arXiv subjects

Alessandro Ferretti

Publications and source records attributed to Alessandro Ferretti.

5 recordsLinked to original sources

arXiv2025

Excitonic optical interface for GHz-THz collective excitations in a van der Waals magnet

Collective spin and lattice excitations in quantum materials span energy scales from GHz to THz, yet establishing a unified optical interface for these modes remains a central challenge. Here we show that excitonic resonances in the van der Waals antiferromagnet CrSBr provide a broadband optical interface for such excitations. Using femtosecond broadband transient reflectivity, we resolve coherent GHz magnon and THz phonon modes that modulate the dielectric response over a wide spectral range. Despite their distinct microscopic origin and frequency scales, both excitations give rise to the same emergent optical signature: a resonance at 1.46 eV that is absent in steady-state spectra and exhibits a characteristic {\pi}-phase inversion, identifying it as a discrete excitonic transition. We attribute this behaviour to boson-driven modulation of the dielectric response, which transiently transfers spectral weight from a nominally dark exciton into an observable channel without requiring a finite equilibrium oscillator strength. Supported by many-body calculations, we assign this feature to a higher-energy exciton with distinct momentum and orbital character and strongly suppressed optical matrix elements. These results establish excitonic resonances in van der Waals magnets as a platform for interfacing collective excitations across GHz, THz and optical frequency scales.

cond-mat.mtrl-sci↗

arXiv2024

Experience gained about Resistive Plate Chambers ageing from the ALICE Muon TRigger/IDentifier detector

The ALICE Muon IDentifier is composed of 72 single-gap bakelite Resistive Plate Chambers, which have been operational since 2009 in maxi-avalanche mode (discrimination threshold:7 mV without amplification) with a Tetrafluoroethane/Isobutane/Sulfur Hexafluoride gas mixture, undergoing counting rates of the order of tens of Hz/cm^2. In this talk, the long-term performance and stability of the RPC system will be discussed, in terms of efficiency, dark current and dark rate. An assessment of potential signs of ageing observed on the detectors will be presented, together with a summary of the most common hardware problems experienced.

physics.ins-det↗

arXiv2021

Studies on tetrafluoropropene-CO2 based gas mixtures for the Resistive Plate Chambers of the ALICE Muon IDentifier

Due to their simplicity and comparatively low cost Resistive Plate Chambers are gaseous detectors widely used in high-energy and cosmic rays physics when large detection areas are needed. However, the best gaseous mixtures are currently based on tetrafluoroethane, which has the undesirable characteristic of a large Global Warming Potential (GWP) of about 1400 and because of this, it is currently being phased out from industrial use. As a possible replacement, tetrafluoropropene (which has a GWP close to 1) has been taken into account. Since tetrafluoropropene is more electronegative than tetrafluoroethane, it has to be diluted with gases with a lower attachment coefficient in order to maintain the operating voltage close to 10 kV. One of the main candidates for this role is carbon dioxide. In order to ascertain the feasibility and the performance of tetrafluoropropene-CO2 based mixtures, an R&D program is being carried out in the ALICE collaboration, which employs an array of 72 Bakelite RPCs (Muon Identifier, MID) to identify muons. Different proportions of tetrafluoropropene and CO2, with the addition of small quantities of isobutane and sulphur hexafluoride, have been tested with 50x50 cm2 RPC prototypes with 2 mm wide gas gap and 2 mm thick Bakelite electrodes. In the presentation, results from tests with cosmic rays will be presented, together with data concerning the current drawn by a RPC exposed to the gamma-ray flux of the Gamma Irradiation Facility (GIF) at CERN.

physics.ins-det↗

arXiv2021

Electron Collision Cross Sections in Tetrafluoropropene HFO1234ze(E) for Gas Mixtures in Resistive Plate Chambers

In recent years, there has been growing interest in tetrafluoropropene HFO1234ze(E) (C$_{3}$H$_{2}$F$_{4}$) for Resistive Plate Chambers (RPCs). This novel gas is considered a promising alternative to the standard mixtures currently used in RPCs, thanks to its low global warming potential. The knowledge of electron collision cross sections in C$_{3}$H$_{2}$F$_{4}$ enables reliable predictions of electron transport coefficients and reaction rates in C$_{3}$H$_{2}$F$_{4}$-based gas mixtures. This allows for optimizing the C$_{3}$H$_{2}$F$_{4}$-based gas mixtures to achieve the desired performance in RPCs. From measurements of electron transport coefficients and reaction rates, a complete set of scattering cross sections for electrons in C$_{3}$H$_{2}$F$_{4}$ has been derived. Validation of the electron collision cross sections is achieved through systematic comparisons of electron swarm parameters with experimental data in both pure C$_{3}$H$_{2}$F$_{4}$ and C$_{3}$H$_{2}$F$_{4}$/CO$_{2}$ gas mixtures. Given the influence of electron attachment in C$_{3}$H$_{2}$F$_{4}$ by the gas density, this work also includes precise calculations of the critical electric field strength in such mixtures. This set of cross sections has been further utilized to compute the effective ionization Townsend coefficient in gas mixtures containing C$_{3}$H$_{2}$F$_{4}$, potentially applicable for RPCs.

physics.plasm-ph↗

arXiv2020

Performance and aging studies for the ALICE muon RPCs

The ALICE muon trigger (MTR) system consists of 72 Resistive Plate Chamber (RPC) detectors arranged in two stations, each composed of two planes with 18 RPCs per plane. The detectors are operated in maxi-avalanche mode using a mixture of 89.7% C$_2$H$_2$F$_4$, 10% i-C$_4$H$_{10}$ and 0.3% SF$_6$. A number of detector performance indicators, such as efficiency and dark current, have been monitored over time throughout the LHC Run2 (2015-18). While the efficiency showed very good stability, a steady increase in the absorbed dark current was observed. Since the end of 2018, the LHC has entered a phase of long shutdown, during which the ALICE experiment will be upgraded to cope with the next phase of data taking, expected in 2021. The MTR is undergoing a major upgrade of the front-end and readout electronics, and will change its functionalities, becoming a Muon Identifier. Only the replacement of the most irradiated RPCs is planned during the upgrade. It is therefore important to perform dedicated studies to gain further insights into the status of the detector. In particular, two RPCs were flushed with pure Ar gas for a prolonged period of time and a plasma was created by fully ionizing the gas. The output gas was analyzed using a Gas Chromatograph combined with a Mass Spectrometer and the possible presence of fluorinated compounds originating from the interaction of the plasma with the inner surfaces of the detector has been assessed using an Ion-Selective Electrode station. This contribution will include a detailed review of the ALICE muon RPC performance at the LHC. The procedure and results of the argon plasma test, described above, are also discussed.

physics.ins-det↗