ARPES – Lightsources.org

Enigmatic Dirac fermions in graphene

2022/08/102022/09/01

Since the discovery of graphene more than 15 years ago, research on graphene-based systems has grown exponentially. Graphene exhibits unique physical properties, for instance, the presence of massless Dirac fermions in a lattice of stronger covalent bonds and frequency-independent optical conductivity, which may help to realize exotic fundamental science and advanced technologies.

So far, graphene has been grown on a multitude of substrates exhibiting interesting properties. In some cases, the graphene layer has minimal link with the substrate. Experiments have revealed enigmatic properties of the Dirac fermions near the band crossing, called Dirac point, at the K point of the Brillouin zone. For example, Angle-Resolved PhotoEmission Spectroscopy (ARPES) data of graphene grown on SiC, shown in Fig. 1a, exhibit large momentum independent intensities near Dirac point as if the top and bottom of the Dirac cone are shifted away from each other. Some studies interpreted these results as a gapped Dirac cone with anomalous in-gap intensities as schematically shown in Fig. 1b. The presence of electron correlation renormalizes the dispersion as shown by red lines. Other proposals involve plasmaron bands where plasmon excitations in addition to photoexcitation of electrons leads to a shifted Dirac cone. The shifted and the pristine Dirac cones appear as a diamond shaped structure around the Dirac point as shown in Fig. 1c.

In order to address this enigmatic scenario, A. Pramanik, S. Thakur and colleagues from India, Italy and Germany performed a detailed polarization dependent ARPES investigation at the BaDElPh beamline at Elettra. Each branch of the Dirac cone was probed selectively using s– and p-polarized synchrotron light. The spectra shown in Fig. 2a,b reveal clearly dispersive bands near the Dirac point.

Read more on the Elettra website

Image: (a) Typical ARPES spectra of graphene on SiC along the ΓKM direction of the Brillouin zone; the origin of the momentum axis is shifted to K point. Schematic of (b) anomalous region and (c) plasmaron scenario around the Dirac point. Red curved lines in (b) show bands in the presence of electron correlation. Red Dirac cone in (c) is due to plasmaron bands.

Buckyballs on gold are less exotic than graphene

2022/07/212022/08/24

C₆₀ molecules on a gold substrate appear more complex than their graphene counterparts, but have much more ordinary electronic properties. This is now shown by measurements with ARPES at BESSY II and detailed calculations.

Graphene consists of carbon atoms that crosslink in a plane to form a flat honeycomb structure. In addition to surprisingly high mechanical stability, the material has exciting electronic properties: The electrons behave like massless particles, which can be clearly demonstrated in spectrometric experiments. Measurements reveal a linear dependence of energy on momentum, namely the so-called Dirac cones – two lines that cross without a band gap – i.e. an energy difference between electrons in the conduction band and those in the valence bands.

Variants in graphene architecture

Artificial variants of graphene architecture are a hot topic in materials research right now. Instead of carbon atoms, quantum dots of silicon have been placed, ultracold atoms have been trapped in the honeycomb lattice with strong laser fields, or carbon monoxide molecules have been pushed into place on a copper surface piece by piece with a scanning tunneling microscope, where they could impart the characteristic graphene properties to the electrons of the copper.

Artificial graphene with buckyballs?

A recent study suggested that it is infinitely easier to make artificial graphene using C₆₀ molecules called buckyballs. Only a uniform layer of these needs to be vapor-deposited onto gold for the gold electrons to take on the special graphene properties. Measurements of photoemission spectra appeared to show a kind of Dirac cone.

Analysis of band structures at BESSY II

“That would be really quite amazing,” says Dr. Andrei Varykhalov, of HZB, who heads a photoemission and scanning tunneling microscopy group. “Because the C₆₀ molecule is absolutely nonpolar, it was hard for us to imagine how such molecules would exert a strong influence on the electrons in the gold.” So Varykhalov and his team launched a series of measurements to test this hypothesis.

In tricky and detailed analyses, the Berlin team was able to study C₆₀ layers on gold over a much larger energy range and for different measurement parameters. They used angle-resolved ARPES spectroscopy at BESSY II, which enables particularly precise measurements, and also analysed electron spin for some measurements.

Read more on the HZB website

Image: Using density functional theory and measurement data from spin-resolved photoemission, the team investigated the origin of the repeating Au(111) bands and resolved them as deep surface resonances. These resonances lead to an onion-like Fermi surface of Au(111).

