Table of contents

Electronic structure, which describes the distribution of electronic states in reciprocal space, is one of the most fundamental concepts in condensed matter physics, since it determines the electrical, optical and magnetic behaviours of materials. Graphene has great promise for both fundamental physics and future applications. Chemical vapour deposition (CVD) is currently the dominant technology for its scaled growth on metal foils. The polycrystalline nature of metal foil makes NanoARPES, one energy-momentum dispersion probe with spatial resolution down to a few tens of nanometers, a unique tool to study the intrinsic electronic structure of polycrystalline graphene films. In this topical review, we present the latest NanoARPES studies on graphene grains and films grown on copper foil by CVD. The comprehensive chemical and electronic images probed by NanoARPES provide deep insight about graphene and point out potential ways to functionalize graphene properties. This knowledge may stimulate us to look into the future of this field from both the material synthesis and the instrumental characterisation.

After the discovery of Dirac electrons in condensed matter physics, more specifically in graphene and its derivatives, their potentialities in the fields of plasmonics and photonics have been readily recognized, leading to a plethora of applications in active and tunable optical devices. Massless Dirac carriers have been further found in three-dimensional topological insulators. These exotic quantum systems have an insulating gap in the bulk and intrinsic Dirac metallic states at any surface, sustaining not only single-particle excitations but also plasmonic collective modes. In this paper we will review the plasmon excitations in different microstructures patterned on Bi₂Se₃ topological insulator thin films as measured by terahertz spectroscopy. We discuss the dependence of the plasmon absorption versus the microstructure shape, wavevector, and magnetic field. Finally we will discuss the topological protection of both the Dirac single-particle and plasmon excitations.

Many chemical reactions that produce a wide range of hydrocarbons and alcohols involve the breaking of C–H bonds in methane. In this paper, we analyzed the decomposition of this molecule on the B5 step-edge type site of Ru surface using first principles calculations based on dispersion-corrected density functional theory. Methane was found to be weakly adsorbed on the surface, characterized by the hybridization of its sp states with Ru–d_xz,yz,zz states. Dissociative adsorption is energetically preferred over molecular methane adsorption, resulting in CH fragment. CH is strongly adsorbed on the surface due to the prevalence of low-energy sp–d bonding interaction over the electron-unoccupied anti-bonding states. This highly stable CH requires higher activation barrier for C–H bond cleavage than CH₄.

The all-optical spin switching induced by an intense (∼TW cm⁻²), near-infrared (775 nm), ultrashort (∼100 fs) circularly-polarized laser pulse is studied based on the spin–orbit coupled Heisenberg model. We find that the magnetic spin momentum undergoes an oscillation in time during the interaction with a driving laser pulse, which can be explained as a classical counterpart of the Rabi oscillation associated with a spin–orbit coupling. The optimal spin reversal is achieved by adjusting the pulse duration to one half the Rabi oscillation period. A successive spin reversal by a delayed pulse is possible if it has the opposite helicity and a shorter duration relative to the first pulse. Moreover, inclusion of an exchange interaction term in the Hamiltonian leads to a precession of the magnetic spin momentum that lasts even after the driving laser pulse turns off. This spin precession is stronger in antiferromagnets than ferrimagnets.

Today, quasiperiodic tilings are well known and have been studied in great detail since they are very useful to describe the properties of metallic and soft matter quasicrystals. A closely related topic are quasiperiodic functions which have also gained large interest recently. Different types of such functions and there interrelation will be presented here. The main topic will be quasiperiodic potentials generated by laser beams and their variability. The distribution of extremal points and local isomorphisms of quasiperiodic functions will also be addressed.

