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Spin-dependent scattering from magnetic impurities inside a superconductor gives rise to Yu-Shiba-Rusinov (YSR) states within the superconducting gap. They can be modeled by the largely equivalent Kondo or Anderson impurity models. The role of the magnetic and nonmagnetic properties of the impurity in relation to the coupling to the substrate is still under debate. Here, we use a scanning tunneling microscope to make a quantitative connection between the energy of a YSR state and the impurity-substrate hybridization. We corroborate the impurity substrate coupling as a key energy scale for surface derived YSR states using the Anderson impurity model. By combining experimental data from YSR state spectra and additional conductance measurements, we can determine on which side of the quantum phase transition the system resides. We thus provide a crucial step towards a more quantitative understanding of the crucial role of impurity substrate coupling utilizing the Anderson model. The physics of Yu-Shiba-Rusinov states which exist in the superconducting gap are a topical area of research and linked to exotic phenomena such as Majorana fermions. Here, the authors use scanning tunnelling microscopy to investigate the influence of impurities on a superconducting surface and the role impurity-substrate hybridisation has on such states.

A single spin in a Josephson junction can reverse the flow of the supercurrent by changing the sign of the superconducting phase difference across it. At mesoscopic length scales, these pi-junctions are employed in various applications, such as finding the pairing symmetry of the underlying superconductor, as well as quantum computing. At the atomic scale, the counterpart of a single spin in a superconducting tunnel junction is known as a Yu-Shiba-Rusinov state. Observation of the supercurrent reversal in that setting has so far remained elusive. Here we demonstrate such a 0 to pi transition of a Josephson junction through a Yu-Shiba-Rusinov state as we continuously change the impurity-superconductor coupling. We detect the sign change in the critical current by exploiting a second transport channel as reference in analogy to a superconducting quantum interference device, which provides our scanning tunnelling microscope with the required phase sensitivity. The measured change in the Josephson current is a signature of the quantum phase transition and allows its characterization with high resolution., Continuously changing the coupling between a magnetic impurity and a superconductor allows the observation of the reversal of supercurrent flow at the atomic scale.

In the field of Dirac materials, spin-orbit coupling (SOC) is usually considered disruptive, since it may lift degeneracies that are not protected by high-symmetry elements. Nonsymmorphic symmetries force degenerate points in the band structure at high-symmetry points that are not disrupted by SOC. The degeneracy is, however, often protected along whole high-symmetry lines or faces resulting in highly anisotropic crossings or nodal lines, which can considerably limit the region, in which the bands are linearly dispersed. It has been theoretically suggested that SOC could circumvent this problem. Here, we show experimentally that SOC can lift the extended protection in nonsymmorphic square-net compounds. We compare ZrSiS and CeSbTe , two materials with drastically different SOC, to show the effect of SOC on the band structure by means of angle-resolved photoemission spectroscopy and density functional theory calculations.

Very recent experiments have reported the tunneling between Yu-Shiba-Rusinov (YSR) bound states at the atomic scale. These experiments have been realized with the help of a scanning tunneling microscope where a superconducting tip is functionalized with a magnetic impurity and is used to probe another magnetic impurity deposited on a superconducting substrate. In this way it has become possible to study for the first time the spin-dependent transport between individual superconducting bound states. Motivated by these experiments, we present here a comprehensive theoretical study of the tunneling processes between YSR bound states in a system in which two magnetic impurities are coupled to superconducting leads. Our theory is based on a combination of an Anderson model with broken spin degeneracy to describe the impurities and nonequilibrium Green's function techniques to compute the current-voltage characteristics. This combination allows us to describe the spin-dependent transport for an arbitrary strength of the tunnel coupling between the impurities. We first focus on the tunnel regime and show that our theory naturally explains the experimental observations of the appearance of current peaks in the subgap region due to both the direct and thermal tunneling between the YSR states in both impurities. Then, we study in detail the case of junctions with increasing transparency, which has not been experimentally explored yet, and predict the occurrence of a large variety of (multiple) Andreev reflections mediated by YSR states that give rise to a very rich structure in the subgap current. In particular, we predict the occurrence of multiple Andreev reflections that involve YSR states in different impurities. These processes have no analogue in single-impurity junctions and they are manifested as current peaks with negative differential conductance for subgap voltages., 16 pages, 9 figures. arXiv admin note: text overlap with arXiv:2005.06491

