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Filling a gap in the literature for a brief course in solid state physics, this is a clear and concise introduction that not only describes all the basic phenomena and concepts, but also discusses such advanced issues as magnetism and superconductivity.

The surface of a topological insulator plays host to an odd number of linearly-dispersing Dirac fermions, protected against back-scattering by time-reversal symmetry. Such characteristics make these materials attractive not only for studying a range of fundamental phenomena in both condensed matter and particle physics, but also for applications ranging from spintronics to quantum computation. Here, we show that the single Dirac cone comprising the topological state of the prototypical topological insulator Bi 2 Se 3 can co-exist with a two-dimensional electron gas (2DEG), a cornerstone of conventional electronics. Creation of the 2DEG is tied to a surface band-bending effect, which should be general for narrow-gap topological insulators. 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. Topological insulators are materials with an insulating interior and a metallic surface. In this study the authors demonstrate that the topological state can coexist with a two-dimensional electron gas state, a feature important in semiconductors used for electronic applications.

LaTe 3 is a non-centrosymmetric material with time reversal symmetry, where the charge density wave is hosted by the Te bilayers. Here, we show that LaTe 3 hosts a Kramers nodal line—a twofold degenerate nodal line connecting time reversal-invariant momenta. We use angle-resolved photoemission spectroscopy, density functional theory with an experimentally reported modulated structure, effective band structures calculated by band unfolding, and symmetry arguments to reveal the Kramers nodal line. Furthermore, calculations confirm that the nodal line imposes gapless crossings between the bilayer-split charge density wave-induced shadow bands and the main bands. In excellent agreement with the calculations, spectroscopic data confirm the presence of the Kramers nodal line and show that the crossings traverse the Fermi level. Furthermore, spinless nodal lines—completely gapped out by spin-orbit coupling—are formed by the linear crossings of the shadow and main bands with a high Fermi velocity. Kramers nodal lines are doubly degenerate nodal lines connecting time-reversal invariant momenta, which are predicted to exist in achiral, non-centrosymmetric crystals with spin-orbit interactions. Here, the authors use ARPES and DFT to demonstrate signatures of Kramers nodal lines in a non-centrosymmetric charge density wave-hosting crystal.

Only a few of the vast range of potential two-dimensional materials (2D) have been isolated or synthesised to date. Typically, 2D materials are discovered by mechanically exfoliating naturally occurring bulk crystals to produce atomically thin layers, after which a material-specific vapour synthesis method must be developed to grow interesting candidates in a scalable manner. Here we show a general approach for synthesising thin layers of two-dimensional binary compounds. We apply the method to obtain high quality, epitaxial MoS 2 films, and extend the principle to the synthesis of a wide range of other materials—both well-known and never-before isolated—including transition metal sulphides, selenides, tellurides, and nitrides. This approach greatly simplifies the synthesis of currently known materials, and provides a general framework for synthesising both predicted and unexpected new 2D compounds. The scalable synthesis of 2D materials critically relies on finding appropriate vapour-phase metal precursors and careful fine-tuning of growth parameters. Here, the authors instead use solid elemental precursors and a single recipe to demonstrate a general approach for synthesising thin epitaxial layers of 20 different 2D binary compounds, including transition metal sulphides, selenides, tellurides, and nitrides.

Charge transfers resulting from weak bondings between two-dimensional materials and the supporting substrates are often tacitly associated with their work function differences. In this context, two-dimensional materials could be normally doped at relatively low levels. Here, we demonstrate how even weak hybridization with substrates can lead to an apparent heavy doping, using the example of monolayer 1H-TaS 2 grown on Au(111). Ab-initio calculations show that sizable changes in Fermi areas can arise, while the transferred charge between substrate and two-dimensional material is much smaller than the variation of Fermi areas suggests. This mechanism, which we refer to as pseudodoping, is associated with non-linear energy-dependent shifts of electronic spectra, which our scanning tunneling spectroscopy experiments reveal for clean and defective TaS 2 monolayer on Au(111). The influence of pseudodoping on the formation of many-body states in two-dimensional metallic materials is analyzed, shedding light on utilizing pseudodoping to control electronic phase diagrams. Weak hybridization of two-dimensional metallic materials with their substrates plays a crucial role in charge transfer and doping characteristics. Here, the authors report heavy doping of monolayer 1H-TaS2 synthesized on Au(111) by ab-initio calculations and STM/STS experiments.

The microscopic motion of water is a central question, but gaining experimental information about the interfacial dynamics of water in fields such as catalysis, biophysics and nanotribology is challenging due to its ultrafast motion, and the complex interplay of inter-molecular and molecule-surface interactions. Here we present an experimental and computational study of the nanoscale-nanosecond motion of water at the surface of a topological insulator (TI), Bi 2 Te 3 . Understanding the chemistry and motion of molecules on TI surfaces, while considered a key to design and manufacturing for future applications, has hitherto been hardly addressed experimentally. By combining helium spin-echo spectroscopy and density functional theory calculations, we are able to obtain a general insight into the diffusion of water on Bi 2 Te 3 . Instead of Brownian motion, we find an activated jump diffusion mechanism. Signatures of correlated motion suggest unusual repulsive interactions between the water molecules. From the lineshape broadening we determine the diffusion coefficient, the diffusion energy and the pre-exponential factor. Water molecular motion on surfaces underpins a range of phenomena in nature. The authors resolve the nanoscale-nanosecond motion of water at a topological insulator’s surface by helium spin-echo spectroscopy and computations, reporting hopping among sites and repulsion between water molecules.

