physics – Lightsources.org

Ultra-fast switching of helicity of circularly polarized light pulses

2020/04/012020/04/03

At the BESSY II storage ring, a joint team of accelerator physicists, undulator experts and experimenters has shown how the helicity of circularly polarized synchrotron radiation can be switched faster – up to a million times faster than before.

They used an elliptical double-undulator developed at HZB and operated the storage ring in the so-called two-orbit mode. This is a special mode of operation that was only recently developed at BESSY II and provides the basis for fast switching. The ultra-fast change of light helicity is particularly interesting to observe processes in magnetic materials and has long been expected by a large user community.
In synchrotron radiation sources such as BESSY II, electron bunches orbit the storage ring at almost the speed of light. They are forced to emit extremely bright light pulses with special properties by periodic magnetic structures (undulators).

Experiments with polarized light pulses

Elliptical undulators can be used to generate also circularly polarized light pulses, which display a feature called helicity: the polarisation goes either clockwise or counterclockwise. Magnetic structures in materials react differently to circularly polarized light: Depending on the helicity of the X-ray pulses, they more or less absorb this radiation.

Image: This picture shows an X-ray image of the electron beam in TRIB-mode where two orbits co-exist: the regular orbit and the second one winding around it closing only after three revolutions.
Credit: F. Armborst/K. Holldack

Ghostly X-ray images could provide key info for analyzing X-ray laser experiments

2019/04/032019/04/04

SLAC researchers say their new method could make it easier to study interactions of ultrabright X-rays with matter

X-ray free-electron lasers (XFELs) produce incredibly powerful beams of light that enable unprecedented studies of the ultrafast motions of atoms in matter. To interpret data taken with these extraordinary light sources, researchers need a solid understanding of how the X-ray pulses interact with matter and how those interactions affect measurements.
Now, computer simulations by scientists from the Department of Energy’s SLAC National Accelerator Laboratory suggest that a new method could turn random fluctuations in the intensity of laser pulses from a nuisance into an advantage, facilitating studies of these fundamental interactions. The secret is applying a method known as “ghost imaging,” which reconstructs what objects look like without ever directly recording their images.

>Read more on the LCLS at SLAC website

Image: SLAC researchers suggest using the randomness of subsequent X-ray pulses from an X-ray laser to study the pulses’ interactions with matter, a method they call pump-probe ghost imaging.
Credit: Greg Stewart/SLAC National Accelerator Laboratory

Godehard Wüstefeld receives the Horst Klein Research Prize

2019/03/252019/03/27

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.

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

Magnetic patterning by electron beam assisted carbon lithography

2019/02/112019/03/11

The exploitation of the unique physical properties of thin films and heterostructures are opening intriguing opportunities for magnetic storage technology. These artificial materials will in fact enable novel architectures for a multitude of magnetic devices and sensors, promoting a significant improvement in storage density, functionality and efficiency. Their usage will also contribute to diminish the consumption of materials that are rare and difficult to extract, being often detrimental to the environment. With these objectives in mind, researchers are now looking with great attention at the combination of thin ferromagnetic layers with 2-dimensional crystals like graphene and transition metal dichalcogenides. Due to their layered structure, these systems exhibit very favorable magnetic properties, which can be tuned through thickness and interfacial interactions. For instance, graphene-cobalt stacks display an enhanced perpendicular magnetic anisotropy, a feature that is especially important for non-volatile memories.
The fabrication of layered materials, however, is still a very challenging process. Not only it requires atomic precision in the deposition of the various layers but also the ability to create nano or microstructures of arbitrary shape. Conventional lithography in conjunction with chemical etching permits nowadays to sculpture the matter with great accuracy, at lateral resolution close to the nanometer. Yet, this approach poses an important limitation, that is, the material can only be shaped by erosion. The ability to vary the chemical composition, by adding atoms for example, is instead very desirable for many applications. To date, this can be done by stimulating the fragmentation of suitable carrier molecules using photons or electrons. So far, various methods based on focused beam induced processing methods have been devised, which can be readily employed to deposit carbonaceous layers and metallic nanostructures. These methods, however, cannot be applied when ultra-clean, ultra-high vacuum (UHV) conditions are needed, as happens for the case of semiconductor industry.