The wavefunctions of a disordered two-dimensional electron gas at the quantum-critical Anderson transition are predicted to exhibit multifractal scaling in their real space amplitude. We experimentally investigate the appearance of these characteristics in the spatially resolved local density of states of a two-dimensional mixed surface alloy Bi_xPb_{1-x}/Ag(111), by combining high-resolution scanning tunneling microscopy with spin and angle-resolved inverse-photoemission experiments. Our detailed knowledge of the surface alloy electronic band structure, the exact lattice structure and the atomically resolved local density of states enables us to construct a realistic Anderson tight binding model of the mixed surface alloy, and to directly compare the measured local density of states characteristics with those from our model calculations. The statistical analyses of these two-dimensional local density of states maps reveal their log-normal distributions and multifractal scaling characteristics of the underlying wavefunctions with a finite anomalous scaling exponent. Finally, our experimental results confirm theoretical predictions of an exact scaling symmetry for Anderson quantum phase transitions in the Wigner-Dyson classes., Comment: 10 pages, 7 figures

Despite plenty of room at the bottom, there is a limit to the miniaturization of every process. For charge transport this is realized by the coupling of single discrete energy levels at the atomic scale. Here, we demonstrate sequential tunneling between parity protected Yu-Shiba-Rusinov (YSR) states bound to magnetic impurities located on the superconducting tip and sample of a scanning tunneling microscope at 10 mK. We reduce the relaxation of the excited YSR state to the bare minimum and find an enhanced lifetime for single quasiparticle levels. Our work offers a way to characterize and to manipulate coupled superconducting bound states, such as Andreev levels, YSR states, or Majorana bound states., 14 pages, 10 figures incl. supplementary information

The following article appeared in Physical Review B 101.13 (2020): 134507. DOI:10.1103/PhysRevB.101.134507. Open access publication funded by the Max Planck Society, As scanning tunneling microscopy is pushed towards fast local dynamics, a quantitative understanding of tunnel junctions under the influence of a fast ac driving signal is required, especially at the ultralow temperatures relevant to spin dynamics and correlated electron states. We subject a superconductor-insulator-superconductor junction to a microwave signal from an antenna mounted in situ and examine the dc response of the contact to this driving signal. Quasiparticle tunneling and the Josephson effect can be interpreted in the framework of Tien-Gordon theory. The situation is more complex when it comes to higher-order effects such as multiple Andreev reflections. Microwave-assisted tunneling unravels these complex processes, providing deeper insights into tunneling than are available in a pure dc measurement, J.C.C. acknowledges funding from the Spanish Ministry of Economy and Competitiveness (MINECO) (Contract No. FIS2017-84057-P). J.A. acknowledges support from the IQST and the German Science Foundation (DFG) under Grant No. AN336/11-1. This work was funded in part by the ERC Consolidator Grant AbsoluteSpin (Grant No. 681164)

We introduce a van der Waals material that exhibits a very high electronic mobility and antiferromagnetism and can be exfoliated., Van der Waals (vdW) materials with magnetic order have been heavily pursued for fundamental physics as well as for device design. Despite the rapid advances, so far, they are mainly insulating or semiconducting, and none of them has a high electronic mobility—a property that is rare in layered vdW materials in general. The realization of a high-mobility vdW material that also exhibits magnetic order would open the possibility for novel magnetic twistronic or spintronic devices. Here, we report very high carrier mobility in the layered vdW antiferromagnet GdTe3. The electron mobility is beyond 60,000 cm2 V−1 s−1, which is the highest among all known layered magnetic materials, to the best of our knowledge. Among all known vdW materials, the mobility of bulk GdTe3 is comparable to that of black phosphorus. By mechanical exfoliation, we further demonstrate that GdTe3 can be exfoliated to ultrathin flakes of three monolayers.

Transition-metal chalcogenides are a promising family of materials for applications as photocathodes in photoelectrochemical (PEC) H2 generation. A long-standing challenge for chalcopyrite semiconductors is characterizing their electronic structure, both experimentally and theoretically, because of their relatively high-energy band gaps and spin-orbit coupling (SOC), respectively. In this work, we present single crystals of CuInTe2, whose relatively small optically measured band gap of 0.9 ± 0.03 eV enables electronic structure characterization by angle-resolved photoelectron spectroscopy (ARPES) in conjunction with first-principles calculations incorporating SOC. ARPES measurements reveal bands that are steeply dispersed in energy with a band velocity of 2.5-5.4 × 105 m/s, almost 50% of the extremely conductive material graphene. Additionally, CuInTe2 single crystals are fabricated into electrodes to experimentally determine the valence band edge energy and confirm the thermodynamic suitability of CuInTe2 for water redox chemistry. The electronic structure characterization and band edge position presented in this work provide kinetic and thermodynamic factors that support CuInTe2 as a strong candidate for water reduction.