Surface states consisting of helical Dirac fermions have been extensively studied in three-dimensional topological insulators. Yet, experiments to date have only investigated fully formed topological surface states (TSS) and it is not known whether preformed or partially formed surface states can exist or what properties they could potentially host. Here, by decorating thin films of Bi₂Se₃ with nanosized islands of the same material, we show for the first time that not only can surface states exist in various intermediate stages of formation but they exhibit unique properties not accessible in fully formed TSS. These include tunability of the Dirac cone mass, vertical migration of the surface state wave-function and the appearance of mid-gap Rashba-like states as exemplified by our theoretical model for decorated TIs. Our experiments show that an interplay of Rashba and Dirac fermions on the surface leads to an intriguing multi-channel weak anti-localization effect concomitant with an unprecedented tuning of the topological protection to transport. Our work offers a new route to engineer topological surface states involving Dirac–Rashba coupling by nano-scale decoration of TI thin films, at the same time shedding light on the real-space mechanism of surface state formation in general.

We report our investigation on the electronic properties of the step edges on a Bi(1 1 1) surface in epitiaxially grown thin films, using scanning tunneling microscopy and spectroscopy. Our results show three differential conductance peaks including the previously reported peak in the spectra recorded at the step edges. Our analysis indicates that all of the three peaks can be ascribed to the van Hove singularities and thus to the band extrema of the one-dimensional edge bands, according to the quasiparticle interference and the Fourier transform patterns. These edge states show an overall penetration length of about 5 nm, but they also show different spatial distributions perpendicular to the edge. The well-determined band extrema may provide information for establishing a better model to describe the electronic topology of the step edge in the Bi(1 1 1) films.

One of the fundamental questions in quantum transport is how charge transfer through complex nanostructures is influenced by quantum coherence. We address this issue for linear triple quantum dots by comparing a Lindblad density matrix description with a Pauli rate equation approach and analyze the corresponding zero-frequency counting statistics of charge transfer. The impact of decaying coherences of the density matrix due to dephasing is also studied. Our findings reveal that the sensitivity to coherence shown by shot noise and skewness, in particular in the limit of large coupling to the drain reservoir, can be used to unambiguously evidence coherent processes involved in charge transport across triple quantum dots. Our analytical results are obtained by using the characteristic polynomial approach to full counting statistics.

We report the results of an investigation of ambipolar transport in a quantum well of 15 nm width in an undoped GaAs/AlGaAs structure, which was populated either by electrons or holes using positive or negative gate voltage V_tg, respectively. More attention was focussed on the low concentration of electrons n and holes p near the metal–insulator transition (MIT). It is shown that the electron mobility ${{\mu}_{e}}$ increases almost linearly with increase of n and is independent of temperature T in the interval 0.3 K–1.4 K, while the hole mobility ${{\mu}_{p}}$ depends non-monotonically on p and T. This difference is explained on the basis of the different effective masses of electrons and holes in GaAs. Intriguingly, we observe that at low p the source–drain current (I_SD)–voltage (V) characteristics, which become non-linear beyond a certain I_SD, exhibit a re-entrant linear regime at even higher I_SD. We find, remarkably, that the departure and reappearance of linear behaviour are not due to non-linear response of the system, but due to an intrinsic mechanism by which there is a reduction in the net number of mobile carriers. This effect is interpreted as evidence of inhomogeneity of the conductive 2D layer in the vicinity of MIT and trapping of holes in 'dead ends' of insulating islands. Our results provide insights into transport mechanisms as well as the spatial structure of the 2D conducting medium near the 2D MIT.

Magnetic tunnel junction (MTJ) micropillars were fabricated with integrated thermometers and a heater line (HL) for thermovoltage measurements. This novel thermometer configuration enabled a direct measurement of ΔT across the MTJ micropillar. The MTJ devices were patterned from a CoFeB/MgO/CoFeB stack, with a 1.2 nm to 1.6 nm MgO wedge across the wafer, resulting in resistance area products in the range of 0.7 kΩ · µm²  <  R  ×  A  <  55 kΩ · µm². This allowed the measurement of thermoelectric properties as a function of the tunnel barrier thickness. The thermometers showed a homogeneous heating behavior for all devices across the wafer. Combining the in-stack temperature measurements and finite element simulations the thermal profile across the MTJ structure and the thermopower were estimated with a noticeable improvement of the measurement accuracy. The studied MTJ structures showed tunneling magnetoresistance (TMR) ratios up to 125%, and tunneling magnetothermopower (TMTP) up to 35%.

