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Accurate channel modeling plays a pivotal role in optimizing communication systems, especially as new frequency bands come into play in next-generation networks. In this regard, fitting field measurements to stochastic models is crucial for capturing the key propagation features and to map these to achievable system performances. In this work, we shed light onto what's the most appropriate alternative for channel fitting, when the ultimate goal is performance analysis. Results show that average-error metrics should be used with caution, since they can largely fail to predict outage probability measures. We show that supremum-error fitting metrics with tail awareness are more robust to estimate both ergodic and outage performance measures, even when they yield a larger average-error fitting.

Silver chalcohalide antiperovskites (CAP), Ag$_{3}$XY (X = S, Se; Y = Br, I), are a family of highly anharmonic inorganic compounds with great potential for energy applications. However, a substantial and unresolved discrepancy exists between the optoelectronic properties predicted by theoretical first-principles methods and those measured experimentally at room temperature, hindering the fundamental understanding and rational engineering of CAP. In this work, we employ density functional theory, tight-binding calculations, and anharmonic Fr\"ohlich theory to investigate the optoelectronic properties of CAP at finite temperatures. Near room temperature, we observe a giant band-gap ($E_{g}$) reduction of approximately $20$-$60$\% relative to the value calculated at $T = 0$ K, bringing the estimated $E_{g}$ into excellent agreement with experimental measurements. This relative $T$-induced band-gap renormalization is roughly twice the largest value previously reported in the literature for similar temperature ranges. Low-energy optical polar phonon modes, which break inversion symmetry and promote the overlap between silver and chalcogen $s$ electronic orbitals in the conduction band, are identified as the primary contributors to this giant $E_{g}$ reduction. Furthermore, when considering temperature effects, the optical absorption coefficient of CAP increases by nearly an order of magnitude for visible light frequencies. These insights not only bridge a crucial gap between theory and experiment but also open pathways for future technologies where temperature, electric fields, or light dynamically tailor optoelectronic behavior, positioning CAP as a versatile platform for next-generation energy applications., Comment: 13 pages, 6 figures

The fifth generation (5G) of mobile communications relies on extremely high data transmissions using a large variety of frequency bands, such as FR1 (sub-6 GHz) and FR2 (mmWave). Future mobile communications envisage using electromagnetic spectrum beyond FR2, i.e. above 100 GHz, known as sub-THz band. These new frequencies open up challenging scenarios where communications shall rely on a major contribution such as the line-of-sight (LoS) component. To the best of the authors' knowledge, for the first time in literature this work studies the human blockage effects over an extremely wide frequency band from 75 GHz to 215 GHz given: (i) the distance between the blocker and the antennas and (ii) the body orientation. Furthermore, the obtained results are modeled with the classical path loss models and compared to 3GPP alternatives. The average losses increase from 42 dB to 56 dB when frequency rises from 75 GHz to 215 GHz. In terms of distance, a 18 dB increment in the received power is found when the Tx--Rx separation is increased from 1 m to 2.5 m. Finally, the blocker orientation induces variations of up to 4.6 dB.

Materials with on-demand control of thermal conductivity are the prerequisites to build thermal conductivity switches, where the thermal conductivity can be turned ON and OFF. However, the ideal switch, while required to develop novel approaches to solid-state refrigeration, energy harvesting, and even phononic circuits, is still missing. It should consist of an active material only, be environment friendly, and operate near room temperature with a reversible, fast, and large switching ratio. Here, we first predict by ab initio electronic structure calculations that ferroelectric domains in barium titanate exhibit anisotropic thermal conductivities. We confirm this prediction by combining frequency-domain thermoreflectance and scanning thermal microscopy measurements on a single crystal of barium titanate. We then use this gained knowledge to propose a lead-free thermal conductivity switch without inactive material, operating reversibly with an electric field. At room temperature, we find a switching ratio of 1.6 $\pm$ 0.3, exceeding the performances of state-of-the-art materials suggested for thermal conductivity switches.

Hybrid organic-inorganic perovskites (HOIP) have emerged in recent years as highly promising semiconducting materials for a wide range of optoelectronic and energy applications. Nevertheless, the rotational dynamics of the organic components and many molecule interdependencies, which may strongly impact the functional properties of HOIP, are not yet fully understood. In this study, we quantitatively analyze the orientational disorder and molecular correlations in the archetypal perovskite CH$_{3}$NH$_{3}$PbI$_{3}$ (MAPI) by performing comprehensive molecular dynamics simulations and entropy calculations. We found that, in addition to the usual vibrational and orientational contributions, rigid molecular rotations around the C-N axis and correlations between neighboring molecules noticeably contribute to the entropy increment associated with the temperature-induced order-disorder phase transition in MAPI, $\Delta S_{t}$. Molecular conformational changes are equally infrequent in the low-$T$ ordered and high-$T$ disordered phases and have a null effect on $\Delta S_{t}$. Conversely, the couplings between the angular and vibrational degrees of freedom are substantially reinforced in the high-$T$ disordered phase and significantly counteract the phase-transition entropy increase resulting from other factors. Furthermore, the tendency for neighboring molecules to be orientationally ordered is markedly local, consequently inhibiting the formation of extensive polar nanodomains both at low and high temperatures. This theoretical investigation not only advances the fundamental knowledge of HOIP but also establishes physically insightful connections with contemporary technological applications like photovoltaics and solid-state cooling., Comment: 15 pages, 10 figures

