Christian R. Ast

Atomic hole doping of graphene.

The application of graphene in nanoscale electronic devices requires the deliberate control of the density and character of its charge carriers. We show by angle-resolved photoemission spectroscopy that substantial hole doping in the conical band structure of epitaxial graphene monolayers can be achieved by the adsorption of bismuth, antimony, or gold. In the case of gold doping the Dirac point is shifted into the unoccupied states. Atomic doping presents excellent perspectives for large scale production.

Giant spin splitting through surface alloying.

Surface alloying is shown to produce electronic states with a very large spin-splitting. We discuss the long range ordered bismuth/silver(111) surface alloy where an energy bands separation of up to one eV is achieved. Such strong spin-splitting enables angular resolved photoemission spectroscopy to directly observe the region close to the band edge, where the density of states shows quasi-one dimensional behavior. The associated singularity in the local density of states has been measured by low temperature scanning tunneling spectroscopy. The implications of this new class of materials for potential spintronics applications as well as fundamental issues are discussed.

Dirac cone protected by non-symmorphic symmetry and three-dimensional Dirac line node in ZrSiS

Materials harbouring exotic quasiparticles, such as massless Dirac and Weyl fermions, have garnered much attention from physics and material science communities due to their exceptional physical properties such as ultra-high mobility and extremely large magnetoresistances. Here, we show that the highly stable, non-toxic and earth-abundant material, ZrSiS, has an electronic band structure that hosts several Dirac cones that form a Fermi surface with a diamond-shaped line of Dirac nodes. We also show that the square Si lattice in ZrSiS is an excellent template for realizing new types of two-dimensional Dirac cones recently predicted by Young and Kane. Finally, we find that the energy range of the linearly dispersed bands is as high as 2 eV above and below the Fermi level; much larger than of other known Dirac materials. This makes ZrSiS a very promising candidate to study Dirac electrons, as well as the properties of lines of Dirac nodes.

Reactive Chemical Doping of the Bi2Se3 Topological Insulator

Using angular resolved photoemission spectroscopy we studied the evolution of the surface electronic structure of the topological insulator Bi(2)Se(3) as a function of water vapor exposure. We find that a surface reaction with water induces a band bending, which shifts the Dirac point deep into the occupied states and creates quantum well states with a strong Rashba-type splitting. The surface is thus not chemically inert, but the topological state remains protected. The band bending is traced back to Se abstraction, leaving positively charged vacancies at the surface. Because of the presence of water vapor, a similar effect takes place when Bi(2)Se(3) crystals are left in vacuum or cleaved in air, which likely explains the aging effect observed in the Bi(2)Se(3) band structure.

Electronic decoupling of an epitaxial graphene monolayer by gold intercalation

The application of graphene in electronic devices requires large-scale epitaxial growth. The presence of the substrate, however, usually reduces the charge-carrier mobility considerably. We show that it is possible to decouple the partially sp(3)-hybridized first graphitic layer formed on the Si-terminated face of silicon carbide from the substrate by gold intercalation, leading to a completely sp(2)-hybridized graphene layer with improved electronic properties.

Silicon surface with giant spin splitting.

We demonstrate a giant Rashba-type spin splitting on a semiconducting substrate by means of a Bi-trimer adlayer on a Si(111) wafer. The in-plane inversion symmetry is broken inducing a giant spin splitting with a Rashba energy of about 140 meV, much larger than what has previously been reported for any semiconductor heterostructure. The separation of the electronic states is larger than their lifetime broadening, which has been directly observed with angular resolved photoemission spectroscopy. The experimental results are confirmed by relativistic first-principles calculations.

Evidence for superconductivity in Li-decorated monolayer graphene

Significance Although superconductivity is well-known in intercalated bulk graphite, the ultimate goal of inducing superconductivity in single-layer graphene has not yet been achieved. We have here developed an experiment that combines ultralow-temperature (5 K) and ultrahigh-vacuum (10−11 torr) sample preparation with high-resolution angle-resolved photoemission spectroscopy (ARPES). We show that decorating monolayer graphene with a layer of lithium atoms enhances the electron–phonon coupling to the point where a superconducting state can be stabilized at low temperature. Measurements of the size of the superconducting gap by ARPES suggest a Tc of about 5.9 K. This result constitutes the first observation, to our knowledge, of superconductivity in monolayer graphene. Given the massive scientific and technological interest in graphene, our findings will have significant cross-disciplinary impact. Monolayer graphene exhibits many spectacular electronic properties, with superconductivity being arguably the most notable exception. It was theoretically proposed that superconductivity might be induced by enhancing the electron–phonon coupling through the decoration of graphene with an alkali adatom superlattice [Profeta G, Calandra M, Mauri F (2012) Nat Phys 8(2):131–134]. Although experiments have shown an adatom-induced enhancement of the electron–phonon coupling, superconductivity has never been observed. Using angle-resolved photoemission spectroscopy (ARPES), we show that lithium deposited on graphene at low temperature strongly modifies the phonon density of states, leading to an enhancement of the electron–phonon coupling of up to λ≃0.58. On part of the graphene-derived π∗-band Fermi surface, we then observe the opening of a Δ≃0.9-meV temperature-dependent pairing gap. This result suggests for the first time, to our knowledge, that Li-decorated monolayer graphene is indeed superconducting, with Tc≃5.9 K.

Illuminating the dark corridor in graphene: Polarization dependence of angle-resolved photoemission spectroscopy on graphene

We have used s- and p-polarized synchrotron radiation to image the electronic structure of epitaxial graphene near the (K) over bar point by angle-resolved photoemission spectroscopy (ARPES). Part of the experimental Fermi surface is suppressed due to the interference of photoelectrons emitted from the two equivalent carbon atoms per unit cell of graphenes honeycomb lattice. Here we show that, by rotating the polarization vector, we are able to illuminate this dark corridor giving access to the complete experimental Fermi surface. Our measurements are supported by first-principles photoemission calculations, which reveal that the observed effect persists in the low-photon-energy regime.

A 10 mK scanning tunneling microscope operating in ultra high vacuum and high magnetic fields

We present design and performance of a scanning tunneling microscope (STM) that operates at temperatures down to 10 mK providing ultimate energy resolution on the atomic scale. The STM is attached to a dilution refrigerator with direct access to an ultra high vacuum chamber allowing in situ sample preparation. High magnetic fields of up to 14 T perpendicular and up to 0.5 T parallel to the sample surface can be applied. Temperature sensors mounted directly at the tip and sample position verified the base temperature within a small error margin. Using a superconducting Al tip and a metallic Cu(111) sample, we determined an effective temperature of 38 ± 1 mK from the thermal broadening observed in the tunneling spectra. This results in an upper limit for the energy resolution of ΔE = 3.5 kBT = 11.4 ± 0.3 μeV. The stability between tip and sample is 4 pm at a temperature of 15 mK as demonstrated by topography measurements on a Cu(111) surface.

Tunable Spin Gaps in a Quantum-Confined Geometry

We have studied the interplay of a giant spin-orbit splitting and of quantum confinement in artificial Bi-Ag-Si trilayer structures. Angle-resolved photoelectron spectroscopy reveals the formation of a complex spin-dependent gap structure, which can be tuned by varying the thickness of the Ag buffer layer. This provides a means to tailor the electronic structure at the Fermi energy, with potential applications for silicon-compatible spintronic devices.