ARPES – Page 2 – Lightsources.org

Functionalized graphdiyne nanowires

2018/02/262018/03/08

… on-surface synthesis and assessment of band structure, flexibility, and information storage potential

With their extraordinary mechanical and electronic properties carbon-based nanomaterials are central in 21^st century research and carry high hopes for future nanotechnology applications. Established sp²-hybridized scaffolds include carbon nanotubes (CNTs), graphene sheets, and graphene nanoribbons. Recently, the interest in carbon allotropes incorporating both sp²– and sp-hybridized atoms rose tremendously, especially for the most popular member, the so-called graphdiyne. According to theory, the related nanomaterials possess characteristics desirable for applications such as molecular electronics, energy storage, gas filtering and light harvesting. However, achieving the targeted materials with high quality remained challenging until now.
Here, we employed covalent on-surface synthesis on well-defined metal substrates under ultra-high vacuum (UHV) conditions to the homocoupling reaction of terminal alkyne compounds and fabricated the first functionalized graphdiyne (f-GDY) nanowires. Combining the substrate templating of the Ag(455) vicinal surface with specifically designed CN-functionalized precursors we achieved the controlled polymerization to atom-precise strands with their length reaching 40 nm. The left panel of Figure 1a depicts a scanning tunneling microscopy (STM) image of an area of the silver surface featuring two step edges where an example of such a f-GDY wire is lying at the lower side of the right step edge. The right panel displays a molecular model of the situation highlighting the structure of the nanowire adsorbed in the lower terrace (darker blue) consisting of covalently coupled monomers (red outline) with the CN moieties pointing towards the atoms of the upper terrace (brighter blue).

Figure: (extract) Synthesis and characterization of functionalized graphdiyne nanowires. a) STM topograph of a f-GDY polymer covering the left step edge. b) ARPES data: Before annealing a non-dispersing feature originates from the HOMO of the monomer. After annealing a dispersing features (blue) can be identified. c) Schematic representation of the deduced intrinsic band structure of the f-GDY nanowires. d) STM topograph of a strongly bent nanowire. e) Information storage thru conformational cis-trans switching of benzonitrile units. Full image here.

Bespoke beamline engineering: the Diamond Sample Manipulator

2018/02/122018/02/19

The Surface and Interfaces village brings together six beamlines with a range of techniques for investigating structural, magnetic and electronic properties of surfaces and interfaces. Many of those beamlines rely on a Sample Manipulator to hold samples securely in an X-ray beam less than a tenth of a millimetre across, whilst also enabling them to move and rotate around multiple axes and rotate around each axis. The differing requirements of each beamline mean that the basic design of the Sample Manipulator is customised for each one.

The I09 beamline, for example, is used for studying atomic structures and electronic properties across a wide variety of surfaces and material interfaces. The Sample Manipulator on I09 makes it possible to use X-ray techniques to study monolayer adsorption and surface reconstructions in a vacuum, crystalline and non-crystalline thin films, nano-particulates, large molecules and complex organic films and magnetism and magnetic thin-films.

Image: The Sample Manipulator in situ as seen through the vacuum window.
Credit: Diamond Light Source.

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

Questioning the universality of the charge density wave nature…

2018/01/312018/02/02

… in electron-doped cuprates

The first superconductor materials discovered offer no electrical resistance to a current only at extremely low temperatures (less than 30 K or −243.2°C). The discovery of materials that show superconductivity at much higher temperatures (up to 138 K or −135°C) are called high-temperature superconductors (HTSC). For the last 30 years, scientists have researched cuprate materials, which contain copper-oxide planes in their structures, for their high-temperature superconducting abilities. To understand the superconducting behavior in the cuprates, researchers have looked to correlations with the charge density wave (CDW), caused by the ordered quantum field of electrons in the material. It has been assumed that the CDW in a normal (non-superconducting) state is indicative of the electron behavior at the lower temperature superconducting state. A team of scientists from SLAC, Japan, and Michigan compared the traits of superconducting and non-superconducting cuprate materials in the normal state to test if the CDW is correlated to superconductivity.

>Read more on the SSRL website

Picture: explanation in detail to read in the full scientific highlight (SSRL website)