Zusammenfassung Teil 2 des Zweiteilers stellt vor, was beim Stromfluss in einem Rastertunnelmikroskop (RTM) bei sehr tiefen Temperaturen im Millikelvin-Bereich passiert. Dabei zeigt sich der Einfluss der Ladungsquantisierung als Konsequenz der Quantenelektrodynamik. Der Tunnelstrom wird zu einem granularen Medium. Die Umgebung beeinflusst dabei zunehmend das Tunneln zwischen RTM-Spitze und Probe. Sind beide im normalleitenden Zustand, macht sich dies in einer dynamischen Coulomb-Blockade der fliesenden Elektronen bemerkbar. Bei Supraleitung, wenn Spitze und Probe einen Josephson-Kontakt formen, haben inelastische Wechselwirkungen mit dem Umfeld Einfluss auf das Verhalten der Cooper-Paare. Das Experiment wird auf eindrucksvolle Weise selbst zum Teil des Experiments.

We study the electronic structure of the nodal line semimetal ZrSiTe both experimentally and theoretically. We find two different surface states in ZrSiTe—topological drumhead surface states and trivial floating band surface states, which can be easily distinguished in ARPES experiments. Using the spectra of Wilson loops, we show that a nontrivial Berry phase that exists in a confined region within the Brillouin zone gives rise to the topological drumhead-type surface states. The Z_{2} structure of the Berry phase induces a Z_{2} “modular arithmetic” of the surface states, allowing surface states derived from different nodal lines to hybridize and gap out, which can be probed by a set of Wilson loops. Our findings are confirmed by ab initio calculations and angle-resolved photoemission experiments, which are in excellent agreement with each other and the topological analysis. This work is the first complete characterization of topological surface states in the family of square-net-based nodal line semimetals, and thus it fundamentally increases the understanding of the topological nature of this growing class of topological semimetals.

We study the band dispersion of graphene with randomly distributed structural defects using two complementary methods, exact diagonalization of the tight-binding Hamiltonian and implementing a self-consistent T matrix approximation. We identify three distinct types of impurities resulting in qualitatively different spectra in the vicinity of the Dirac point. First, resonant impurities, such as vacancies or 585 defects, lead to stretching of the spectrum at the Dirac point with a finite density of localized states. This type of spectrum has been observed in epitaxial graphene by photoemission spectroscopy and discussed extensively in the literature. Second, nonresonant (weak) impurities, such as paired vacancies or Stone-Wales defects, do not stretch the spectrum but provide a line broadening that increases with energy. Finally, disorder that breaks sublattice symmetry, such as vacancies placed in only one sublattice, open a gap around the Dirac point and create an impurity band in the middle of this gap. We find good agreement between the results of the two methods and also with the experimentally measured spectra., 11 pages, 5 figures, including supplementary information

New developments in the field of topological matter are often driven by materials discovery, including novel topological insulators, Dirac semimetals and Weyl semimetals. In the last few years, large efforts have been performed to classify all known inorganic materials with respect to their topology. Unfortunately, a large number of topological materials suffer from non-ideal band structures. For example, topological bands are frequently convoluted with trivial ones, and band structure features of interest can appear far below the Fermi level. This leaves just a handful of materials that are intensively studied. Finding strategies to design new topological materials is a solution. Here we introduce a new mechanism that is based on charge density waves and non-symmorphic symmetry to design an idealized Dirac semimetal. We then show experimentally that the antiferromagnetic compound GdSb$_{0.46}$Te$_{1.48}$ is a nearly ideal Dirac semimetal based on the proposed mechanism, meaning that most interfering bands at the Fermi level are suppressed. Its highly unusual transport behavior points to a thus far unknown regime, in which Dirac carriers with Fermi energy very close to the node seem to gradually localize in the presence of lattice and magnetic disorder.