Control of atomic-scale interfaces between materials with distinct electronic structures is crucial for the design and fabrication of most electronic devices. In the case of two-dimensional materials, disparate electronic structures can be realized even within a single uniform sheet, merely by locally applying different vertical gate voltages. Here, we utilize the inherently nano-structured single layer and bilayer graphene on silicon carbide to investigate lateral electronic structure variations in an adjacent single layer of tungsten disulfide (WS 2 ). The electronic band alignments are mapped in energy and momentum space using angle-resolved photoemission with a spatial resolution on the order of 500 nm (nanoARPES). We find that the WS 2 band offsets track the work function of the underlying single layer and bilayer graphene, and we relate such changes to observed lateral patterns of exciton and trion luminescence from WS 2 . Sharp atomic interfaces between materials dictate the interface’s electronic properties. The authors use angle-resolved photoemission spectroscopy with a spatial resolution of ~500 nm to investigate the nanoscale electronic band structure and band alignment in a lateral heterostructure composed of WS 2 placed on alternating nano-stripes of monolayer and bilayer graphene.

Fermi surfaces are essential for predicting, characterizing and controlling the properties of crystalline metals and semiconductors. Angle-resolved photoemission spectroscopy (ARPES) is the only technique directly probing the Fermi surface by measuring the Fermi momenta (k F ) from energy- and angular distribution of photoelectrons dislodged by monochromatic light. Existing apparatus is able to determine a number of k F -vectors simultaneously, but direct high-resolution 3D Fermi surface mapping remains problematic. As a result, no such datasets exist, strongly limiting our knowledge about the Fermi surfaces. Here we show that using a simpler instrumentation it is possible to perform 3D-mapping within a very short time interval and with very high resolution. We present the first detailed experimental 3D Fermi surface as well as other experimental results featuring advantages of our technique. In combination with various light sources our methodology and instrumentation offer new opportunities for high-resolution ARPES in the physical and life sciences. The Fermi surface is related to the energy distribution of electrons in a solid, and governs physical properties of metals and semiconductors. A new type of angle-resolved photoemission spectroscopy, probing the Fermi surface and combining short recording time with high resolution, is now presented.

The 'impossible' plasmon At the surface of a metal, collective electronic excitations such as surface plasmons are possible, and these can be exploited for a wide range of applications in microscopy and photonics. Now a new type of plasmon has been observed, one previously considered an impossibility on a metal surface. It has a much lower energy than the conventional type, so it was thought that it would be destroyed by the underlying bulk electrons in the metal. But this new plasmon, which has acoustic (or linear) dispersion, has an electronic structure that can coexist with the electron continuum of the bulk. The acoustic plasmon is likely to exist on many metal surfaces, and will be of particular relevance to nano-optics and photonics applications. A plasmon of low energy that was therefore thought to be impossible to support on a metal surface is observed; the underlying bulk electrons would simply destroy it. It is found that this new plasmon, which has an acoustic, linear dispersion (energy dependence on momentum), can exist due to a particular, non-local aspect, of its energy structure. Nearly two-dimensional (2D) metallic systems formed in charge inversion layers 1 and artificial layered materials 2 , 3 permit the existence of low-energy collective excitations 4 , 5 , called 2D plasmons, which are not found in a three-dimensional (3D) metal. These excitations have caused considerable interest because their low energy allows them to participate in many dynamical processes involving electrons and phonons 3 , and because they might mediate the formation of Cooper pairs in high-transition-temperature superconductors 6 . Metals often support electronic states that are confined to the surface, forming a nearly 2D electron-density layer. However, it was argued that these systems could not support low-energy collective excitations because they would be screened out by the underlying bulk electrons 7 . Rather, metallic surfaces should support only conventional surface plasmons 8 —higher-energy modes that depend only on the electron density. Surface plasmons have important applications in microscopy 9 , 10 and sub-wavelength optics 11 , 12 , 13 , but have no relevance to the low-energy dynamics. Here we show that, in contrast to expectations, a low-energy collective excitation mode can be found on bare metal surfaces. The mode has an acoustic (linear) dispersion, different to the dependence of a 2D plasmon, and was observed on Be(0001) using angle-resolved electron energy loss spectroscopy. First-principles calculations show that it is caused by the coexistence of a partially occupied quasi-2D surface-state band with the underlying 3D bulk electron continuum and also that the non-local character of the dielectric function prevents it from being screened out by the 3D states. The acoustic plasmon reported here has a very general character and should be present on many metal surfaces. Furthermore, its acoustic dispersion allows the confinement of light on small surface areas and in a broad frequency range, which is relevant for nano-optics and photonics applications.

Establishing the relation between ubiquitous antiferromagnetism in the parent compounds of unconventional superconductors and their superconducting phase is important for understanding the complex physics in these materials. Going from bulk systems to thin films additionally affects their phase diagram. For Fe 1+ y Te, the parent compound of Fe 1+ y Se 1− x Te x superconductors, bulk-sensitive neutron diffraction revealed an in-plane oriented diagonal double-stripe antiferromagnetic spin structure. Here we show by spin-resolved scanning tunnelling microscopy that the spin direction at the surfaces of bulk Fe 1+ y Te and thin films grown on the topological insulator Bi 2 Te 3 is canted out of the high-symmetry directions of the surface unit cell resulting in a perpendicular spin component, keeping the diagonal double-stripe order. As the magnetism of the Fe d -orbitals is intertwined with the superconducting pairing in Fe-based materials, our results imply that the superconducting properties at the surface of the related superconducting compounds might be different from the bulk. Knowledge of the spin structure in parent compounds of unconventional superconductors is crucial for an understanding of the complex physics in these materials. Here, the authors report canted spin structure on the surface as well as on the thin film form of Fe 1+ y Te, different from the bulk.