Davide Sangalli

Effect of spin-orbit interaction on the optical spectra of single-layer, double-layer, and bulk MoS2

We present converged ab-initio calculations of the optical absorption spectra of single-layer, bi-layer, and bulk MoS

Photo-Induced Bandgap Renormalization Governs the Ultrafast Response of Single-Layer MoS2

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Double excitations in finite systems.

. Both the quasiparticle-energy calculations (on the level of the GW approximation) and the calculation of the absorption spectra (on the level of the Bethe-Salpeter equation) explicitly include spin-orbit coupling, using the full spinorial Kohn-Sham wave-functions as input. Without excitonic effects, the absorption spectra would have the form of a step-function, corresponding to the joint-density of states of a parabolic band-dispersion in 2D. This profile is deformed by a pronounced bound excitonic peak below the continuum onset. The peak is split by spin-orbit interaction in the case of single-layer and (mostly) by inter-layer interaction in the case of double-layer and bulk MoS

Validating the neuro vr-based virtual version of the multiple errands test: Preliminary results

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Double excitations in correlated systems: A many–body approach

. The resulting absorption spectra are thus very similar in the three cases but the interpretation of the spectra is different. Differences in the spectra can be seen around 3 eV where the spectra of single and double-layer are dominated by a strongly bound exciton.

Ultra-fast carriers relaxation in bulk silicon following photo-excitation with a short and polarized laser pulse

Transition metal dichalcogenides (TMDs) are emerging as promising two-dimensional (2D) semiconductors for optoelectronic and flexible devices. However, a microscopic explanation of their photophysics, of pivotal importance for the understanding and optimization of device operation, is still lacking. Here, we use femtosecond transient absorption spectroscopy, with pump pulse tunability and broadband probing, to monitor the relaxation dynamics of single-layer MoS2 over the entire visible range, upon photoexcitation of different excitonic transitions. We find that, irrespective of excitation photon energy, the transient absorption spectrum shows the simultaneous bleaching of all excitonic transitions and corresponding red-shifted photoinduced absorption bands. First-principle modeling of the ultrafast optical response reveals that a transient bandgap renormalization, caused by the presence of photoexcited carriers, is primarily responsible for the observed features. Our results demonstrate the strong impact of many-body effects in the transient optical response of TMDs even in the low-excitation-density regime.

Nonequilibrium optical properties in semiconductors from first principles: A combined theoretical and experimental study of bulk silicon

Time-dependent density-functional theory (TDDFT) is widely used in the study of linear response properties of finite systems. However, there are difficulties in properly describing excited states, which have double- and higher-excitation characters, which are particularly important in molecules with an open-shell ground state. These states would be described if the exact TDDFT kernel were used; however, within the adiabatic approximation to the exchange-correlation (xc) kernel, the calculated excitation energies have a strict single-excitation character and are fewer than the real ones. A frequency-dependent xc kernel could create extra poles in the response function, which would describe states with a multiple-excitation character. We introduce a frequency-dependent xc kernel, which can reproduce, within TDDFT, double excitations in finite systems. In order to achieve this, we use the Bethe-Salpeter equation with a dynamically screened Coulomb interaction W(omega), which can describe these excitations, and from this we obtain the xc kernel. Using a two-electron model system, we show that the frequency dependence of W does indeed introduce the double excitations that are instead absent in any static approximation of the electron-hole screening.

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

The purpose of this study was to establish ecological validity and initial construct validity of the virtual reality version of the Multiple Errands Test based on NeuroVR software as an assessment tool for executive functions. In particular, the Multiple Errands Test is an assessment of executive functions in daily life which consists of tasks that abide by certain rules and is performed in a shopping mall-like setting where there are items to be bought and information to be obtained. The study population included three groups: post-stroke participants (n = 9), healthy young participants (n = 10), and healthy older participants (n = 10). The general purpose of the study was investigated through the following specific objectives: (1) to examine the relationships between the performance of three groups of participants in the Virtual Multiple Errands Test (VMET) and in the traditional neuropsychological tests employed to assess executive functions; and (2) to compare the performance of post-stroke participants to those of healthy young and older controls in the Virtual Multiple Errands Test and in the traditional neuropsychological tests employed to assess executive functions. Correlations between Virtual Multiple Errands Test variables and some traditional executive functions measures provide preliminary support for the ecological and construct validity of the VMET; further performance obtained at the Virtual Multiple Errands Test provided a distinction between the clinical and healthy population, and between the two age control groups. These results suggest a possible future application of such an ecological approach for cognitive assessment and rehabilitation of stroke patients and elderly population with age-related cognitive decline.

Vibrational properties of sp carbon atomic wires in cluster-assembled carbon films

A coherent approach to the description of double excitations in correlated materials is presented: We derive stringent mathematical conditions on the algebraical structure of the Bethe-Salpeter and time-dependent density functional theory kernels that avoid the occurrence of spurious and nonphysical excitations. We discuss how these conditions need to be respected at any level of approximation, including the commonly used local density and static screening approximations. We propose a correlated kernel for the Bethe-Salpeter equation, and we illustrate several aspects of our approach with numerical calculations for model molecular systems.

Dielectrics in a time-dependent electric field: A real-time approach based on density-polarization functional theory

A novel approach based on the merging of the out--of--equilibrium Greens function method with the ab-initio, Density--Functional--Theory is used to describe the ultra--fast carriers relaxation in Silicon. The results are compared with recent two photon photo--emission measurements. We show that the interpretation of the carrier relaxation in terms of L -> X inter--valley scattering is not correct. The ultra--fast dynamics measured experimentally is, instead, due to the scattering between degenerate