Robust quantum systems rely on having a protective environment with minimized relaxation channels. Superconducting gaps play an important role in the design of such environments. The interaction of localized single spins with a conventional superconductor generally leads to intrinsically extremely narrow Yu-Shiba-Rusinov (YSR) resonances protected inside the superconducting gap. However, this may not apply to superconductors with nontrivial, energy dependent order parameters. Exploiting the Fe-doped two-band superconductor NbSe$_2$, we show that due to the nontrivial relation between its complex valued and energy dependent order parameters, YSR states are no longer restricted to be inside the gap. They can appear outside the gap (i. e. inside the coherence peaks), where they can also acquire a substantial intrinsic lifetime broadening. T-matrix scattering calculations show excellent agreement with the experimental data and relate the intrinsic YSR state broadening to the imaginary part of the host's order parameters. Our results suggest that non-thermal relaxation mechanisms contribute to the finite lifetime of the YSR states, even within the superconducting gap, making them less protected against residual interactions than previously assumed. YSR states may serve as valuable probes for nontrivial order parameters promoting a judicious selection of protective superconductors., Comment: 11 pages, 8 figures, including supporting information

By establishing magnetic order in a square lattice compound, we introduce the first magnetic “new fermion.”, Recent interest in topological semimetals has led to the proposal of many new topological phases that can be realized in real materials. Next to Dirac and Weyl systems, these include more exotic phases based on manifold band degeneracies in the bulk electronic structure. The exotic states in topological semimetals are usually protected by some sort of crystal symmetry, and the introduction of magnetic order can influence these states by breaking time-reversal symmetry. We show that we can realize a rich variety of different topological semimetal states in a single material, CeSbTe. This compound can exhibit different types of magnetic order that can be accessed easily by applying a small field. Therefore, it allows for tuning the electronic structure and can drive it through a manifold of topologically distinct phases, such as the first nonsymmorphic magnetic topological phase with an eightfold band crossing at a high-symmetry point. Our experimental results are backed by a full magnetic group theory analysis and ab initio calculations. This discovery introduces a realistic and promising platform for studying the interplay of magnetism and topology. We also show that we can generally expand the numbers of space groups that allow for high-order band degeneracies by introducing antiferromagnetic order.

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 et al. Nat. Phys. 8, 131-134 (2012)]. While experiments have indeed demonstrated 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 $\lambda\!\simeq\!0.58$. On part of the graphene-derived $\pi^*$-band Fermi surface, we then observe the opening of a $\Delta\!\simeq\!0.9$ meV temperature-dependent pairing gap. This result suggests for the first time, to our knowledge, that Li-decorated monolayer graphene is indeed superconducting with $T_c\!\simeq\!5.9 K$., Comment: Accepted. A high-resolution version with supplementary material can be found at http://qmlab.ubc.ca/ARPES/PUBLICATIONS/Articles/Graphene_Li.pdf

The Rashba-type spin-splitting found in many two-dimensional electron systems at surfaces is a band splitting in momentum, which is most easily extracted from angular resolved photoemission data. Scanning tunneling microscopy as a real-space resolving technique relies on quasiparticle interference to extract momentum information about the experimental electronic structure. However, the lifted spin degeneracy in the Rashba split bands imposes a selection rule that makes it impossible to extract the Rashba splitting from single scattering events, e.g. scattering from a point defect in STM data. Nevertheless, going beyond single scattering events, the Rashba-type spin splitting can be extracted from STM data, which will be discussed in this review.

Tuning the electronic properties of graphene by adatom deposition unavoidably introduces disorder into the system, which directly affects the single-particle excitations and electrodynamics. Using angle-resolved photoemission spectroscopy (ARPES) we trace the evolution of disorder in graphene by thallium adatom deposition and probe its effect on the electronic structure. We show that the signatures of quasiparticle scattering in the photoemission spectral function can be used to identify thallium adatoms, although charged, as efficient short-range scattering centers. Employing a self-energy model for short-range scattering, we are able to extract a δ-like scattering potential δ = -3.2 ± 1 eV. Therefore, isolated charged scattering centers do not necessarily act just as good long-range (Coulomb) scatterers but can also act as efficient short-range (δ-like) scatterers; in the case of thallium, this happens with almost equal contributions from both mechanisms.

A promising route to the realization of Majorana fermions is in non-centrosymmetric superconductors, in which spin-orbit-coupling lifts the spin degeneracy of both bulk and surface bands. A detailed assessment of the electronic structure is critical to evaluate their suitability for this through establishing the topological properties of the electronic structure. This requires correct identification of the time-reversal-invariant momenta. One such material is BiPd, a recently rediscovered non-centrosymmetric superconductor which can be grown in large, high-quality single crystals and has been studied by several groups using angular resolved photoemission to establish its surface electronic structure. Many of the published electronic structure studies on this material are based on a reciprocal unit cell which is not the actual Brillouin zone of the material. We show here the consequences of this for the electronic structures and show how the inferred topological nature of the material is affected., Comment: Originally a comment on 1505.03466/1610.03431 and 1609.08947. 4 pages plus an appendix with literature crystal structure refinements