Credit: © HZB

Rich electronic features of a kagome superconductor

2022/04/262022/05/04

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 K 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

The fourth signature of the superconducting transition in cuprates

2022/01/262022/02/03

The results cap 15 years of detective work aimed at understanding how these materials transition into a superconducting state where they can conduct electricity with no loss.

When an exciting and unconventional new class of superconducting materials was discovered 35 years ago, researchers cheered.

Like other superconductors, these materials, known as copper oxides or cuprates, conducted electricity with no resistance or loss when chilled below a certain point – but at much higher temperatures than scientists had thought possible. This raised hopes of getting them to work at close to room temperature for perfectly efficient power lines and other uses.

Research quickly confirmed that they showed two more classic traits of the transition to a superconducting state: As superconductivity developed, the material expelled magnetic fields, so that a magnet placed on a chunk of the material would levitate above the surface. And its heat capacity – the amount of heat needed to raise their temperature by a given amount – showed a distinctive anomaly at the transition.

But despite decades of effort with a variety of experimental tools, the fourth signature, which can be seen only on a microscopic scale, remained elusive: the way electrons pair up and condense into a sort of electron soup as the material transitions from its normal state to a superconducting state.

Now a research team at the Department of Energy’s SLAC National Accelerator Laboratory and Stanford University has finally revealed that fourth signature with precise, high-resolution measurements made with angle-resolved photoemission spectroscopy, or ARPES, which uses light to eject electrons from the material. Measuring the energy and momentum of those ejected electrons reveals how the electrons inside the material behave.

In a paper published in Nature, the team confirmed that the cuprate material they studied, known as Bi2212, made the transition to a superconducting state in two distinct steps and at very different temperatures.

Read more on the SLAC website

Image: How can you tell if a material is a superconductor? Four classic signatures are illustrated here. Left to right: 1) It conducts electricity with no resistance when chilled below a certain temperature. 2) It expels magnetic fields, so a magnet placed on top of it will levitate. 3) Its heat capacity – the amount of heat needed to raise its temperature by a given amount – shows a distinctive anomaly as the material transitions to a superconducting state. 4) And at that same transition point, its electrons pair up and condense into a sort of electron soup that allows current to flow freely. Now experiments at SLAC and Stanford have captured this fourth signature in cuprates, which become superconducting at relatively high temperatures, and shown that it occurs in two distinct steps and at very different temperatures. Knowing how that happens in fine detail suggests a new and very practical direction for research into these enigmatic materials.

Credit: Greg Stewart, SLAC National Accelerator Laboratory

Disorder brings out quantum physical talents

2021/09/012021/09/07

Quantum effects are most noticeable at extremely low temperatures, which limits their usefulness for technical applications. Thin films of MnSb₂Te₄, however, show new talents due to a small excess of manganese. Apparently, the resulting disorder provides spectacular properties: The material proves to be a topological insulator and is ferromagnetic up to comparatively high temperatures of 50 Kelvin, measurements at BESSY II show. This makes this class of material suitable for quantum bits, but also for spintronics in general or applications in high-precision metrology.

Quantum effects such as the anomalous quantum Hall effect enable sensors of highest sensitivity, are the basis for spintronic components in future information technologies and also for qubits in quantum computers of the future. However, as a rule, the quantum effects relevant for this only show up clearly enough to make use of them at very low temperatures near absolute zero and in special material systems.

Read more on the HZB website

Image: The Dirac cone is typical for topological insulators and is practically unchanged on all 6 images (ARPES measurements at BESSY II). The blue arrow additionally shows the valence electrons in the volume. The synchrotron light probes both and can thus distinguish the Dirac cone at the surface (electrically conducting) from the three-dimensional volume (insulating).

Credit: © HZB

Unusual electronic properties taking shape

2021/06/132021/06/29

In a recent study, an international team led by researchers from The Pennsylvania State University in the US investigated the one-dimensional (1D) material tantalum selenide iodide (TaSe₄ )₂I. Its electronic properties had been theoretically predicted but not observed experimentally before the study conducted at the Bloch beamline. Evaporating iodine atoms turn out to drive unforeseen electronic changes.

