In one dimension, there are only two ways to move: left or right. This leads to some peculiar properties for one-dimensional systems on the atomic scale. In our paper we present a one-dimensional conductor forming on a bismuth surface, which effectively separates the electrons going through it according to their spin, a kind of rotation around the electron's axis. It turns out that electrons going to the left have exactly the opposite spin as electrons going to the right. Such a situation could have useful applications in the field of spintronics, a novel type of electronics which is based on the electron's spin rather than its charge and which could lead to more effective computers or even quantum computing. The state is in several ways similar to so-called edge states appearing in the recently discovered quantum spin Hall effect but instead of being found for a sandwich structure of semiconductors at very low temperatures, it is found on a simple, clean surface, is truly one-dimensional  and, most remarkably, even exists at room temperature.

J.W. Wells et. al., Phys. Rev. Lett. 102, 096802 (2009)

Surface plasmon is the name for an electron density wave which propagates on the surface of a conductor, very similar to water waves on the surface of the sea. These electron waves have recently been studied intensely due to the vast range of possible technical applications, especially in nano-optics, microscopy, light generation and optical data processing.

Unlike water waves, a lot of energy is needed to kick off a surface plasmon wave. Surface plasmons have therefore been thought to be unimportant in many processes, e.g. chemical reactions, where not enough energy is available to create them. Many scientists working in the field were therefore surprised by a theoretical discovery made by scientists at the Donostia International Physics Centre in Spain who predicted the existence of a new kind of surface plasmon, the so-called acoustic surface plasmon, which could be excited with very small energies. The theoretical prediction and the fascination of this completely new phenomenon sparked an intense experimental effort to proof the existence of the acoustic surface plasmon, an endeavour which has now finally succeeded on the (0001) surface of the metal beryllium.

The figure shows a visualisation of the acoustic surface plasmon. The lower part shows a static electronic wave induced by a charge close to the surface. The upper part shows what happens when the charge close to the surface is not fixed but moved: the novel type of propagating electron wave is generated and superimposed on the static wave.

As with many new discoveries, it is difficult to appraise their possible impact. The key element of the new plasmon is its acoustic character, i.e. that plasmon waves can be excited with very small energies (like water waves), allowing them to participate in many dynamical processes at surfaces such as chemical reactions. Another interesting possible application lies in the interaction of the new plasmon with light, which could be confined to nano-scale areas on the surface of a conductor.

B. Diaconescu et al., Nature 448, 57 (2007).

Understanding the conductance of a material is one of the most fundamental tasks in solid state physics. Precise conductance measurements are usually carried out using a (macroscopic) four point probe. Recently, microscopic versions of four point probes have been developed which allow surface-sensitive conductance measurements and, in principle, conductance measurments on nano-structures. Using such a microscopic four point probe, we have studied the Si(111)(7x7) surface, the electronic character of which has been disputed for a long time. Its temperature dependent conductivity was measured with a microscopic four point probe between room temperature and 100 K. At room temperature the measured conductance corresponds to that expected from the bulk doping level. However, as the temperatures is lowered below approximately 200 K, the conductance decreases by several orders of magnitude in a small temperature range and it saturates at low temperatures, irrespective of bulk doping. This abrupt transition is interpreted as the switching from bulk to surface conduction, an interpretation which is supported by a numerical model for the measured four point probe conductance. The value of the surface conductance is considerably lower than that of a good metal.

The figure below shows the temperature-dependent conductance of Si(111)(7x7) for samples with a different bulk doping as measured with a microscopic four point probe. At high temperature, the measurement is bulk sensitive and the conductance depends on the bulk doping. At low temperatures, the conductance is solely due to the surface and does not depend on the bulk doping. The inset shows an image of a typical microscopic four point probe as viewed through an optical microscope.

J. Wells et al. Phys. Rev. Lett., 97, 206803 (2006).

Quasiparticle interference is of fundamental importance to screening in metals. On surfaces and on high-temperature superconductors, interference patterns observed by scanning tunnelling microscopy (STM) can also be used to study the local electronic structure. Here we show that the spin of the quasiparticles can have a profound effect on the interference patterns. On Bi(110), a system with non spin-degenerate surface state bands, the patterns are not simply related to the dispersion of the electronic states. In fact, the features which are expected on grounds of the surface state and Fermi contour topology are absent and the only observable interference patterns can be assigned to spin-conserving scattering events.

The figure below shows (a) the quasiparticle interference pattern observed on Bi(110) together with (b) a Fourier transform in order to extract the main periodicities. (c) and (d) explain the main scattering events which lead to the structures in (b), taking into account the spin of the quasiparticles. evident.

J. I. Pascual et al., Phys. Rev. Lett. 93, 196802 (2004).

Using first-principles calculations and angle-resolved photoemission, we have recently shown that the spin-orbit interaction leads to a strong splitting of the surface state bands on low-index surfaces of Bi. The dispersion of the states and the corresponding Fermi surfaces are profoundly modified in the whole surface Brillouin zone. This has implications with respect to a proposed surface charge density wave on Bi(111) as well as to the surface screening, surface spin-density waves, electron (hole) dynamics in surface states, and to possible applications to the spintronics.

The figure shows the calculated surface state dispersion (red markers) together with the measured dispersion for some selected Bi surfaces (photoemission intensity as colour map). The agreement between experiment and calculation is excellent, but only if the spin-orbit interaction is taken into account.

