arXiv subjects
Publications and source records attributed to M. Monti.
The Upstream Tracker is a new, high-precision silicon strip detector installed in the LHCb experiment as part of the overall LHCb Upgrade I project. The detector is an integral part of the tracking system, and plays a crucial role in the reconstruction of charged particles in the new fully software-based trigger. An overview of the Upstream Tracker detector design, its commissioning, and performance are presented.
Ultrashort light pulses can drive transient photocurrents in solids, the basis of modern terahertz (THz) emitters. Ferroelectrics have recently emerged in this context as sources that require neither the bias voltage of photoconductive antennas nor the magnetic field of spintronic emitters, yet the ultrafast photocurrent generating the emission has remained debated. Here, we resolve this question in epitaxial BiFeO$_3$ thin films with engineered ferroelectric domain configurations, from single-domain to periodic stripe patterns. The stripe-domain film emits a THz field four times stronger than the single-domain configurations, reaching about half the output of a metallic spintronic reference emitter while remaining unsaturated at our highest excitation fluences. Resolving the emitted THz field in amplitude and phase as a function of the pump light polarization and the crystal orientation, we separate the coexisting photocurrents by their symmetry. This identifies the ultrafast screening of the ferroelectric polarization by photocarriers as the dominant emission mechanism in the in-plane-polarized films, enhanced in multidomain films by the strong built-in fields at the domain walls, while an out-of-plane polarization favors the bulk photovoltaic current. Our results establish the ferroelectric domain configuration as a design parameter setting both the strength and the microscopic origin of light-driven THz emission.
Among the global efforts to directly detect dark matter, the only positive claim so far relies on NaI(Tl) crystal detectors, making this technology of particular interest. ASTAROTH is a project aimed at developing the next generation of such detectors by reading out their scintillation light with SiPM matrices operated at cryogenic temperatures. This paper describes the innovative design of the ASTAROTH cryostat, consisting of a double-walled copper-steel cryogenic chamber that cools the detectors by means of a liquid argon bath. The detectors are thermalized in a helium atmosphere at a temperature tunable from 87 to 150 K. The design has been validated in terms of heat transfer efficiency and mechanical stress, developing a nonlinear multiphysics model. The mechanical properties of OFHC copper were experimentally evaluated on dedicated tensile samples. The simulation results show that the structural integrity is guaranteed. At the highest operating temperature, the region with the steepest temperature gradient exhibits stresses that slightly exceed the yield strength of copper (localized strain-hardened condition). Following construction, the cryostat was commissioned and has been in regular operation for over 30 cooling cycles, with no signs of degradation. The temperature can be tuned across the full target range and remains stable within 0.1 K. These results demonstrate that this is a viable design for next-generation dark matter detectors, as well as for a variety of applications requiring uniform and tunable gas-conducted cooling of instrumentation.
High quality stoichiometric magnetite (Fe3O4) films grown by infrared pulsed laser deposition (IR-PLD) on different surfaces were investigated in order to study the influence of the substrate, orientation, and thickness on their magnetic behavior. Different single crystal (001)-oriented substrates, i.e., SrTiO3(001), MgAl2O4(001) and MgO(001), have been used for the preparation of epitaxial Fe3O4(001) films. By comparison, polycrystalline magnetite films were obtained on both single crystal Al2O3(0001) and amorphous Si/SiO2 substrates. The thickness has been varied between 50 - 400 nm. All films consist of nanocrystalline stoichiometric magnetite with very small strain (<1\%) and present the Verwey transition (Tv) between 110-120 K, i.e., close to bulk magnetite (122 K). In general, Tv depends on both microstructure and thickness, increasing mainly as the thickness increases. Room temperature angular-dependent measurements reveal an in-plane fourfold symmetry magnetic behavior for all films grown on (001)-oriented surfaces, and with the easy axes lying along the Fe3O4 [010] and [100] directions. Remarkably, the fourfold magnetic symmetry shows up to 400 nm thick films. In turn, the films grown on single crystal Al2O3 (0001) and on amorphous Si/SiO2 surfaces display an isotropic magnetic behavior. Coercive field (Hc) depends on microstructure and film thickness. The largest (lowest) Hc value corresponds to the thinner film grown on a single crystal SrTiO3(001) (amorphous Si/SiO2) surface, which present the largest (lowest) strain (crystallinity). Moreover, the coercivity follows an inverse law with film thickness. Our results demonstrate that we can artificially control the magnetic behavior of stoichiometric IR-PLD grown Fe3O4 films by exploiting substrate-induced anisotropy and thickness-controlled coercivity, that might be relevant to incorporate magnetite in future spintronic devices.
We report on the R\&D for a first prototype of a silicon tracker based on an alternative approach for fast track finding. The working principle is inspired from neurobiology, in particular by the processing of visual images by the brain as it happens in nature. It is based on extensive parallelisation of data distribution and pattern recognition. In this work we present the design of a practical device that consists of a telescope based on single-sided silicon detectors; we describe the data acquisition system and the implementation of the track finding algorithms using available digital logic of commercial FPGA devices. Tracking performance and trigger capabilities of the device are discussed along with perspectives for future applications.
The mechanisms of magnesium hydride formation and thermal decomposition are directly examined using in-situ imaging.