Doped epitaxial graphene close to the Lifshitz transition

Graphene, an spbonded sheet of carbon atoms, is still attracting lots of interest almost 15 years after its discovery. Angle-resolved photoemission spectroscopy (ARPES) is a uniquely powerful method to study the electronic structure of graphene and it has been used extensively to study the coupling of electrons to lattice vibrations (phonons) in doped graphene. This electron-phonon coupling (EPC) manifests as a so-called “kink” feature in the electronic band structure probed by ARPES. What is much less explored is the effect of EPC on the phonon structure. A very accurate probe of the phonons in graphene is Raman spectroscopy.
M.G. Hell and colleagues from Germany, Italy, Indonesia, and Japan combined ARPES (carried out at the BaDelPhbeamline – see Figure 1) with low energy electron diffraction (LEED) and Raman spectroscopy (carried out at the University of Cologne in Germany) in a clever way to fully understand the coupled electron-phonon system in alkali metal doped graphene. LEED revealed ordered (1×1), (2×2), and (sqrt3xsqrt3)R30°adsorbate patterns with increasing alkali metal deposition. The ARPES analysis yielded not only the carrier concentration but also the EPC coupling constant. Ultra-High Vacuum (UHV) Raman spectra carried out using identically prepared samples with the very same carrier concentrations provided the EPC induced changes in the phonon frequencies.

>Read more on the Elettra Sincrotrone Trieste website

Image:  Top: ARPES spectra along the Γ-K-M high symmetry direction of the hexagonal Brillouin zone for Cs doped graphene/Ir(111) with increasing Cs deposition. The Dirac energy ED and the observed LEED reconstruction are also indicated. Bottom: Corresponding Fermi surfaces at the indicated charge carrier concentration. 

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

The human behind the beamline

Happy Birthday, Felix Bloch – 23rd October 1905

Felix Bloch was born on this day (23rd October) in 1905 in Zürich, Switzerland. He got a Ph.D. in 1928 studying under Werner Heisenberg. In his thesis, he established the quantum theory of solids describing how electrons moved through crystalline materials using Bloch waves. The phenomena he described are observed today using the technique ARPES which is carried out at the Bloch beamline at MAX IV.

>Read more on the MAX IV Laboratory website

Image: Detail of a Max Bloch illustration. To discover the entire illustration click here.
Credit: Emelie Hilner.

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

Lattice Coupling conspires in the correlated cuprate high-Tc superconductivity

For the cuprate high temperature superconductivity (high-Tc) research over the past three decades, the biggest challenge is to identify the relevant low energy degrees of freedom that are critical to formulating the correct theoretical model for high-Tc superconductivity. The main difficulty lies in the closeness between various relevant energy scales. For low energy processes that are comparable to the superconducting gap energy ∆sc, there are the spin exchange energy J, the lattice vibration (phonon) energy Ωph, and the van Hove singularity energy E(π,0). However, anomalous isotope effects on Tc and superfluid density in the cuprates cannot be captured by traditional phonon-mediated superconductivity theories. Historically, a purely electronic Hamiltonian – the Hubbard model – was widely regarded to encapsulate all the core physics of the high-Tc phenomena.

In a recent paper published in Science, scientists from Stanford University and from Stanford Institute for Materials and Energy Sciences (SIMES), in collaboration with material scientists from Japan and theoreticians from Japan, the Netherlands, and Berkeley, reinstated the substantial role of the lattice vibration in the cuprate high-Tc superconductivity – however, in a subtle way that is highly intertwined with the electronic correlations. They finely straddled 18 differently hole-doped high-Tc compound Bi2Sr2CaCu2O8+δ within 8% change of hole carrier concentration, a doping range where Tc evolves from 47 K to 95 K through a putative quantum critical point, around which the electronic correlation effect experiences a sudden change. Then systematic experiments were carried out using the angle-resolved photoemission spectroscopy (ARPES) facility at SSRL Beam Line 5-4. Here, the high-resolution ARPES end station provided critical information of both the superconducting gap and the electron-lattice coupling.

>Read more on the Stanford Synchrotron Radiation Lightsource website

Image: Intertwined growth of the superconductivity and the electron-phonon coupling tuned by the hole concentration. The red line is an illustration of the Tc in Bi-2212 (Tcmax = 95 K). The blue shade and line represent the single-layer Bi-2201 system, where the coupling to the B1g mode is weak and T max is only 38 K. The yellow ball represents the optimally doped tri-layer Bi-2223 where Tcmax is 108 K. The top-right inset shows the intertwined relation between the pseudogap and the EPC under strong electronic correlation. The Madelung potential and the lattice stacking along the c-axis are schematically depicted for the single- layer, bi-layer and tri-layer systems. The dark grey blocks represent the CuO2n- plane, and the light grey blocks represent the charge reservoir layers (Ca2+, SrO, BiO+). The orange dots mark the CuO2n- planes that experience to the first order a non-zero out-of-plane electric field.
Credit: Science, doi: 10.1126/science.aar3394

The electronic structure of a “Kagome” material

Functionalized graphdiyne nanowires

… 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 21st 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 sp2and 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).

>Read more on the Elettra Sincrotrone website

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

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.

>Read more on the Diamond Light Source website

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

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

Questioning the universality of the charge density wave nature…

… 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)