Challenges for first-principles methods in theoretical and computational physics: multiple excitations in many-electrons systems and the Aharonov-Bohm effect in carbon nanotubes

In the first part of the thesis we will describe double excitations in the absorption spectrum. Double excitations are a peculiar effect of interacting systems which does not have a counterpart in non-interacting ones. The optical absorption spectrum of a system is obtained by shining light on it. At the microscopic level photons hit the electrons which sit in the ground state and change their configuration. If the light source is not too intense this can be described in linear response; that is only "one photon" processes are involved, only one electron per time can be influenced. Here is where the interaction comes in. The hit electron is linked to the others and so other process take place, one of these is the appearance of multiple excitations. In the second part of the thesis we focus on the application of more standard techniques to the description of carbon nanotubes (CNTs). In particular we focus on the effects of magnetic fields on CNTs. CNTs are quasi 1D-systems composed by carbon atoms which have been discovered in 1952. Under the effect of a magnetic field electrons delocalized on a cylindrical surface display a peculiar behaviour, known as Aharonov-Bohm effect. The Aharonov-Bohm is a pure quantum mechanical effect which does not have any counterpart in classical physics. In CNTs the Aharonov-Bohm modify the electronic gap and so can be used to tune the electronic properties. Though a model able to account for such process is available in the literature, in the present work we will describe the effect of magnetic fields "ab-initio". In the description of CNTs we will use standard approximations which are by far much more accurate and general than any approximation introduced in phenomenological descriptions based on model systems.Comment: PhD thesis. http://phd.fisica.unimi.i

Non-equilibrium Bethe-Salpeter equation for transient photo-absorption spectroscopy

In this work we propose an accurate first-principle approach to calculate the transient photo--absorption spectrum measured in Pump\&\,Probe experiments. We formulate a condition of {\em adiabaticity} and thoroughly analyze the simplifications brought about by the fulfillment of this condition in the non--equilibrium Green's function (NEGF) framework. Starting from the Kadanoff-Baym equations we derive a non--equilibrium Bethe--Salpeter equation (BSE) for the response function that can be implemented in most of the already existing {\em ab--initio} codes. In addition, the {\em adiabatic} approximation is benchmarked against full NEGF simulations in simple model hamiltonians, even under extreme, nonadiabatic conditions where it is expected to fail. We find that the non--equilibrium BSE is very robust and captures important spectral features in a wide range of experimental configurations.Comment: 13 pages, 5 captioned figure

First-principles approach to excitons in time-resolved and angle-resolved photoemission spectra

We show that any {\em quasi-particle} or GW approximation to the self-energy does not capture excitonic features in time-resolved (TR) photoemission spectroscopy. In this work we put forward a first-principles approach and propose a feasible diagrammatic approximation to solve this problem. We also derive an alternative formula for the TR photocurrent which involves a single time-integral of the lesser Green's function. The diagrammatic approximation applies to the {\em relaxed} regime characterized by the presence of quasi-stationary excitons and vanishing polarization. The main distinctive feature of the theory is that the diagrams must be evaluated using {\em excited} Green's functions. As this is not standard the analytic derivation is presented in detail. The final result is an expression for the lesser Green's function in terms of quantities that can all be calculated {\em ab initio}. The validity of the proposed theory is illustrated in a one-dimensional model system with a direct gap. We discuss possible scenarios and highlight some universal features of the exciton peaks. Our results indicate that the exciton dispersion can be observed in TR {\em and} angle-resolved photoemission.Comment: 15 pages, 8 figure

Optical properties of periodic systems within the current-current response framework: pitfalls and remedies

We compare the optical absorption of extended systems using the density-density and current-current linear response functions calculated within many-body perturbation theory. The two approaches are formally equivalent for a finite momentum $\mathbf{q}$ of the external perturbation. At $\mathbf{q}=\mathbf{0}$, however, the equivalence is maintained only if a small $q$ expansion of the density-density response function is used. Moreover, in practical calculations this equivalence can be lost if one naively extends the strategies usually employed in the density-based approach to the current-based approach. Specifically we discuss the use of a smearing parameter or of the quasiparticle lifetimes to describe the finite width of the spectral peaks and the inclusion of electron-hole interaction. In those instances we show that the incorrect definition of the velocity operator and the violation of the conductivity sum rule introduce unphysical features in the optical absorption spectra of three paradigmatic systems: silicon (semiconductor), copper (metal) and lithium fluoride (insulator). We then demonstrate how to correctly introduce lifetime effects and electron-hole interactions within the current-based approach.Comment: 17 pages, 6 figure