Materials with unusual electronic properties such as charge density waves or topological states push the understanding of the fundamentals of quantum matter. They are also exciting candidates for the next generations of energy-efficient electronic and spintronic devices.

In the present study, the researchers found that the electronic properties of (TaSe₄ )₂I were different from the theoretical prediction. The band structure of a material can loosely be compared to a map of the material’s electronic properties. (TaSe₄ )₂I has something called Dirac bands, which is often found in this type of materials. The prediction said that the Dirac bands would split due to Weyl physics, which is not the case. The bands split with temperature, and the driver behind it is iodine atoms evaporating from the material’s surface.

Read more on the MAX IV website

Image: Surface charge induced Dirac band splitting in 1D material (TaSe₄ )₂I

Synthesis of mesoscale ordered 2D π-conjugated polymers with semiconducting properties

2020/05/222020/06/04

Two-dimensional materials can exhibit intriguing electronic properties that stem from their geometry. The best-known example is graphene’s Dirac cone that gives rise to massless electrons, which originates from the all-carbon hexagonal lattice. Two-dimensional conjugated polymers (2DCPs) can be considered as analogues of graphene, yet offering greater potential to design geometry and properties by carefully selecting their building blocks. Strikingly, 2DCPs on a kagome lattice (i.e. a trihexagonal tiling) can show both Dirac cones and flat bands, with highly-massive charge carriers.

Despite experimental efforts spanning more than a decade, the poor crystallinity of the synthesized polymers made the study of the electronic properties of 2DCPs a scientific niche reserved to theorists. A collaboration between the “Istituto di Struttura della Materia” of the Italian CNR three Canadian universities (INRS, McGill and Lakehead) realized the milestone of the synthesis of a long-range ordered 2D polymeric network, enabling the measurement of their Dirac cone and flat band features by angle-resolved photoelectron spectroscopy (ARPES). This achievement paves the way to study the intriguing electronic properties of this new class of materials, which make them promising for applications in future electronic and optoelectronic technologies.

Beyond graphene: monolayer arsenene observed for the first time

2020/02/272020/02/27

An article recently published in 2D Materials shows the first experimental evidence of the successful formation of arsenene, an analogue of graphene with noteworthy semiconducting properties.

This material shows a great potential for the development of new nanoelectronics. Crucial sample preparation and electron spectroscopy experiments were performed at the Bloch beamline at MAX IV.

The discovery of graphene, the single-layer carbon honeycomb material worth the Nobel Prize in Physics in 2010, surely has had a revolutionary impact on research. It triggered a whole new field of study within two-dimensional (2D) materials. However, its application in developing new 2D electronics has been hindered by its lack on an intrinsic band gap. Researchers therefore started to turn their attention to other elements in the periodic table and set their eyes on group V, to which arsenic belongs.
“The aim of the study was to show that arsenene can be formed. Our article is the first to report this”, says Roger Uhrberg, professor at Linköping University and spokesperson for the Bloch beamline at MAX IV. Arsenene, a single-layer buckled honeycomb structure of arsenic, had been previously predicted by various theoretical studies, but this is the first experimental observation that verifies its existence.

>Read more on the MAX IV website

Image: A view of the Bloch beamline at MAX IV. The Bloch beamline consists of two branchlines, and is dedicated to high resolution photoelectron spectroscopy, encompassing angle-resolved (ARPES), spin resolved (spin-ARPES) and core-level spectroscopy.

Visualizing electrostatic gating effects in two-dimensional heterostructures

2019/07/242019/08/08

Electronic and optoelectronic devices utilise electric fields to manipulate material properties, controlling band structures and band alignments across heterostructures that combine metals, semiconductors and insulators. With two-dimensional materials, 2D heterostructures (2DHS) can be fabricated with atomic precision by simply stacking layers. In these, applied out-of-plane electric fields are a powerful tool that can be used to degenerately dope semiconductors, modify electronic structure through the Stark effect, and alter band-alignments between layers. As a result, out-of-plane electric fields have been used to engineer functional architectures such as high-efficiency light-emitting diodes and tunnelling transistors, and to probe many-body phenomena.
Despite the fundamental importance of electric-field control over band structure, direct experimental measurements are challenging and have been limited. Whilst gate electrodes are routinely applied for electrical transport investigations, and many studies have reported electric-field dependent light-emission from 2DHS, these depend upon but do not directly reveal the single-particle electronic structure. Angle resolved photoemission spectroscopy (ARPES) has proven to be a powerful tool for probing the momentum-resolved valence band structure of 2D materials such as graphene and semiconducting transition metal dichalcogenides (MX₂). But it is challenging to apply conventional ARPES, which typically averages over lengthscales > 100 µm, to 2DHS which are usually only a few µm across. Using the high spatial resolution and flux of the Spectromicroscopy beamline at Elettra, we have shown that submicrometre spatially resolved ARPES (µARPES) can determine band parameters and band alignments across 2DHS of mechanically exfoliated flakes. These heterostructures are similar to those used for optical spectroscopy and transport measurements, opening the way to study operating devices.

