Strain engineering of germanium surface states: an ab-initio study.

Nanostructures have received growing interests in the last decade as consequence of their peculiar and fascinating properties, and applications often superior to their bulk counterparts. In the last few years surfaces and interfaces strongly influenced the electronic properties of semiconductor nanostructures. Nanomembranes (NMs) provide the opportunity for such quantitative investigations; they are two-dimensional crystalline films that can be shaped with precise surface orientations and sizes. My work connects with this widespread focus on two dimensional materials, with the aim of working out a novel understanding of the electronic band-gap engineering of the (2x1)-reconstructed Ge(001) surface, by means of ab-initio density functional theory numerical techniques. The truncation of the germanium crystalline structure along the [001] direction generates the (001)-oriented surface of germanium. At room temperature, this (ideal) surface is energetically unstable so that a process of atomic rearrangement occours which leads to (2x1) reconstructions, where (2x1) means that the surface periodicity is doubled along the [110] or the [$\bar1$10] crystallographic directions. In fact, on ideally-terminated (001) surfaces of the diamond lattice, each surface atom exhibits two dangling bonds, partly occupied. The process of formation of dimers on the Ge (001)-oriented surface is driven by halving the number of surface dangling bonds due to the establishment of a chemical bond between two neighbour Ge atoms on the surface. The angular orientation of the dimer has been longly debated, but eventually it was experimentally observed that it is tilted with respect to the surface plane; the asymmetry of the dimers plays a central role in the electronic properties of the (2x1) Ge (001) surface: the semiconductive character of the surface is the most important effect due to this asymmetry. The termination of a crystal with a surface and the successive rearrangment of surface atoms leads to a change of the electronic band structure with respect to the bulk material. In fact, electronic surface-localized states may exist at the semiconductor surface. In previous works the study of these surface states was confined to the unstrained (2x1) Ge (001) surface. Indeed, for low-dimensional as well as for three-dimensional crystalline materials a rich family of properties and functionalities can be tuned by strain-engineering, i.e., varying structural parameters, because their electronic and geometrical structures are sensitive to strain. In this work I study for the first time, by means of ab-initio techniques, the strain-engineering of the structural and electronic properties of the (2x1)-reconstructed Ge (001) surface. Such techniques have revealed to be invaluable tools to investigate crystalline structures (three or low-dimensional), interpret experimental measurements, give fundamental support in many solid state physics fields and design new devices, also due to the everlasting increase of computational capabilities. Nevertheless, these numerical techniques need to be handled and tuned carefully according to the working situations. For this reason, before focusing on the (2x1) Ge (001) surface electronic band-engineering, we found necessary to start from the study of the Ge bulk crystal, under relaxed or strained conditions, and of the unstrained (2x1) Ge (001) surface. These steps are necessary to rightly tune, by a fair comparison with known experimental results, the choices of the computational and working procedures to be finally used in the study of the strained (2x1) Ge (001) surface. The main conclusion of this work is the evidence that it is possible to manipulate the Ge(001) surface states relative to the bulk band structure. We demonstrate that by appropriate compressive strain it is possible to obtain pure surface states which can be exploited for surface transport experiments and optical transitions. The choice of germanium for the present investigation is that it is an ideal material to achieve integrated optical sources for silicon photonics, in particular on the (001) silicon surface which is at the heart of modern semiconductor devices

Theory of infrared double-resonance Raman spectrum in graphene: the role of the zone-boundary electron-phonon enhancement

We theoretically investigate the double-resonance Raman spectrum of monolayer graphene down to infrared laser excitation energies. By using first-principles density functional theory calculations, we improve upon previous theoretical predictions based on conical models or tight-binding approximations, and rigorously justify the evaluation of the electron-phonon enhancement found in Ref. [Venanzi, T., Graziotto, L. et al., Phys. Rev. Lett. 130, 256901 (2023)]. We proceed to discuss the effects of such enhancement on the room temperature graphene resistivity, hinting towards a possible reconciliation of theoretical and experimental discrepancies.Comment: 19 pages, 18 figure

The Danger of a “Geyser Disease” Effect: Structural Fragility of the Tourism-Led Recovery in Iceland

Peer reviewe

Born effective charges and vibrational spectra in super and bad conducting metals

Interactions mediated by electron-phonon coupling are responsible for important cooperative phenomena in metals such as superconductivity and charge-density waves. The same interaction mechanisms produce strong collision rates in the normal phase of correlated metals, causing sizeable reductions of the dc conductivity and reflectivity. As a consequence, low-energy excitations like phonons, which are crucial for materials characterization, become visible in optical infrared spectra. A quantitative assessment of vibrational resonances requires the evaluation of dynamical Born effective charges, which quantify the coupling between macroscopic electric fields and lattice deformations. We show that the Born effective charges of metals crucially depend on the collision regime of conducting electrons. In particular, we describe, within a first principles framework, the impact of electron scattering on the infrared vibrational resonances, from the undamped, collisionless regime to the overdamped, collision-dominated limit. Our approach enables the interpretation of vibrational reflectance measurements of both super and bad conducting metals, as we illustrate for the case of strongly electron-phonon coupled superhydride H$_3$S

EFFECT OF WIND LOADS ON NON REGULARLY SHAPED HIGH-RISE BUILDINGS

Wind loads have historically been recognized as one of the most important issue in high-rise buildings analysis and design. In particular, in regions of low seismic intensity, a high-rise building lateral design is controlled by wind loads. In wind analysis, Computational Fluid Dynamics (CFD) and/or wind tunnel testing are required to calculate the external pressures acting on a building. In this paper, two case studies are presented to show how the wind loads are calculated and applied in design. The first case study is based on the CFD results for the New Marina Casablanca Tower in Casablanca, Morocco. The second case study considers the results from the wind tunnel test studies conducted for the Al- Hamra tower, in Kuwait City, Kuwait. The New Marina Casablanca tower is a 167m tall concrete building, with a unique twisting shape generated from the relative rotation of two adjacent floors. Sloped columns are introduced in the perimeter to follow the tower outer geometry and to support the concrete slabs spanning between the central core and the perimeter frame. The effects of wind loads on the twisted geometry has been studied in details since the pressure coefficients are not easily identified for such a complex form. In addition, the effect of the wind loads on the structure presented unique challenges that required innovative structural solutions. The Al-Hamra tower is a 412m tall concrete building with a sculpted twisting form which optimizes the views to the Arabian Gulf while minimizing the solar heat gain. The complex form is realized using sloped walls and vertical columns on the perimeter and a central concrete core. The unique shape of the tower presented several design challenges related to the wind loads on the structure. This paper will discuss the unique challenges and solutions associated with wind loads effect on buildings of unique form

Enhanced coupling between massive fermions and zone-boundary phonons probed by infrared resonance Raman in bilayer graphene

Few-layer graphene possesses low-energy carriers which behave as massive fermions, exhibiting intriguing properties in both transport and light scattering experiments. By lowering the excitation energy of resonance Raman spectroscopy down to 1.17 eV we target these massive quasiparticles in the low-energy split bands close to the K point. The low excitation energy suppresses some of the Raman processes which are resonant in the visible, and induces a clearer frequency-separation of the sub-structures of the resonant 2D peak. Studying the different intensities of the sub-structures and comparing experimental measurements with fully ab initio theoretical calculations, in the case of bilayer graphene we unveil an enhanced coupling between the massive fermions and the lattice vibrations at the K point, in analogy to what found for the massless fermions of monolayer graphene, and also suggesting that what governs the enhancement is the vicinity of the electron-hole pair momentum to K rather than how small the electron-hole pair energy is.Comment: 14 pages, 10 figure