arXiv subjects

F. Ercolessi

Publications and source records attributed to F. Ercolessi.

15 recordsLinked to original sources

arXiv2026

Electron identification and hadron discrimination using Cherenkov radiation in air and SiPMs

This paper presents a method to identify electrons using the Cherenkov light emitted when a charged particle travels in air and photons are detected with a Silicon PhotoMultiplier (SiPM). The analysis is based on a photon-counting approach using SPAD cells and uses data collected during a test beam at CERN PS. The results are well described by a simple Monte Carlo simulation, which further demonstrates that a very good electron identification and a strong pion/hadron rejection could be obtained over a wide momentum range.

hep-ex↗

arXiv2025

Measurements of efficiency, timing and response to irradiation for direct detection of charged particles with SiPMs

In this paper the efficiency of direct charged particle detection with different Silicon PhotoMultiplier (SiPM) sensors has been measured to be close to 100%. Time resolution of about 20 ps has also been confirmed for sensors with an active area of around 3x3 mm$^\text{2}$ and a single-cell area of 40 $\mu$m$^\text{2}$. In addition, the SiPM performance after irradiation, in terms of timing response and dark count rate, has been evaluated for sensors with a 1x1 mm$^\text{2}$ area, demonstrating that SiPMs can maintain excellent timing capabilities and a low dark count rate when an appropriate threshold is applied to the signal.

physics.ins-det↗

arXiv2023

Measurements of the Cherenkov effect in direct detection of charged particles with SiPMs

In this paper, different Silicon PhotoMultiplier (SiPM) sensors have been tested with charged particles to characterize the Cherenkov light produced in the sensor protection layer. A careful position scan of the SiPM response has been performed with different prototypes, confirming the large number of firing cells and proving almost full efficiency, with the SiPM filling factor essentially negligible. This study also allowed us to study the time resolution of such devices as a function of the number of firing cells, reaching values below 20 ps. These measurements provide significant insight into the capabilities of SiPM sensors in direct detection of charged particles and their potential for several applications.

physics.ins-det↗

arXiv2022

Understanding the direct detection of charged particles with SiPMs

In this paper evidence that the increased response of SiPM sensors to the passage of charged particles is related mainly to Cherenkov light produced in the protection layer is reported. The response and timing properties of sensors with different protection layers have been studied.

physics.ins-det↗

arXiv2022

Direct detection of charged particles with SiPMs

The direct response of Silicon PhotoMultipliers being traversed by a MIP charged particle have been studied in a systematic way for the first time. Using beam test data, time resolution and the crosstalk probability have been measured. A characterization of the SiPM by means of a laser beam is also reported. The results obtained for different sensors indicate a measured time resolution around 40-70 ps. Although particles are expected to traverse only one SPAD per event, crosstalk measurements on different sensors indicate an unexpected higher value with respect to the one related to the sensor noise.

physics.ins-det↗

arXiv2005

Melting and nonmelting of solid surfaces and nanosystems

We present an extensive but concise review of our present understanding, largely based on theory and simulation work from our group, on the equilibrium behavior of solid surfaces and nanosystems close to the bulk melting point. In the first part we define phenomena, in particular surface melting and nonmelting, and review some related theoretical approaches, from heuristic theories to computer simulation. In the second part we describe the surface melting/nonmelting behavior of several different classes of solids, ranging from van der Waals crystals, to valence semiconductors, to ionic crystals and metals. In the third part, we address special cases such as strained solids, the defreezing of glass surfaces, and rotational surface melting. Next, we digress briefly to surface layering of a liquid metal, possibly leading to solid-like or hexatic two dimensional phases floating on the liquid. In the final part, the relationship of surface melting to the premelting of nanoclusters and nanowires is reviewed.

cond-mat.mtrl-sci↗

arXiv2002

Bending strain-driven modification of surface reconstructions: Au(111)

Strain can affect the morphology of a crystal surface, and cause modifications of its reconstruction even when weak, as in the case of mechanical bending. We carried out calculations of strain-dependent surface free energy and direct bending simulations demonstrating the change of incommensurate reconstruction in Au(111) under strain, in good agreement with recent data. Time-dependent strain should cause a sliding of the topmost layer over the second, suggesting an interesting case of nanofriction. Bending strain could also be used to fine tune the spacing of selectively absorbed nanoclusters.

cond-mat.mtrl-sci↗

arXiv2000

Surface molecular dynamics simulation with two orthogonal surface steps: how to beat the particle conservation problem

Due to particle conservation, Canonical Molecular Dynamics (MD) simulations fail in the description of surface phase transitions involving coverage or lateral density changes. However, a step on the surface can act effectively as a source or a sink of atoms, in the simulation as well as in real life. A single surface step can be introduced by suitably modifying planar Periodic Boundary Conditions (PBC), to accommodate the generally inequivalent stacking of two adjacent layers. We discuss here how, through the introduction of two orthogonal surface steps, particle number conservation may no longer represent a fatal constraint for the study of these surface transitions. As an example, we apply the method for estimating temperature-induced lateral density increase of the reconstructed Au (001) surface; the resulting anisotropic cell change is consistent with experimental observations. Moreover, we implement this kind of scheme in conjunction with the variable curvature MD method, recently introduced by our group.