The crystallization process of liquid metals is studied using ab initio molecular dynamics simulations. The evolution of short-range order during quenching in Pb and Zn liquids is compared with body-centered cubic (bcc) Nb and V, and hexagonal closed-packed (hcp) Mg. We found that the fraction and type of the short-range order depends on the system under consideration, in which the icosahedral symmetry seems to dominate in the body-centered cubic metals. Although the local atomic structures in stable liquids are similar, liquid hcp-like Zn, bcc-like Nb and V can be deeply supercooled far below its melting point before crystallization while the supercooled temperature range in liquid Pb is limited. Further investigations into the nucleation process reveal the process of polymorph selection. In the body-centered cubic systems, the polymorph selection occurs in the supercooled state before the nucleation is initiated, while in the closed-packed systems it starts at the time of onset of crystallization. Atoms with bcc-like lattices in all studied supercooled liquids are always detected before the polymorph selection. It is also found that the bond orientational ordering is strongly correlated with the crystallization process in supercooled Zn and Pb liquids.

Elastic moduli, hardness (both at room temperature) and thermal expansion (4.2–670 K) have been experimentally determined for polycrystalline CePt₃Si and its prototype compound CePt₃B as well as for single-crystalline CePt₃Si. Resonant ultrasound spectroscopy was used to determine elastic properties (Young's modulus E and Poisson's ratio ν) via the eigenfrequencies of the sample and the knowledge of sample mass and dimensions. Bulk and shear moduli were calculated from E and ν, and the respective Debye temperatures were derived. In addition, ab initio DFT calculations were carried out for both compounds. A comparison of parameters evaluated from DFT with those of experiments revealed, in general, satisfactory agreement. Positive and negative thermal expansion values obtained from CePt₃Si single crystal data are fairly well explained in terms of the crystalline electric field model, using CEF parameters derived recently from inelastic neutron scattering. DFT calculations, in addition, demonstrate that the atomic vibrations keep almost unaffected by the antisymmetric spin–orbit coupling present in systems with crystal structures having no inversion symmetry. This is opposite to electronic properties, where the antisymmetric spin–orbit interaction has shown to distinctly influence features like the superconducting condensate of CePt₃Si.

Nitrogen-vacancy (NV) centers are defects in diamonds, which, due to their electronic structure, have been extensively studied as magnetic field sensors. Such field detection applications usually employ the NV centers to detect field components aligned with the direction of the internally-defined spin axis of the NV center. In this work we detect magnetic fields which are slightly misaligned with the NV center axis. In particular, we demonstrate that the NV center can measure the square of the angle between the magnetic field and the NV center axis with high sensitivity which diverges as the external field approaches a value pre-defined by the NV center's internal parameters, in agreement with predictions. These results show that NV centers could be used as sensitive transducers for making quantum nondemolition (QND) measurements on systems such as nanomechanical oscillators.

Electron edge states in gated bilayer graphene in the quantum valley Hall (QVH) effect regime can carry both charge and valley currents. We show that an interlayer potential splits the zero-energy level and opens a bulk gap, yielding counter-propagating edge modes with different valleys. A rich variety of valley current states can be obtained by tuning the applied boundary potential and lead to the QVH effect, as well as to the unbalanced QVH effect. A method to individually manipulate the edge states by the boundary potentials is proposed.

There have been growing efforts to find new two-dimensional (2D) materials with anisotropic properties due to their potential applications in electronics. Although in such a search, a symmetry based analysis can be useful, it has not been reported so far. Using group theory we have found sufficient conditions for the existence of a linear dispersion in one direction and quadratic one in perpendicular direction, in the vicinity of points of symmetry in the Brillouin zone (BZ) of any non-magnetic, 2D material with negligible spin–orbit coupling. We have formulated a set of symmetry conditions that lead to the semi-Dirac dispersion and analyzed all possible symmetries of 2D materials. In four, out of all eighty symmetry groups, combined time-reversal and crystal symmetry leads, at given points in the BZ, to such dispersion. The result is valid irrespectively of strength of electronic correlations in the system, model used to calculate the band structure, or the actual crystal structure that realizes given groups. We have illustrated our findings using a tight-binding example.