Skeletal Class III Surgical-Orthodontic Treatment and Remote Digital Monitoring during COVID-19 Pandemic: A Case Report

Moderate to severe cases of skeletal Class III malocclusion, where residual growth is no longer present and an orthodontic camouflage would not achieve satisfactory outcomes, are good candidates for a combined surgical-orthodontic approach. We present the case of a 34-year-old healthy male with skeletal Class III malocclusion, where aesthetics and masticatory function were further worsened by maxillary and mandibular transverse discrepancy, hyperdivergent pattern, moderate dental crowding, occlusal contacts present only on molars, negative overjet and overbite. The management of the case included a pre-surgical phase of surgically assisted rapid palatal expansion (SARPE) and an orthodontic treatment with fixed multibracket appliance, a surgical phase consisting in Le Fort I osteotomy and bilateral sagittal split osteotomy (BSSO), and a myofunctional physical therapy targeting orofacial muscles following the orthognathic surgery. The pre-surgical phase was additionally integrated with a system of remote digital monitoring, such as Dental Monitoring®, to early detect any orthodontic emergency. As in-office visits were abruptly interrupted because of COVID-19 pandemic, the remote digital system also permitted to regularly monitor the patient at long-distance. In conclusion, a case of skeletal Class III malocclusion was successfully managed with a multidisciplinary approach which involved orthognathic surgery, orthodontic treatment, and myofunctional physical therapy. The additional integration of remote digital technologies, such as Dental Monitoring®, may provide a continuity of care to orthodontic patients in times of COVID-19 pandemic, when the regularity of non-urgent chairside appointments might be disrupted

Exciton-Exciton transitions involving strongly bound Frenkel excitons: an ab initio approach

In pump-probe spectroscopy, two laser pulses are employed to garner dynamical information from the sample of interest. The pump initiates the optical process by exciting a portion of the sample from the electronic ground state to an accessible electronic excited state, an exciton. Thereafter, the probe interacts with the already excited sample. The change in the absorbance after pump provides information on transitions between the excited states and their dynamics. In this work we study these exciton-exciton transitions by means of an ab initio real time propagation scheme based on dynamical Berry phase formulation. The results are then analyzed taking advantage of a Fermi-golden rule approach formulated in the excitonic basis-set and in terms of the symmetries of the excitonic states. Using bulk LiF and 2D hBN as two prototype materials, we discuss the selection rules for transitions involving strongly bound Frenkel excitons, for which the hydrogen model cannot be used

Stabilization of tetragonal/cubic phase in Fe doped Zirconia grown by atomic layer deposition

Achieving high temperature ferromagnetism by doping transition metals thin films is seen as a viable approach to integrate spin-based elements in innovative spintronic devices. In this work we investigated the effect of Fe doping on structural properties of ZrO2 grown by atomic layer deposition (ALD) using Zr(TMHD)4 for Zr and Fe(TMHD)3 for Fe precursors and ozone as oxygen source. The temperature during the growth process was fixed at 350{\deg}C. The ALD process was tuned to obtain Fe doped ZrO2 films with uniform chemical composition, as seen by time of flight secondary ion mass spectrometry. The control of Fe content was effectively reached, by controlling the ALD precursor pulse ratio, as checked by X-ray photoemission spectroscopy (XPS) and spectroscopic ellipsometry. From XPS, Fe was found in Fe3+ chemical state, which maximizes the magnetization per atom. We also found, by grazing incidence X-ray diffraction, that the inclusion of Fe impurities in ZrO2 induces amorphization in thin ZrO2 films, while stabilizes the high temperature crystalline tetragonal/cubic phase after rapid thermal annealing at 600{\deg}C.Comment: 11 pages, 7 figures, 1 Tabl

V-CHAT - Versatile Chat

We focus our research on text based messaging systems and the impact they have when they are implemented in a work structure and how this kind of software can improve workflow and interaction between firms and clients both in B2B and B2C relations. In recent years many online text chat systems have been developed and use. Internet Relay Chat or IRC, ICQ, AOL and MSN are some examples of the most popular tools for real-time communications via the Internet and have been used both for doing serious work, decision making and for entertainment purpose. Messaging programs tend to imitate each other and they imitate other chat programs either. Each new release simply adds new features that keep the use of this software nothing but a cheaper alternative to phone. Nowadays Instant Messaging, chat system and video conference software must evolve and specialize into something more flexible, sophisticated and secure