We present the design and calibration measurements of a scanning tunneling microscope setup in a 3He ultrahigh-vacuum cryostat operating at 400 mK with a hold time of 10 days. With 2.70 m in height and 4.70 m free space needed for assembly, the cryostat fits in a one-story lab building. The microscope features optical access, an xy table, in situ tip and sample exchange, and enough contacts to facilitate atomic force microscopy in tuning fork operation and simultaneous magneto-transport measurements on the sample. Hence, it enables scanning tunneling spectroscopy on microstructured samples which are tuned into preselected transport regimes. A superconducting magnet provides a perpendicular field of up to 14 T. The vertical noise of the scanning tunneling microscope amounts to 1 pmrms within a 700 Hz bandwidth. Tunneling spectroscopy using one superconducting electrode revealed an energy resolution of 120 mueV. Data on tip-sample Josephson contacts yield an even smaller feature size of 60 mueV, implying that the system operates close to the physical noise limit., 12 pages, 11 figures

Probing absolute values of spin polarization at the nanoscale offers insight into the fundamental mechanisms of spin-dependent transport. Employing the Zeeman splitting in superconducting tips (Meservey-Tedrow-Fulde effect), we introduce a novel spin-polarized scanning tunneling microscopy that combines the probing capability of the absolute values of spin polarization with precise control at the atomic scale. We utilize our novel approach to measure the locally resolved spin polarization of magnetic Co nanoislands on Cu(111). We find that the spin polarization is enhanced by 65% when increasing the width of the tunnel barrier by only 2.3 angstrom due to the different decay of the electron orbitals into vacuum.

The properties of bismuth have long defied expectation, casting it just outside the bounds of almost every category. Now topology joins the list, as its electronic structure once deemed trivial turns out to have higher-order topology.

Layered heavy-metal square-lattice compounds have recently emerged as potential Dirac fermion materials due to bonding within those sublattices. We report quantum transport and spectroscopic data on the layered Sb square-lattice material LaCuSb$_{2}$. Linearly dispersing band crossings, necessary to generate Dirac fermions, are experimentally observed in the electronic band structure observed using angle-resolved photoemission spectroscopy (ARPES), along with a quasi-two-dimensional Fermi surface. Weak antilocalization that arises from two-dimensional transport is observed in the magnetoresistance, as well as regions of linear dependence, both of which are indicative of topologically non-trivial effects. Measurements of the Shubnikov-de Haas (SdH) quantum oscillations show low effective mass electrons on the order of 0.065$m_{e}$, further confirming the presence of Dirac fermions in this material.

We have studied the temporal evolution of a quantum system subjected to strong dissipation at ultra-low temperatures where the system-bath interaction represents the leading energy scale. In this regime, theory predicts the time evolution of the system to follow a generalization of the classical Smoluchowski description, the quantum Smoluchowski equation, thus, exhibiting quantum Brownian motion characteristics. For this purpose, we have investigated the phase dynamics of a superconducting tunnel junction in the presence of high damping. We performed current-biased measurements on the small-capacitance Josephson junction of a scanning tunneling microscope placed in a low impedance environment at milli-Kelvin temperatures. We can describe our experimental findings by a quantum diffusion model with high accuracy in agreement with theoretical predications based on the quantum Smoluchowski equation. In this way we experimentally demonstrate that quantum systems subjected to strong dissipation follow quasi-classical dynamics with significant quantum effects as the leading corrections., 5 pages, 4 figures

In this work, we present a model of the surface states of nonsymmorphic semimetals. These are derived from surface mass terms that lift the high degeneracy imposed in the band structure by the nonsymmorphic bulk symmetries. Reflecting the reduced symmetry at the surface, the bulk bands are strongly modified. This leads to the creation of two-dimensional floating bands, which are distinct from Shockley states, quantum well states or topologically protected surface states. We focus on the layered semimetal ZrSiS to clarify the origin of its surface states. We demonstrate an excellent agreement between DFT calculations and ARPES measurements and present an effective four-band model in which similar surface bands appear. Finally, we emphasize the role of the surface chemical potential by comparing the surface density of states in samples with and without potassium coating. Our findings can be extended to related compounds and generalized to other crystals with nonsymmorphic symmetries., Comment: 8 pages, 5 figures

