even more of a drawback in the case of flat and extended specimens, which can be found in various fields of nanoscience.

The generalized geometry of laminography, characterized by a tilted axis of rotation concerning the incident X-ray beam resulting in a constant material thickness during rotation, has proven to be particularly suitable for the investigation of laterally extended and thin objects. The combination of soft X-rays and laminography provides the unique potential of bridging the gap between investigations of elaborate nanostructured thin film samples and taking advantage of the characteristic absorption contrast mechanisms in the soft X-ray range.

>Read more on the Swiss Light Source at PSI website

Image: 3D model constructed from soft X-ray laminography measurements of the front tip of the wing scale from a European peacock butterfly.