>Read more on the Elettra website

Figure 1. (left) Scheme of the protocol for printing chemo-magnetic patterns in ultrathin Co on Re(0001). (a) The film is exposed to CO at room temperature. The irradiation with a focused electron beam (yellow) stimulates the dissociation of the molecule, which results in the accumulation of atomic carbon on the surface. (b) Subsequently, the sample is annealed above 170 °C to desorb molecularly adsorbed CO from the non-irradiated surface regions. (c) LEEM image of an e-beam irradiated disk. Disk diameter: 1 μm; Co thickness: 4 atomic layers; irradiation energy: 50 eV; CO dose: 9.75 L; (d) Intensity profile across the orange line in the LEEM image in (c) and fit using a step function convoluted with a Gaussian of full width at half-maximum of 30 nm. The dashed blue lines indicate the 15–85% distance between minimum and maximum intensity. (e) XMCD-PEEM image of the same region at the Co L3 edge. (f) Intensity profiles across the blue and orange dashed lines in the XMCD-PEEM image in (e). The magnetic stripes indicate out-of-plane magnetic anisotropy. The stripe period is 120 nm. Adapted with permission from [1].
Copyright (2018) American Chemical Society.

The 2018 Julian David Baumert Ph.D. Thesis Award

2018/06/212018/06/21

Maxwell Terban received the 2018 Julian Baumert Ph.D. Thesis Award at this year’s Joint CFN and NSLS-II Users’ Meeting.

Maxwell Terban, a postdoctoral researcher at the Max-Plank Institute for Solid State Research, Stuttgart, is this year’s recipient of the Julian Baumert Ph.D. Thesis Award. Terban was selected for developing new research methods, based around a technique called pair distribution function (PDF), for extracting and analyzing structural signatures from materials. His research incorporated measurements from the now-closed National Synchrotron Light Source (NSLS) and the recently opened National Synchrotron Light Source II (NSLS-II)—a U.S. Department of Energy (DOE) Office of Science User Facility located at Brookhaven National Laboratory.

Each year, the Baumert Award is given to a researcher who has recently conducted a thesis project that included measurements at NSLS or NSLS-II. The award was established in memory of Julian David Baumert, a young Brookhaven physicist who worked on x-ray studies of soft-matter interfaces at NSLS.

Terban holds a bachelor’s degree in chemical engineering from the University of Massachusetts, Amherst, and a master’s degree in materials science and engineering from Columbia University. He graduated with a Ph.D. in materials science and engineering from Columbia University in 2018, and completed his doctoral dissertation under the guidance of Simon Billinge, a professor of materials science and engineering and applied physics and mathematics at Columbia.

Image: Maxwell Terban, a postdoctoral researcher at the Max-Plank Institute for Solid State Research, Stuttgart, is this year’s recipient of the Julian Baumert Ph.D. Thesis Award.

Perovskites, the rising star for energy harvesting

2018/06/052018/06/18

Perovskites are promising candidates for photovoltaic cells, having reached an energy harvesting of more than 20% while it took silicon three decades to reach an equivalent. Scientists from all over the world are exploring these materials at the ESRF.

Photovoltaic (PV) panels exist in our society since several years now. The photovoltaic market is currently dominated by wafer-based photovoltaics or first generation PVs, namely the traditional crystalline silicon cells, which take a 90% of the market share.

Although silicon (Si) is an abundant material and the price of Si-PV has dropped in the past years, their manufacturing require costly facilities. In addition, their fabrication typically takes place in countries that rely on carbon-intensive forms of electricity generation (high carbon footprint).

But there is room for hope. There is a third generation of PV: those based on thin-film cells. These absorb light more efficiently and they currently take 10% of the market share.

Image: The CEA-CNRS team on ID01. From left to right: Peter Reiss, from CEA-Grenoble/INAC, Tobias Schulli from ID01, Tao Zhou from ID01, Asma Aicha Medjahed, Stephanie Pouget (both from CEA-Grenoble/INAC) and David Djurado, from the CNRS.
Credits: C. Argoud.

Edges and corners increase efficiency of catalytic converters

2018/03/262019/11/29

X-rays reveal oxide islands on noble metal nanoparticles

Catalytic converters for cleaning exhaust emissions are more efficient when they use nanoparticles with many edges. This is one of the findings of a study carried out at DESY’s X-ray source PETRA III. A team of scientists from the DESY NanoLab watched live as noxious carbon monoxide (CO) was converted into common carbon dioxide (CO2) on the surface of noble metal nanoparticles like those used in catalytic converters of cars. The scientists are presenting their findings in the journal Physical Review Letters. Their results suggest that having a large number of edges increases the efficiency of catalytic reactions, as the different facets of the nanoparticles are often covered by growing islands of a nano oxide, finally rendering these facets inactive. At the edges, the oxide islands cannot connect, leaving active sites for the catalytic reaction and an efficient oxygen supply.
Catalytic converters usually use nanoparticles because these have a far greater surface area for a given amount of the material, on which the catalytic reaction can take place. For the study presented here, the scientists at DESY’s NanoLab grew platinum-rhodium nanoparticles on a substrate in such a way that virtually all the particles were aligned in the same direction and had the same shape of truncated octahedrons (octahedrons resemble double pyramids). The scientists then studied the catalytic properties of this sample under the typical working conditions of an automotive catalytic converter, with different gaseous compositions in a reaction chamber that was exposed to intense X-rays from PETRA III on the P09 beamline.

