Built-in reduction of statistical fluctuations of partitioning objects

Our theoretical and numerical investigation of the movement of an object that partitions a microtubule filled with small particles indicates that vibrations warranted by thermal equilibrium are reached only after a time that increases exponentially with the number of particles involved. This points to a basic mechanical process capable of breaching, on accessible time scales, the ultimate ergodic constraints that force randomness on bound microscale and nanoscale systems

Photoemission Spectra from Reduced Density Matrices: the Band Gap in Strongly Correlated Systems

We present a method for the calculation of photoemission spectra in terms of reduced density matrices. We start from the spectral representation of the one-body Green's function G, whose imaginary part is related to photoemission spectra, and we introduce a frequency-dependent effective energy that accounts for all the poles of G. Simple approximations to this effective energy give accurate spectra in model systems in the weak as well as strong correlation regime. In real systems reduced density matrices can be obtained from reduced density-matrix functional theory. Here we use this approach to calculate the photoemission spectrum of bulk NiO: our method yields a qualitatively correct picture both in the antiferromagnetic and paramagnetic phases, contrary to mean-field methods, in which the paramagnet is a metal

Reduced Density-Matrix Functional Theory: correlation and spectroscopy

In this work we explore the performance of approximations to electron correlation in reduced density-matrix functional theory (RDMFT) and of approximations to the observables calculated within this theory. Our analysis focuses on the calculation of total energies, occupation numbers, removal/addition energies, and spectral functions. We use the exactly solvable Hubbard molecule at 1/4 and 1/2 filling as test systems. This allows us to analyze the underlying physics and to elucidate the origin of the observed trends. For comparison we also report the results of the $GW$ approximation, where the self-energy functional is approximated, but no further hypothesis are made concerning the approximations of the observables. In particular we focus on the atomic limit, where the two sites of the molecule are pulled apart and electrons localize on either site with equal probability, unless a small perturbation is present: this is the regime of strong electron correlation. In this limit, using the Hubbard molecule at 1/2 filling with or without a spin-symmetry-broken ground state, allows us to explore how degeneracies and spin-symmetry breaking are treated in RDMFT. We find that, within the used approximations, neither in RDMFT nor in $GW$ the signature of strong correlation are present in the spin-singlet ground state, whereas both give the exact result for the spin-symmetry broken case. Moreover we show how the spectroscopic properties change from one spin structure to the other. Our findings can be generalized to other situations, which allows us to make connections to real materials and experiment

A new approach for roughness representation within urban dispersion models

The effects of green infrastructure on pollutant concentrations are greatly variable, essentially depending on the surrounding built-up environment and on local meteorological conditions. To simulate the effects of the presence of trees at urban scale, a reliable methodology is the Computational Fluid Dynamics (CFD) approach, however it needs high calculation costs. An alternative integral dispersion model is given by provided that a suitable parameterization for vegetation is included. In this work, we have developed and demonstrated a novel methodology, based on aerodynamic parameters, to include the aerodynamic effect of trees in an operational dispersion model, the ADMS-Urban. The aerodynamic parameters were derived using the morphometric method starting from open data containing information on buildings and trees. The new roughness parameter calculation method has produced the urban spatially varying roughness (USVR) and it was evaluated in different scenarios at the urban and neighborhood scale. The numerical outputs of the simulations were compared with observations from reference air quality stations collected within an ad-hoc intensive field campaign conducted in 2017 in the city of Bologna, Italy. The results of the comparison highlight that the introduction of the aerodynamic effects of buildings lead to great improvements in the performance of the model at both spatial scales and for the different study sites considered in this study. Conversely, the inclusion of trees in the calculation produces significant improvements only when conducting studies at high spatial resolution and for densely vegetated areas

Reduced statistical fluctuations of the position of an object partitioning in two its environment

Through hard‐disk simulations and theoretical considerations on the movement of an object that partitions a microtubule filled with small particles, we find that the vibrations typical of thermal equilibrium are reached after a time that increases exponentially with the number of particles involved. The result is a mechanism capable of breaching, on accessible time scales, the ergodic constraints in nano‐scale systems

Characteristic Scales for Turbulent Exchange Processes in a Real Urban Canopy

AbstractAn experimental field campaign is designed to unveil mechanisms responsible for turbulent exchange processes when mechanical and thermal effects are entwined. The focus is an urban street canyon with a mean aspect ratio H/W of 1.65 in the business centre of a mid-size Italian city (H is the mean building height and W is the mean canyon width). The exchange processes can be characterized by time scales and time-scale ratios specific to either mechanical or thermal process. Time scales describe the mixing caused by momentum and heat exchange within different canyon layers, while their rates are surrogates of their efficacy. Given that homogeneous mixing does not always occur within the canyon, several time scales are estimated at different levels, showing that mechanical and thermal processes may both contribute to enhance mixing. By computing mechanical time scales, it is found that the fastest mixing occurs at the canyon rooftop level for perpendicular or oblique wind directions, while slow mixing occurs for parallel directions. Thermal processes are faster than the mechanical ones and are particularly efficient for perpendicular wind directions. By calculating the time-scale ratios, exchange processes are found to facilitate mixing for most wind directions and to regulate the pollutant-concentration variability in the canyon. This variability can be associated with the local-circulation regime, demarcated as thermally driven or inertially driven using a buoyancy parameter, i.e., the ratio between thermal and inertial forcings. Using this approach, a generalization of the results is proposed, enabling the extension of the current investigation to different street-canyon aspect ratios

Lino Leonardi, Critica del testo, Firenze, Le Monnier Università, 2022

Recensione al manuale "Critica del testo" di Lino Leonardi