cond-mat.mtrl-sci↗

arXiv2000

(Meta-)stable reconstructions of the diamond(111) surface: interplay between diamond- and graphite-like bonding

Off-lattice Grand Canonical Monte Carlo simulations of the clean diamond (111) surface, based on the effective many-body Brenner potential, yield the $(2\times1)$ Pandey reconstruction in agreement with \emph{ab-initio} calculations and predict the existence of new meta-stable states, very near in energy, with all surface atoms in three-fold graphite-like bonding. We believe that the long-standing debate on the structural and electronic properties of this surface could be solved by considering this type of carbon-specific configurations.

cond-mat.mtrl-sci↗

arXiv2000

Realistic simulations of Au(100): Grand Canonical Monte Carlo and Molecular Dynamics

The large surface density changes associated with the (100) noble metals surface hex-reconstruction suggest the use of non-particle conserving simulation methods. We present an example of a surface Grand Canonical Monte Carlo applied to the transformation of a square non reconstructed surface to the hexagonally covered low temperature stable Au(100). On the other hand, classical Molecular Dynamics allows to investigate microscopic details of the reconstruction dynamics, and we show, as an example, retraction of a step and its interplay with the surface reconstruction/deconstruction mechanism.

cond-mat.mtrl-sci↗

arXiv2000

Islands, craters, and a moving surface step on a hexagonally reconstructed (100) noble metal surface

Deposition/removal of metal atoms on the hex reconstructed (100) surface of Au, Pt and Ir should present intriguing aspects, since a new island implies hex -> square deconstruction of the substrate, and a new crater the square -> hex reconstruction of the uncovered layer. To obtain a microscopic understanding of how islands/craters form in these conditions, we have conducted simulations of island and crater growth on Au(100), whose atomistic behavior, including the hex reconstruction on top of the square substrate, is well described by mean s of classical many-body forces. By increasing/decreasing the Au coverage on Au(100), we find that island/craters will not grow unless they exceed a critical size of about 8-10 atoms. This value is close to that which explains the nonlinear coverage dependence observed in molecular adsorption on the closely related surface Pt (100). This threshold size is rationalized in terms of a transverse step correlation length, measuring the spatial extent where reconstruction of a given plane is disturbed by the nearby step.

cond-mat.mtrl-sci↗

arXiv2000

Reentrant layering in rare gas adsorption: preroughening or premelting?

The reentrant layering transitions found in rare gas adsorption on solid substrates have conflictually been explained either in terms of preroughening (PR), or of top layer melting-solidification phenomena. We obtain adsorption isotherms of Lennard-Jones particles on an attractive substrate by off lattice Grand Canonical Monte Carlo (GCMC) simulation, and reproduce reentrant layering. Microscopic analysis, including layer-by-layer occupancies, surface diffusion and pair correlations, confirms the switch of the top surface layer from solid to quasi-liquid across the transition temperature. At the same time, layer occupancy is found at each jump to switch from close to full to close to half, indicating a disordered flat (DOF) surface and establishing preroughening as the underlying mechanism. Our results suggest that top layer melting is essential in triggering preroughening, which thus represents the threshold transition to surface melting in rare gas solids.

cond-mat.mtrl-sci↗

arXiv1998

The Puzzling Stability of Monatomic Gold Wires

We have examined theoretically the spontaneous thinning process of tip-suspended nanowires, and subsequently studied the structure and stability of the monatomic gold wires recently observed by Transmission Electron Microscopy (TEM). The methods used include thermodynamics, classical many-body force simulations, Local Density (LDA) and Generalized Gradient (GGA) electronic structure calculations as well as ab-initio simulations including the two tips. The wire thinning is well explained in terms of a thermodynamic tip suction driving migration of surface atoms from the wire to the tips. For the same reason the monatomic wire becomes progressively stretched. Surprisingly, however, all calculations so far indicate that the stretched monatomic gold wire should be unstable against breaking, contrary to the apparent experimental stability. The possible reasons for the observed stability are discussed.

cond-mat.mtrl-sci↗

arXiv1994

Premelting of Thin Wires

Recent work has raised considerable interest on the nature of thin metallic wires. We have investigated the melting behavior of thin cylindrical Pb wires with the axis along a (110) direction, using molecular dynamics and a well-tested many-body potential. We find that---in analogy with cluster melting---the melting temperature $T_m (R)$ of a wire with radius $R$ is lower than that of a bulk solid, $T_m^b$, by $T_m (R) = T_m^b -c/R$. Surface melting effects, with formation of a thin skin of highly diffusive atoms at the wire surface, is observed. The diffusivity is lower where the wire surface has a flat, local (111) orientation, and higher at (110) and (100) rounded areas. The possible relevance to recent results on non-rupturing thin necks between an STM tip and a warm surface is addressed.

cond-mat↗

arXiv1992

Multiple jumps and vacancy diffusion in a face-centered cubic metal

The diffusion of monovacancies in gold has been studied by computer simulation. Multiple jumps have been found to play a central role in the atomic dynamics at high temperature, and have been shown to be responsible for an upward curvature in the Arrhenius plot of the diffusion coefficient. Appropriate saddle points on the potential energy surface have been found, supporting the interpretation of vacancy multiple jumps as distinct migration mechanisms.

cond-mat↗