Image: With increasing oxygen (red) concentration, an oxide sandwich forms on the surface of the metallic nanoparticles, inhibiting the desired reaction of carbon monoxide to carbon dioxide. At the edges, however, the oxide sandwich brakes up, leaving free active sites for catalysis. The more edges the nanoparticles posses, the more efficient will the catalytic converter work.
Credit: DESY, Lucid Berlin

Tailoring the surface chemical reactivity of transition‐metal dichalcogenide PtTe2 crystals

2018/03/262018/03/28

Recently, the PtX2 (X=S, Se, Te) class of transition-metal dichalcogenides has emerged as one of the most promising among layered materials “beyond graphene” for the presence of high room-temperature electron mobility and, moreover, bulk type-II Dirac fermions, arising from a tilted Dirac cone.
Information on the ambient stability of PtTe2 is a crucial step in order to evaluate the feasibility of its exploitation in technology. Moreover, the possibility to tune surface chemical reactivity by appropriate surface modification is an essential step for its employment for diverse applications, especially in catalysis.
By means of experiments with several surface-science spectroscopies and density functional theory, an international team of researchers from Italy, Republic of Korea, and Taiwan (coordinated by Graphene Labs of Istituto Italiano di Tecnologia) has investigated the reactivity of the PtTe2 surface toward most common ambient gases (oxygen and water), under the framework of the European Graphene Flagship-Core1 project.
To assess the surface chemical reactivity of PtTe2, X-ray photoelectron spectroscopy (XPS) carried out at the APE-HE beamline has been combined with high-resolution electron energy loss (HREELS) experiments and with density functional theory.
From the analysis of Te 3d core-level spectra in XPS and from the featureless vibrational spectrum in HREELS, it has been demonstrated that as-cleaved defect-free PtTe2 surface is inert toward most common ambient gases (oxygen and water).
In the evaluation of the ambient stability of PtTe2, the possible influence of Te vacancies on surface chemical reactivity deserves particular attention. As a matter of fact, Te vacancies may appear on non-stoichiometric samples during the growth process. To check the influence of Te vacancies on ambient stability of PtTe2, Te vacancies have been intentionally introduced in stoichiometric PtTe2 samples by Ar-ion sputtering. After exposing to O2 the PtTe2 surface defected by ion sputtering, with a Pt:Te ratio of 39:61, spectral features related to Te(IV) species appear, arising from the formation of Te=O bonds in a tellurium-oxide phase. The Te(IV) components are the most intense lines in the Te 3d XPS spectra for the case of air-exposed defected samples (see Figure 1). Concerning reactivity to water, it adsorbs molecularly even at room temperature on defected PtTe2. These findings also imply that the presence of Te vacancies is able to jeopardize the ambient stability of uncapped PtTe2-based devices, with a subsequent necessity to reduce the amount of Te vacancies for a successful technological exploitation of PtTe2.

>Read more on the Elettra website

Figure: XPS spectra of Te-3d core levels acquired for: defected PtTe₂ (green curve), the same surface exposed to 10⁶ L of O₂(black curve) and air-exposed defected PtTe₂ (yellow curve). The photon energy is 745 eV.

Scientists have a new way to gauge the growth of nanowires

2018/03/192018/03/27

In a new study, researchers from the U.S. Department of Energy’s Argonne and Brookhaven National Laboratories observed the formation of two kinds of defects in individual nanowires, which are smaller in diameter than a human hair.

These nanowires, made of indium gallium arsenide, could be useful for a wide range of applications in a field scientists have termed optoelectronics, which encompasses devices that work by converting light energy into electrical impulses. Fiber optic relays are a good example.

The effectiveness of these devices, however, can be affected by tiny defects in their components. These defects, which can change both the optical and electronic properties of these materials, interest scientists who seek to tailor them to boost the functionality of future optoelectronics, including materials that will be able to manipulate quantum information.

