Atomically thin two-dimensional semiconductors have emerged as an interesting class of material systems, both for applications and fundamental studies. For optoelectronic applications like displays, light sources, and photovoltaics, transition-metal-dichalcogenides (TMDs) are an appealing system, as they combine great mechanical strength with high carrier mobility and a direct optical band gap. In this rapidly developing field, attention has recently shifted towards the realization of nanostructures. The generation of localized states, either induced via defects or via systematic confinement engineering, opens the possibility to deterministically generate single-photons or, more generally, provide sources of quantum light. For this purpose, flakes of TMDs have been placed on nanowires, over gold edges and over etched holes to form single-photon emitters. We focus on a different platform, which consists of TMD nanobubbles that develop if air is enclosed during the stacking of layers. The physics governing all of these examples has predominantly been discussed in terms of strain engineering. Due to the high bending rigidity, strain induces large variation of the band gap that can lead to a transition from a direct to an indirect band gap. Another, much less discussed, mechanism is the change of the dielectric environment that also induces strong bandgap variations. We report on results of atomistic tight- binding calculations of different sizes and height-to-diameter ratios of these nanostructures and demonstrate that the formation of confined quantum-dot-like single-particle states is caused by an interplay of strain and dielectric screening.

We show that the strain pockets are caused by a crumpling of the material due to its high bending rigidity and discuss the implications of the underlying physics to other TMD-based nanostructures. We also investigate semiconducting TMDs under high excitation and/or high doping conditions. Under such excitation conditions, optical transitions between the first and higher conduction bands are possible, that are analogous to intersubband transitions in conventional quantum well devices. We discuss the carrier density and temperature dependence and show that due to the large Coulomb renormalizations in layered materials, the optical transitions can be tuned into the technologically relevant 1550nm telecom wavelength range. The high absorbances found for single-layer TMDs opens the possibility to utilize TMDCs in novel devices ranging from quantum cascade lasers to novel infrared photodetectors.

 

Inviting: D. Reiter

Atomic monolayers (ML) of Pb/Si(111) have recently been found to be superconducting below 𝑇_C ≈ 1.8 K, but the mechanism behind the evolution of these 1d (or 2d) states is yet not understood. In the range from 6/5 ML to 4/3 ML, supercells consisting of linear combinations of (√7 × √3) and (√3 × √3) unit cells are formed (so-called Devil’s staircase regime). This allows us to tune the spin-orbit interaction (SOI), the electronic and atomic structure via adsorption of minute amounts of Pb. In this talk I present (spin resolved)-ARPES measurements at low 𝑇 (> 𝑇_C ) to evaluate the influence of SOI on the Pb surface states. The SOI gives rise to manifold types of spin-splittings of the surface bands as can be derived from the spin-texture around the surface high symmetry points. Moreover, breaking the high symmetry of the perfect (√3 × √3) surface reconstruction feeds back onto the measured spin-polarization.

Attosecond science, studies of electron dynamics in their natural time scale, stems from the development of mostly extreme ultraviolet sources through high-order harmonic generation in gas-phase systems. Recently, being ignited by the researches done in Stanford, high-order harmonic generation in solids has been discovered and been actively investigated. The coherent, extreme ultraviolet radiation from solids can be utilized not only as a new source for technical applications but it also offers a great tool to study electronic properties of solids. In this talk, we briefly review the developments in this emerging field and we report our newest results. In details, we demonstrate the first polarimetry measurement of high-order harmonic generation from solids and use it to uncover the non-vanishing Berry curvature underlying the generation of even harmonics in quartz, in orthogonal polarization with respect to the linearly incident electric field. First ab initio calculation of Berry curvature of quartz has been carried out and it shows a high degree of agreement to the experimentally retrieved Berry curvature which concludes an important spectroscopic application. Furthermore, we extend high-order harmonic generation in condensed matter by reporting on the unambiguous, systematic, experimental investigations of the high-order harmonic generation in liquids, the third phase of matter. By utilizing a liquid flat-jet as a target for light-matter interaction, coherent, intense extreme ultraviolet radiation is recorded in the form of multiple odd-order harmonics reaching up to 27 order and extending beyond 20 electron Volt. The intensity scaling and the ellipticity measurement show the non-perturbative, solid-like nature of the radiation. Highest cut-off energy photons were obtained using ethanol by comparison to water and other liquids. Our investigation serves as a promising first step in utilizing the new source of coherent extreme ultraviolet radiation as well as exploring electron dynamics in liquid-phase of matter.

The availability of intense and short light pulses has opened up a new and active research field. This allows to address fundamental questions on the time evolution of the electron dynamics leading to electron emission. We demonstrate in our studies that electron pair emission from surfaces holds the promise to unravel the time scale of electron dynamics. This can be achieved without atto-second light sources.

Specifically, we studied the Auger decay following the emission of a core-electron due to photon absorption. With coincidence spectroscopy, we demonstrate an extensive energy sharing between the Ag 4p photoelectron and the NVV Auger electron exceeding 10 eV. This energy width provides access to the time scale of the emission process. We convert this to a timescale of 60 as over which the fluctuations takes place. This value is in fair agreement with the theoretical calculation of the timescale to fill an exchange-correlation hole. [1]

The neutralization of ions near a surface is known to be an efficient process and leads to electron emission via Auger-type pathways. In the case of the He²⁺ ions the double ionization energy of 79 eV becomes available. We demonstrate that the neutralization of a single He²⁺ ion near an Ir(100) surface leads to the emission of an electron pair. Via coincidence spectroscopy we give evidence that a sizable amount of these electron pairs originate from a correlated single step neutralization of the ion involving a total of 4 electrons from the metal. These correlated electron pairs cannot be explained in the common picture of two consecutive and independent neutralization steps. We infer a characteristic time scale for the correlated electron dynamics in the metal of 40-400 as. [2]