Materials with strong spin-orbit coupling (SOC) have in recent years become a subject of intense research due to their potential applications in spintronics and quantum information technology. In particular, in systems which break inversion symmetry, SOC facilitates the Rashba-Dresselhaus effect, leading to a lifting of spin degeneracy in the bulk and intricate spin textures of the Bloch wave functions. Here, by combining angular resolved photoemission (ARPES) and low temperature scanning tunneling microscopy (STM) measurements with relativistic first-principles band structure calculations, we examine the role of SOC in single crystals of noncentrosymmetric BiPd. We report the detection of several Dirac surface states, one of which exhibits an extremely large spin splitting. Unlike the surface states in inversion-symmetric systems, the Dirac surface states of BiPd have completely different properties at opposite faces of the crystal and are not trivially linked by symmetry. The spin-splitting of the surface states exhibits a strong anisotropy by itself, which can be linked to the low in-plane symmetry of the surface termination., Comment: 6 page letter, + supplementary material

The interrelation between spin and charge in semiconductors leads to interesting effects, e.g., the Rashba-type spin–orbit splitting or the exchange enhancement. These properties are proposed to be used in applications such as spin transistors or spin qubits. Probing them on the local scale with the ultimate spatial resolution of the scanning tunneling microscope addresses their susceptibility to disorder directly. Here we review the results obtained on two-dimensional electron systems (2DESs) in semiconductors. We describe the preparation and characterization of an adequate 2DES which can be probed by scanning tunneling microscopy. It is shown how the electron density and the disorder within the 2DES can be tuned and measured. The observed local density of states of weakly and strongly disordered systems is discussed in detail. It is shown that the weakly disordered 2DES exhibits quantum Hall effect in magnetic field. The corresponding local density of states across a quantum Hall transition is mapped showing the development from localized states to extended states and back to localized states in real space. Decoupling the 2DES from screening electrons of the bulk of the III–V semiconductor leads to a measurable exchange enhancement of up to 0.7 meV which depends on the local spin polarization of the 2DES. At stronger confinement potential, i.e. larger doping, the Rashba spin splitting with α as large as 7×10 −11 eV m is observed as a beating in the density of states in magnetic field. The Rashba spin splitting varies with position by about ±50% being largest at potential hills.

The Dirac-like electronic structure of graphene originates from the equivalence of the two basis atoms in the honeycomb lattice. We show that the characteristic parameters of the initial state wave function (sublattice symmetry and isospin) can be determined using angle-resolved photoemission spectroscopy (ARPES) with circularly polarized synchrotron radiation. At a photon energy of hν = 52 eV, transition matrix element effects can be neglected allowing us to determine sublattice symmetry and isospin with high accuracy using a simple theoretical model.

We present Angle-Resolved Photoelectron Spectroscopy (ARPES) data on spin-orbit split states in two XAg2 surface alloys grown on an Ag(111) substrate, and of the Si(111)-AgBiAg2 trilayer system. We briefly discuss the origin of the unusually large energy and momentum splitting, and the possibility of tuning by surface engineering the spin polarization at the Fermi level. [DOI: 10.1380/ejssnt.2009.264]

Graphene is an excellent candidate for the next generation of electronic materials due to the strict two-dimensionality of its electronic structure as well as the extremely high carrier mobility. A prerequisite for the development of graphene based electronics is the reliable control of the type and density of the charge carriers by external (gate) and internal (doping) means. While gating has been successfully demonstrated for graphene flakes and epitaxial graphene on silicon carbide, the development of reliable chemical doping methods turns out to be a real challenge. In particular hole doping is an unsolved issue. So far it has only been achieved with reactive molecular adsorbates, which are largely incompatible with any device technology. Here we show by angle-resolved photoemission spectroscopy that atomic doping of an epitaxial graphene layer on a silicon carbide substrate with bismuth, antimony or gold presents effective means of p-type doping. Not only is the atomic doping the method of choice for the internal control of the carrier density. In combination with the intrinsic n-type character of epitaxial graphene on SiC, the charge carriers can be tuned from electrons to holes, without affecting the conical band structure.

Superconductivity is commonly described as a macroscopic quantum phenomenon. However, it arises from microscopic mechanisms occurring at the nanometer scale as illustrated, for example, by the non-trivial pairing in unconventional superconductors. More recently, also local interactions with superconductors in the context of Majorana fermions became of interest. A very direct way to study the atomic scale properties of superconductors is given by the combination of the Josephson effect with scanning tunneling microscopy (STM), also referred to as JSTM. Here, the critical Josephson current serves as a direct local probe of the superconducting ground state and may reveal valuable information that is often inaccessible when studying quasi-particle excitation spectra. We show that we can extract local values of the critical Josephson current from JSTM measurements in the dynamical Coulomb blockade regime. Furthermore, we experimentally determine the regime of sequential Cooper pair tunneling, which is in accordance to theoretical predictions. Our study presents new insights on the tunneling mechanisms in Josephson junctions and lays the basis for the implementation of JSTM as a versatile probe for superconductivity.