>Read more on the NSLS-II website and the Advanced Photon Source website

Image: Argonne and Brookhaven researchers observed two kinds of defects forming in individual nanowires, depicted here. These nanowires are smaller in diameter than a human hair.
Credit: Megan Hill/Northwestern University

Twin Orbit operation successfully tested at BESSY II

2018/03/152018/03/15

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.

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

New technique simplifies creation of nanoparticle ‘magic-sized clusters’

2018/03/122018/03/21

One of the cool things about nanoparticles is also what makes them so difficult to work with: the fact that their properties are dependent on their size.

A critical challenge in translating nanomaterials from the laboratory into commercial applications, such as lighting or optical memory storage, is making a batch of nanoparticles all the same size. Two Cornell research groups have joined forces to lay out a solution for this issue.

Researchers in the labs of Richard Robinson and Tobias Hanrath – using X-ray analysis at the Cornell High Energy Synchrotron Source (CHESS) – have developed a new nanosynthetic pathway to achieve ultra-pure and highly stable groups of same-sized particles – known as “magic-sized clusters.”

Their paper, “Mesophase Formation Stabilizes High-Purity Magic-Sized Clusters,” published online Jan. 27 in the Journal of the American Chemical Society, and will be on a cover of the March 14 print edition. Lead authors are Curtis Williamson, doctoral student in both the Robinson and Hanrath groups, and Douglas Nevers, doctoral student in the Hanrath Group. Lena Kourkoutis, assistant professor of applied and engineering physics, also contributed.

Image: Schematic of the magic-sized clusters hexagonal mesophase. The mesophase (left) is an assembly of nanofibers (center), which are composed of magic-sized clusters (right).
Credit: Richard Robinson

Marianne Liebi winner of Swedish L’Oréal-Unesco For Women in Science 2018

2018/03/072018/03/15

L’Oréal-Unesco For Women in Science Prize is awarded in Sweden for the third time. The purpose of the prize is to pay attention to and reward young women who have shown great potential in science, while offering positive female role-models. Researchers Marianne Liebi, Chalmers, and Ruth Pöttgen, Lund University, get L’Oréal-Unesco For Women in Science Award, supported by Sweden’s young academy 2018.

Marianne Liebi gets the award “for the constructive use of advanced imaging methods for biomaterials with the aim of understanding the connection between molecular and mechanical properties”. Marianne Liebi uses powerful X-ray technology to study how, for example, the smallest building blocks, collagen fibrils, the bone tissue, look and are organised. The goal is to develop a mimicking, biomimetic material, where nature’s own design principles are imitated and applied to develop artificial bone and cartilage.
“It’s important to show that in research, it does not matter where you come from or who you are – what matters is passion and dedication. At best, this kind of award will not be needed in the future, it would be aimed at all young researchers. It would not matter who you were, says Marianne Liebi.

Photo: Researchers Ruth Pöttgen (left), Lund University, and Marianne Liebi (right), Chalmers, get L’Oréal-Unesco For Women in Science Award 2018, supported by Young Academy Sweden.
Credit: Emma Burendahl

Precise layer growth in a superlattice controls electron coupling and magnetism

2018/03/022018/03/12

Two-dimensional (2-D) crystalline films often exhibit interesting physical characteristics, such as unusual magnetic or electric properties. By layering together distinct crystalline thin films, a so-called “superlattice” is formed. Due to their close proximity, the distinct layers of a superlattice may significantly affect the properties of other layers. In this research, single 2-D layers of strontium iridium oxide were sandwiched between either one, two, or three layers of strontium titanium oxide to form three distinct superlattices. Researchers then used x-ray scattering at the U.S. Department of Energy’s Advanced Photon Source (APS) to probe the magnetic structure of each superlattice. The x-ray data revealed that the number of layers of the titanium-based material produced a dramatic difference in the magnetic behavior of the iridium-based layer. These findings are especially significant because the iridium compound is one of the perovskites, a class of materials known for their unique electric, magnetic, optical, and other properties that have proven useful in sensor and energy-related devices, and which are being intensively investigated for their application towards improved electronics and other technologies.

