Polytechnic University of Milan

About: Polytechnic University of Milan is a education organization based out in Milan, Italy. It is known for research contribution in the topics: Finite element method & Population. The organization has 18231 authors who have published 58416 publications receiving 1229711 citations. The organization is also known as: PoliMi & L-NESS.

Nonequilibrium dynamics of photoexcited electrons in graphene : collinear scattering, Auger processes, and the impact of screening

Abstract: We present a combined analytical and numerical study of the early stages (sub-100-fs) of the nonequilibrium dynamics of photoexcited electrons in graphene. We employ the semiclassical Boltzmann equation with a collision integral that includes contributions from electron-electron (e-e) and electron–optical phonon interactions. Taking advantage of circular symmetry and employing the massless Dirac fermion (MDF) Hamiltonian, we are able to perform an essentially analytical study of the e-e contribution to the collision integral. This allows us to take particular care of subtle collinear scattering processes—processes in which incoming and outgoing momenta of the scattering particles lie on the same line—including carrier multiplication (CM) and Auger recombination (AR). These processes have a vanishing phase space for two-dimensional MDF bare bands. However, we argue that electron-lifetime effects, seen in experiments based on angle-resolved photoemission spectroscopy, provide a natural pathway to regularize this pathology, yielding a finite contribution due to CM and AR to the Coulomb collision integral. Finally, we discuss in detail the role of physics beyond the Fermi golden rule by including screening in the matrix element of the Coulomb interaction at the level of the random phase approximation (RPA), focusing in particular on the consequences of various approximations including static RPA screening, which maximizes the impact of CM and AR processes, and dynamical RPA screening, which completely suppresses them.

Numerical modeling of 1D arterial networks coupled with a lumped parameters description of the heart

Abstract: The investigations on the pressure wave propagation along the arterial network and its relationships with vascular physiopathologies can be supported nowadays by numerical simulations. One dimensional (1D) mathematical models, based on systems of two partial differential equations for each arterial segment suitably matched at bifurcations, can be simulated with low computational costs and provide useful insights into the role of wave reflections. Some recent works have indeed moved in this direction. The specific contribution of the present paper is to illustrate a 1D numerical model numerically coupled with a model for the heart action. Typically, the action of the heart on the arterial system is modelled as a boundary condition at the entrance of the aorta. However, the left ventricle (LV) and the vascular network are a strongly coupled single mechanical system. This coupling can be relevant in the numerical description of pressure waves propagation, particularly when dealing with pathological situations. In this work, we propose a simple lumped parameter model for the heart and show how it can be coupled numerically with a 1D model for the arteries. Numerical results actually confirm the relevant impact of the heart-arteries coupling in realistic simulations.

Three-photon bosonic coalescence in an integrated tritter.

Abstract: The main features of quantum mechanics reside in interference deriving from the superposition of different quantum states. While current quantum optical technology enables two-photon interference both in bulk and integrated systems, simultaneous interference of more than two particles, leading to richer quantum phenomena, is still a challenging task. Here we report the experimental observation of three-photon interference in an integrated three-port directional coupler realized by ultrafast laser writing. By exploiting the capability of this technique to produce three-dimensional structures, we realized and tested in the quantum regime a three-port beam splitter, namely a tritter, which allowed us to observe bosonic coalescence of three photons. These results open new important perspectives in many areas of quantum information, such as fundamental tests of quantum mechanics with increasing number of photons, quantum state engineering, quantum sensing and quantum simulation. For the development and application of quantum technologies, devices capable of implementing more than two-photon processes are vital. Towards this aim, Spagnolo et al.build a three-port beam splitter and demonstrate mutual interference between the three photons.