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., The family of topological materials has been growing rapidly but most members bare limitations hindering the study of exotic behaviour of topological particles. Here, Schoop et al. report a Fermi surface with a diamond-shaped line of Dirac nodes in ZrSiS, providing a promising candidate for studying two-dimensional Dirac fermions.

Direct and inverse Auger scattering are amongst the primary processes that mediate the thermalization of hot carriers in semiconductors. These two processes involve the annihilation or generation of an electron-hole pair by exchanging energy with a third carrier, which is either accelerated or decelerated. Inverse Auger scattering is generally suppressed, as the decelerated carriers must have excess energies higher than the band gap itself. In graphene, which is gapless, inverse Auger scattering is instead predicted to be dominant at the earliest time delays. Here, $, 16 pages, 8 figures

Theoretical and experimental studies agree that Bi1-xSbx (0.07

The peak width in angle-resolved photoemission spectra from 3D valence bands in Bi(1 1 1) has been analyzed as a function of final-state energy. Over the photon energy range of 9–100 eV valence band transitions display pronounced oscillations with minima at the Γ and T points. The maxima in-between these points are additionally modulated through transitions into multiple final-state bands. Model calculations show that the 3D linewidth is a function of the initial- and final-state widths as well as the respective band velocities v ⊥ = ℏ − 1 ∂ E / ∂ k ⊥ . Extending the model calculations past the commonly used linear approximation by using experimentally determined dispersion relations E ( k ) of initial- and final-state bands significantly improves the linewidth changes in regions near the symmetry points.

Angular resolved photoemission spectroscopy is used to map the Fermi surface of Sb(1 1 1). In the region close to the T-point, we find multiple Fermi surface features. Utilizing the tuneability of Synchrotron radiation shows that these structures are not multiply connected, but can be associated with the bulk hole-Fermi surface and electron pockets originating from two 2D-bands crossing the Fermi level at K F ∼0.1 and ∼0.05 A −1 . The energy dispersion of the latter band follows that of a rotational paraboloid with a circular Fermi surface. The cross section of the bulk hole-pocket in the mirror plane is compared with theoretical predictions.

Angle-resolved photoemission spectra of the Sb 4d and Bi 5d core levels show spectral features split-off in energy by Δ E ∼170–320 meV from the main peaks. The appearance of a second component in the Bi 5d spectrum was previously assigned to a surface related binding energy shift. However, use of synchrotron radiation ranging from 30 to 150 eV and angular scans along different symmetry directions of the (1 1 1) surface Brillouin zone show that the split-off peaks disperse as a function of the electron momentum k . Additionally, the peak width and intensity of the split-off peak oscillates between final state symmetry points. Since the d 5/2 and d 3/2 components show the same features symmetry arguments rule out crystal-field effects as the source of the observed peak splitting. The data can be explained by a k -dependent loss function consisting of several characteristic direct and indirect low energy interband transitions excited in various parts of the bulk Brillouin zone.

The spin texture of the unoccupied bands of the surface alloy Bi/Ag(111) is investigated with spin- and angle-resolved inverse photoemission and first-principles calculations. Surprisingly, the measured spin character does not always reflect the calculated spin texture of the bands. With the help of photoemission calculations within the one-step model, however, the discrepancy is traced back to the influence of the orbital symmetry of the respective states in combination with the experimental geometry. In particular, the calculations show that the spin texture of a surface band with mixed orbital symmetries may neither be recovered with s- nor p- nor unpolarized light. In general, spin information from direct or inverse photoemission experiments on electronic states with mixed orbital symmetries at spin-orbit-influenced surfaces has to be taken with a pinch of salt, while it remains reliable for states with pure symmetry.

We study the band structure of the Bi2Se3 topological insulator (1 1 1) surface using angle-resolved photoemission spectroscopy. We examine the situation where two sets of quantized subbands exhibiting different Rashba spin splitting are created via bending of the conduction (CB) and the valence (VB) bands at the surface. While the CB subbands are strongly Rashba spin split, the VB subbands do not exhibit clear spin splitting. We find that CB and VB experience similar band bending magnitudes, which means, a spin-splitting discrepancy due to different surface potential gradients can be excluded. On the other hand, by comparing the experimental band structure to first-principles LMTO band structure calculations, we find that the strongly spin-orbit coupled Bi 6p orbitals dominate the orbital character of CB, whereas their admixture to VB is rather small. The spin-splitting discrepancy is, therefore, traced back to the difference in spin-orbit coupling between CB and VB in the respective subbands' regions.