Using first-principles calculations, we show that both face-centered cubic (fcc) Ag (1 1 0) ultrathin films and body-centered cubic (bcc) Eu(1 1 0) ultrathin films exhibit thickness selective stability. However, the origin of such thickness selection is different. While the thickness selective stability in fcc Ag(1 1 0) films is mainly due to the well-known quantum well states ascribed to the quantum confinement effects in free-electron-like metal films, the thickness selection in bcc Eu(1 1 0) films is more complex and also strongly correlated with the occupation of the surface and surface resonance states.

The standard treatment of quantum corrections to semiclassical electronic conduction assumes that charge carriers propagate many wavelengths between scattering events, and succeeds in explaining multiple phenomena (weak localization magnetoresistance (WLMR), universal conductance fluctuations, Aharonov–Bohm oscillations) observed in polycrystalline metals and doped semiconductors in various dimensionalities. We report apparent WLMR and conductance fluctuations in H_xVO₂, a poor metal (in violation of the Mott–Ioffe–Regel limit) stabilized by the suppression of the VO₂ metal-insulator transition through atomic hydrogen doping. Epitaxial thin films, single-crystal nanobeams, and nanosheets show similar phenomenology, though the details of the apparent WLMR seem to depend on the combined effects of the strain environment and presumed doping level. Self-consistent quantitative analysis of the WLMR is challenging given this and the high resistivity of the material, since the quantitative expressions for WLMR are derived assuming good metallicity. These observations raise the issue of how to assess and analyze mesoscopic quantum effects in poor metals.

We studied the influence of the induced strain and applied electric field on the ground state of ferroelectric Ba_0.7Sr_0.3TiO₃ thin films, deposited on the cubic (0 0 1) substrate. The dependence of the pyroelectric coefficient on the applied field is calculated for the different values of the induced strain. We found that tuning of the misfit strain in the film under the dielectric bolometer mode by the proper selection of substrate makes it possible to create the structures with very large values of the pyroelectric coefficient.

A multi-scale approach for the theoretical description of deformed phosphorene is presented. This approach combines a valence-force model to relate macroscopic strain to microscopic displacements of atoms and a tight-binding model with distance-dependent hopping parameters to obtain electronic properties. The resulting self-consistent electromechanical model is suitable for large-scale modeling of phosphorene devices. We demonstrate this for the case of inhomogeneously deformed phosphorene drum, which may be used as an exciton funnel.

The experimental observation of topological magnon bands and thermal Hall effect in a kagomé lattice ferromagnet Cu(1–3, bdc) has inspired the search for topological magnon effects in various insulating ferromagnets that lack an inversion center allowing a Dzyaloshinskii–Moriya (DM) spin–orbit interaction. The star lattice (also known as the decorated honeycomb lattice) ferromagnet is an ideal candidate for this purpose because it is a variant of the kagomé lattice with additional links that connect the up-pointing and down-pointing triangles. This gives rise to twice the unit cell of the kagomé lattice, and hence more interesting topological magnon effects. In particular, the triangular bridges on the star lattice can be coupled either ferromagnetically or antiferromagnetically which is not possible on the kagomé lattice ferromagnets. Here, we study DM-induced topological magnon bands, chiral edge modes, and thermal magnon Hall effect on the star lattice ferromagnet in different parameter regimes. The star lattice can also be visualized as the parent material from which topological magnon bands can be realized for the kagomé and honeycomb lattices in some limiting cases.

The geometry and structure of an interface ultimately determines the behavior of devices at the nanoscale. We present a generic method to determine the possible lattice matches between two arbitrary surfaces and to calculate the strain of the corresponding matched interface. We apply this method to explore two relevant classes of interfaces for which accurate structural measurements of the interface are available: (i) the interface between pentacene crystals and the (1 1 1) surface of gold, and (ii) the interface between the semiconductor indium-arsenide and aluminum. For both systems, we demonstrate that the presented method predicts interface geometries in good agreement with those measured experimentally, which present nontrivial matching characteristics and would be difficult to guess without relying on automated structure-searching methods.