>Read more on the Elettra website

Illustration: Direct momentum-resolved electronic structure measurements of in-operando microelectronic devices.

Progress on low energy electronics

2018/12/112018/12/12

Soft X-ray experiments used to characterise new thin film topological Dirac Semimetal

A large international collaboration including scientists from Monash University, the ARC Centre for Future Low Energy Electronics (FLEET), the Monash Centre for Anatomically Thin Materials and the Australian Synchrotron reported today in Nature on the development of an advanced material that is able to switch between an electrically conductive state to an insulating state, simply by applying an electric field.
The work represents a step towards the development of a new generation of ultra-low energy electronics at room temperature.
Co-author Dr Anton Tadich, a beamline scientist at the Soft X-ray beamline and Partner Investigator with FLEET, collaborated with investigators from Monash University, Singapore and Lawrence Berkeley National Lab on the use of photoemission techniques at the Australian Synchrotron X-ray Photoelectron Spectroscopy (XPS) and the Advanced Light Source in the US Angle Resolved Photoelectron Spectroscopy, (ARPES).
The chemical composition and growth mechanisms of thin films of the topological Dirac semi-metal sodium bismuthide Na₃Bi on a silicon substrate was investigated using XPS at the Australian Synchrotron’s Soft X-ray beamline.

Doped epitaxial graphene close to the Lifshitz transition

2018/11/142018/12/06

Graphene, an sp²bonded 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.

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 E_D and the observed LEED reconstruction are also indicated. Bottom: Corresponding Fermi surfaces at the indicated charge carrier concentration.

Graphene on the way to superconductivity

2018/11/102018/11/15

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.

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

2018/10/232018/10/24

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.

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

2018/10/152018/10/19

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.

Image: https://pubs.acs.org/doi/10.1021/acs.nanolett.8b01305

Lattice Coupling conspires in the correlated cuprate high-Tc superconductivity

2018/10/152018/10/25

For the cuprate high temperature superconductivity (high-T_c) 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-T_c 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 Ω_ph, and the van Hove singularity energy E_(π,0). However, anomalous isotope effects on T_c 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-T_c 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-T_c superconductivity – however, in a subtle way that is highly intertwined with the electronic correlations. They finely straddled 18 differently hole-doped high-T_c compound Bi₂Sr₂CaCu₂O_8+δ within 8% change of hole carrier concentration, a doping range where T_c 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.

Image: Intertwined growth of the superconductivity and the electron-phonon coupling tuned by the hole concentration. The red line is an illustration of the T_c in Bi-2212 (T_c^max = 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 T_c^max 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 CuO₂^n- plane, and the light grey blocks represent the charge reservoir layers (Ca²⁺, SrO, BiO⁺). The orange dots mark the CuO₂^n- 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

2018/06/152018/07/12

The greater understanding of kagome materials afforded by this work helps open up a new path toward goals such as ultralow-power electronic devices and quantum computing.

In a “kagome” material, the atoms in a layer form a lattice resembling a traditional Japanese basket-weaving pattern (kago means “basket” and me means “eye,” a reference to the large holes characteristic of this open weave). The quasi-hexagonal symmetry is reminiscent of the hexagonal lattice of graphene, a material well known for its unusual electronic properties.

Recent theoretical developments have suggested that, under some conditions, a kagome material could, like graphene, exhibit a wide range of novel physics. In these phenomena, certain electronic excitations (“massive Dirac fermions”) play a major role. Despite predictions, however, the presence of these electronic modes in kagome compounds has evaded direct experimental observation.

Image: Artistic interpretation of kagome crystals.
Images by F. Frankel/MIT; illustration overlays by C. Turner/MIT