We have studied the BiCu${}_{2}$/Cu(111) surface alloy using low-temperature scanning tunneling microscopy and spectroscopy. We observed standing waves caused by scattering off defects and step edges. Different from previous studies on similar Rashba-type surfaces, we identified multiple scattering vectors that originate from various intraband as well as interband scattering processes. A detailed energy-dependent analysis of the standing-wave patterns enables a quantitative determination of band dispersions, including the Rashba splitting. The results are in good agreement with angle-resolved photoelectron spectroscopy (ARPES) data and demonstrate the usefulness of this strategy to determine the band structure of Rashba systems. The lack of other possible scattering channels will be discussed in terms of spin conservation and hybridization effects. The results open new possibilities to study spin-dependent scattering on complex spin-orbit coupled surfaces.

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.

The earth's crust and outer space are rich sources of technologically relevant materials which have found application in a wide range of fields. Well-established examples are diamond, one of the hardest known materials, or graphite as a suitable precursor of graphene. The ongoing drive to discover novel materials useful for (opto)electronic applications has recently drawn strong attention to topological insulators. Here, we report that Kawazulite, a mineral with the approximate composition Bi$_2$(Te,Se)$_2$(Se,S), represents a naturally occurring topological insulator whose electronic properties compete well with those of its synthetic counterparts. Kawazulite flakes with a thickness of a few tens of nanometers were prepared by mechanical exfoliation. They exhibit a low intrinsic bulk doping level and correspondingly a sizable mobility of surface state carriers of more than $1000 \text{cm}^2/(\text{V s})$ at low temperature. Based on these findings, further minerals which due to their minimized defect densities display even better electronic characteristics may be identified in the future.

We present a tight-binding calculation for a two-dimensional electron gas (2DEG) including the spin-orbit interaction as well as an electric field perpendicular to the system in order to model the Bychkov-Rashba spin splitting. The associated potential gradient introduces two contributions to the tight-binding matrix: an on-site contribution coupling orbitals of the same atom and a nearest-neighbor contribution. At the $\overline{\ensuremath{\Gamma}}$ point the first-order Rashba constant ${\ensuremath{\alpha}}_{R}$ only depends on this nearest-neighbor contribution regardless of the lattices considered (square, hexagonal, honeycomb). Applying the model to graphene reveals that this nearest-neighbor contribution induces a significant increase in the zeroth-order Rashba constant ${\ensuremath{\lambda}}_{R}$ and introduces a spin-splitting component, which varies linearly in momentum.

International audience; The charge carrier dynamics of photoexcited bismuth generates a Drude response that evolves over time. Our data show that the plasma frequency of bismuth displays an initial increase and a subsequent decay. We have performed ab initio calculations on bulk bismuth within the density functional theory and show that this peculiar behavior is due to local extrema in the valence and conduction bands. It follows that most of the carriers first accumulate in these extrema and reach the Fermi level only 0.6 ps after the photoexcitation. © 2012 American Physical Society.

Recently, a giant spin splitting has been observed in surface alloys on noble metal (111) surfaces as a result of a strong structural modification at the surface as well as the large atomic spin-orbit interaction (SOI) of the alloy atoms. These surface alloys are an ideal playground to manipulate both the size of the spin splitting as well as the position of the Fermi level as it is possible to change the atomic SOI as well as the relaxation by varying alloy atoms and substrates. Using spin- and angle-resolved photoemission spectroscopy in combination with quantitative low energy electron diffraction we have studied the mixed binary Bi(x)Sb(1-x)/Ag(111) and the mixed ternary Bi(x)Pb(y)Sb(1-x-y)/Ag(111) surface alloys where we observed a continuous evolution of the band structure with x and y., 6 pages, 5 figures, 1 table

We have used s- and p-polarized synchrotron radiation to image the electronic structure of epitaxial graphene near the K-point by angular 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 graphene's honeycomb lattice. Here we show that by rotating the polarization vector, we are able to illuminate this 'dark corridor' indicating that the present theoretical understanding is oversimplified. Our measurements are supported by first-principles photoemission calculations, which reveal that the observed effect persists in the low photon energy regime., 5 pages, 4 figures

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 sp3-hybridized first graphitic layer formed on the Si-terminated face of silicon carbide from the substrate by gold intercalation, leading to a completely sp2-hybridized graphene layer with improved electronic properties., 7 pages, 4 figures, 1 table