>Read more on the Advance Photon Source website
Image: Fig. 1. Illustration of superlattices. Panel (a) shows the Sr2IrO4 crystalline superlattice, with alternating layers of SrIrO3 and SrO. The SrIrO3 layers are perovskites, depicted as diamond-like shapes formed by six oxygen atoms; inside each diamond is a gold-colored iridium ion (cation), while pink strontium ions lay near the diamond’s ends. The SrIrO3 layers are separated by non-perovskite (inert) SrO layers, depicted as pink bars. Panel (b) shows the more-recently developed SrIrO3/SrTiO3 superlattice used for this research. Three distinct SrIrO3/SrTiO3 superlattices were created, having 1, 2, or 3 layers of inert SrTiO3 layers (colored green) separating the active SrIrO3 layers. Bold green boxes highlight the number of inert SrTiO3 layers in the three distinct lattices. While both SrIrO3 (gold diamonds) and SrTiO3 (green diamonds) are perovskites, the green-colored SrTiO3 layers buffer the active SrIrO3 layers. (The entire image is visible here)

The Molecular Scale Structure of Electrolyte-Metal Oxide Interface

2018/02/282018/03/08

Li-ion batteries (LIBs) are key components of portable electronic devices, as well as in electric vehicles, military and medical equipment, backup power supplies, and even grid storage. However, the energy storage capacity and rate capability of current LIBs is still too low to meet the increasing demand of key markets. For the latter, the properties of the electrolyte-electrode interface play a decisive role.

From a more general point of view, interfaces, or surfaces, are the outer boundary of any condensed matter. Due to the resulting symmetry breaking, the arrangement of atoms or molecules at the interface often varies significantly from that in the bulk. Studies of the molecular scale structural properties of liquids at interfaces are intriguing, as these give insights into the fundamental molecule–molecule and molecule–substrate interactions. Investigations have included layering of ionic liquids [1], layering of metallic [2] and non-metallic liquids [3], and the (potential-dependent) structure of water adsorbed on solid surfaces [4]. However, basic insights into how a non-aqueous electrolyte–salt solution organizes at a solid interface, in particular from experiments, is still missing [5]. In many technological applications, the atomic scale properties of interfaces govern the functionality of the system. A prominent example is the importance of the structure and molecular arrangement of the liquid at the functional solid–liquid interface in batteries. More specifically, in LIBs, the arrangement of the electrolyte molecules directly at the electrode interface, and the electric double layer (EDL) formation are expected to govern the interfacial ion transport during charge/discharge, as well as affect the origin and properties of the solid electrolyte interphase (SEI).

Image: (a) Fresnel-normalized XRR (symbols) of the sapphire/LiPF₆:EC:DMC and corresponding model fits (lines). (b) Fit-derived electron density profiles. (c) Comparison of the XRR- and MD-derived (blue) density profiles. The MD-derived profile is smeared by the XRR-derived roughness. All curves are spaced vertically for clarity. (d) Periodicity at the solid/liquid interface vs. LiPF₆ concentration. (e) Normalized correlation lengths. (f) Schematic illustration of the proposed origin induced increased layer spacing with increasing salt-concentration.

Liquid crystal molecules form nano rings

2018/02/062018/02/07

Quantised self-assembly enables design of materials with novel properties

At DESY’s X-ray source PETRA III, scientists have investigated an intriguing form of self-assembly in liquid crystals: When the liquid crystals are filled into cylindrical nanopores and heated, their molecules form ordered rings as they cool – a condition that otherwise does not naturally occur in the material. This behavior allows nanomaterials with new optical and electrical properties, as the team led by Patrick Huber from Hamburg University of Technology (TUHH) reports in the journal Physical Review Letters.

The scientists had studied a special form of liquid crystals that are composed of disc-shaped molecules called discotic liquid crystals. In these materials, the disk molecules can form high, electrically conductive pillars by themselves, stacking up like coins. The researchers filled discotic liquid crystals in nanopores in a silicate glass. The cylindrical pores had a diameter of only 17 nanometers (millionths of a millimeter) and a depth of 0.36 millimeters.

There, the liquid crystals were heated to around 100 degrees Celsius and then cooled slowly. The initially disorganised disk molecules formed concentric rings arranged like round curved columns. Starting from the edge of the pore, one ring after the other gradually formed with decreasing temperature until at about 70 degrees Celsius the entire cross section of the pore was filled with concentric rings. Upon reheating, the rings gradually disappeared again.

Image: Stepwise self-organisation of the cooling liquid crystals. (Extract, see the entire image here)
Credit: A. Zantop/M. Mazza/K. Sentker/P. Huber, Max-Planck Institut für Dynamik und Selbstorganisation/Technische Universität Hamburg; Quantized Self-Assembly of Discotic Rings in a Liquid Crystal Confined in Nanopores, Physical Review Letters, 2018; CC BY 4.

40-year controversy in solid-state physics resolved

2018/02/062018/02/07

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