Yu. M. Koroteev et al. Phys. Rev. Lett. 93, 046403 (2004).

It is generally assumed that electrons in deep atomic core states are highly localised and do not participate in the bonding of molecules and solids. This implies well-defined core level binding energies and the absence of any splitting and band-like dispersion, a fact which is exploited in several powerful experimental techniques, such as x-ray photoemission spectroscopy. Violations of this assumption have only been found for a few small molecules in the gas phase such as acetylene or nitrogen with much stronger bonding and shorter bonding distances than present in solids. In this work we report the observation of a sizeable band-like dispersion of the C 1s core level in graphene, a single-layer honeycomb net of carbon atoms that is presently attracting considerable attention in the scientific community.

The dispersion is observed as an emission-angle dependent binding energy modulation (see sketch below) and it is shown that under appropriate conditions only the bonding or anti-bonding states can be observed.

S. Lizzit et al. Nature Physics s 6, 345 (2010).

Graphene, a single layer of graphite, has recently attracted considerable attention owing to its remarkable electronic and structural properties and its possible applications in many

emerging areas such as graphene-based electronic devices. The charge carriers in graphene behave like massless Dirac fermions, and graphene shows ballistic charge transport, turning it into an ideal material for circuit fabrication. However, graphene lacks a bandgap around the Fermi level, which is the defining concept for semiconductor materials and essential for controlling the conductivity by electronic means. Theory predicts that a tunable bandgap may be engineered by periodic modulations of the graphene lattice but experimental evidence for this is so far lacking. In this paper, we demonstrate the existence of a bandgap opening in graphene, induced by the patterned adsorption of atomic hydrogen onto the moiré superlattice positions of graphene grown on an Ir(111) substrate. The figure illustrates the structure and electronic structure of the clean and hydrogen-covered graphene.

R. Balog et al. Nature Materials 9, 315 (2010).

The gap between valence band and conduction band of a semiconductor is one of its most fundamental properties. The presence and size of the gap is what makes a semiconductor device work. The function of many devices relies on band bending, an electrostatic effect in which the energy of both valence and conduction bands changes in the vicinity of a surface or an interface. In extreme cases, the bending can be so strong that new quantum well states appear at the interface, forming a two-dimensional electron gas. Here we show that the presence of such states, and the high electron density associated with it, can give rise to a considerable shrinkage of the band gap. This effect can be expected to be important for the engineering of actual semiconductor devices. The figure shows the spectrum of a quantum well state at a surface of CdO.

P. D. C. King et al., Phys. Rev. Lett. 104, 256803 (2010).

Topological insulators are a recently discovered class of materials with fascinating properties: While the inside of the solid is insulating, fundamental symmetry considerations require the surfaces to be metallic. The metallic surface states show an unconventional spin texture, electron dynamics and stability. Recently, surfaces with only a single Dirac cone dispersion have received particular attention. These are predicted to play host to a number of novel physical phenomena such as Majorana fermions, magnetic monopoles and unconventional superconductivity. Such effects will mostly occur when the topological surface state lies in close proximity to a magnetic or electric field, a (superconducting) metal, or if the material is in a confined geometry. We show that a band bending near to the surface of the topological insulator Bi2Se3 gives rise to the formation of a two-dimensional electron gas (2DEG). The 2DEG, renowned from semiconductor surfaces and interfaces where it forms the basis of the integer and fractional quantum Hall effects, two-dimensional superconductivity, and a plethora of practical applications, coexists with the topological surface state in Bi2Se3. This leads to the unique situation where a topological and a non-topological, easily tunable and potentially superconducting, metallic state are confined to the same region of space. The figure below shows angle-resolved photoemission data revealing the 2DEG and the topological state.

M. Bianchi et al., Nature Communications 1:128 (2010), DOI: 10.1038/ncomms1131

see also arXiv:1009.2879v1

Electrostatic manipulation of the electron spin lies at the heart of the emergent field of spintronics. Currently, however, even the prototypical method for demonstrating such control via a Rashba spin-splitting of a two-dimensional electron gas (2DEG) is difficult to implement due to the small spin-orbit interaction in most semiconductors. In this paper, we report a Rashba splitting of a 2DEG in the topological insulator Bi2Se3, which is at least an order of magnitude larger than in other semiconductors. We further demonstrate that the spin-splitting can be controlled electrostatically, opening a route towards room-temperature operation of nanoscale spintronic devices. The figure below shows how the Rashba splitting of the the 2DEG evolves with time, from a 2DEG with no apparent splitting, to a strong splitting.

P. D. C. King et al., Physical Review Letters (in press)

see also arXiv:1103.3220v1), click on the image to view the corresponding movie.

In the highlight below (Large and tunable Rashba spin splitting....), we show that the clean surface of the topological insulator Bi2Se3, undergoes a time-dependent band bending that is strong enough to give rise to a substantial Rashba splitting in a two-dimensional electron gas (2DEG) near the surface. In this paper, we probe the origin of this band bending and show that it can be induced by exposing the surface to carbon monoxide. Most unusually, we do not only observe a quantization of the bulk conduction band states into 2DEGs but also a quantization of the valence band states into a ladder of quantum wells (shown in the figure below). We argue that such a simultaneous quantization of conduction band and valence band is only possible because of the special electronic structure of bulk Bi2Se3.

Marco Bianchi et al., Physical Review Letters, 107, 086802 (2011).

see also (arXiv:1104.4930v1), click on the image to view the quantum well formation as a movie.