Silver-based chalcohalide anti-perovskites (CAP), Ag$_{3}$BC (B = S, Se; C = Cl, Br, I), represent an emerging family of energy materials with intriguing optoelectronic, vibrational and ionic transport properties. However, the structural features and phase stability of CAP remain poorly investigated to date, hindering their fundamental understanding and potential integration into technological applications. Here we employ theoretical first-principles methods based on density functional theory to fill this knowledge gap. Through crystal structure prediction techniques, ab initio molecular dynamics simulations, and quasi-harmonic free energy calculations, we unveil a series of previously overlooked energetically competitive phases and temperature-induced phase transitions for all CAP. Specifically, we identify a new cubic $P2_{1}3$ structure as the stable phase of all CAP containing S both at zero temperature and $T \neq 0$ K conditions. Consequently, our calculations suggest that the cubic $Pm\overline{3}m$ phase identified in room-temperature X-ray diffraction experiments is likely to be metastable. Furthermore, for CAP containing Se, we propose different orthorhombic ($Pca2_{1}$ and $P2_{1}2_{1}2_{1}$) and cubic ($I2_{1}3$) structures as the ground-state phases and reveal several phase transformations induced by temperature. This theoretical investigation not only identifies new candidate ground-state phases and solid-solid phase transformations for all CAP but also provides insights into potential stability issues affecting these highly anharmonic superionic materials., Comment: 15 pages, 12 figures

We introduce and characterize the operational diversity order (ODO) in fading channels, as a proxy to the classical notion of diversity order at any arbitrary operational signal-to-noise ratio (SNR). Thanks to this definition, relevant insights are brought up in a number of cases: (i) We quantify that in dominant line-of-sight scenarios an increased diversity order is attainable compared to that achieved asymptotically, even in the single-antenna case; (ii) this effect is attenuated, but still visible, in the presence of an additional dominant specular component; (iii) the decay slope in Rayleigh product channels increases very slowly, never fully achieving unitary slope for a finite SNR.

Perfect error correcting codes allow for an optimal transmission of information while guaranteeing error correction. For this reason, proving their existence has been a classical problem in both pure mathematics and information theory. Indeed, the classification of the parameters of $e-$error correcting perfect codes over $q-$ary alphabets was a very active topic of research in the late 20th century. Consequently, all parameters of perfect $e-$error correcting codes were found if $e \ge 3$, and it was conjectured that no perfect $2-$error correcting codes exist over any $q-$ary alphabet, where $q > 3$. In the 1970s, this was proved for $q$ a prime power, for $q = 2^r3^s$ and for only $7$ other values of $q$. Almost $50$ years later, it is surprising to note that there have been no new results in this regard and the classification of $2-$error correcting codes over non-prime power alphabets remains an open problem. In this paper, we use techniques from the resolution of generalised Ramanujan--Nagell equation and from modern computational number theory to show that perfect $2-$error correcting codes do not exist for $172$ new values of $q$ which are not prime powers, substantially increasing the values of $q$ which are now classified. In addition, we prove that, for any fixed value of $q$, there can be at most finitely many perfect $2-$error correcting codes over an alphabet of size $q$., Comment: 12 pages, 2 tables. The new version includes the comments by the anonymous referees

In this paper, we study the integer solutions of a family of Fermat-type equations of signature $(2, 2n, n)$, $Cx^2 + q^ky^{2n} = z^n$. We provide an algorithmically testable set of conditions which, if satisfied, imply the existence of a constant $B_{C, q}$ such that if $n > B_{C,q}$, there are no solutions $(x, y, z, n)$ of the equation. Our methods use the modular method for Diophantine equations, along with level lowering and Galois theory., Comment: 20 pages

Solid-state cooling presents an energy-efficient and environmentally friendly alternative to traditional refrigeration technologies that rely on thermodynamic cycles involving greenhouse gases. However, conventional caloric effects face several challenges that impede their practical application in refrigeration devices. Firstly, operational temperature conditions must align closely with zero-field phase transition points; otherwise, the required driving fields become excessively large. But phase transitions occur infrequently near room temperature. Additionally, caloric effects typically exhibit strong temperature dependence and are sizeable only within relatively narrow temperature ranges. In this study, we employ first-principles simulation methods to demonstrate that light-driven phase transitions in polar oxide perovskites have the potential to overcome such limitations. Specifically, for the prototypical ferroelectric KNbO$_{3}$ we illustrate the existence of giant photocaloric effects induced by light absorption ($\Delta S_{\rm PC} \sim 100$~J~K$^{-1}$~kg$^{-1}$ and $\Delta T_{\rm PC} \sim 10$~K) across a vast temperature range of several hundred Kelvin, encompassing room temperature. These findings are expected to be generalizable to other materials exhibiting similar polar behavior., Comment: 5 pages, 5 figures

This paper presents a maintenance policy for a modular system formed by K independent modules (n-subsystems) subjected to environmental conditions (shocks). For the modeling of this complex system, the use of the Matrix-Analytical Method (MAM) is proposed under a layered approach according to its hierarchical structure. Thus, the operational state of the system (top layer) depends on the states of the modules (middle layer), which in turn depend on the states of their components (bottom layer). This allows a detailed description of the system operation to plan maintenance actions appropriately and optimally. We propose a hierarchical decision-based maintenance strategy with periodic inspections as follows: at the time of the inspection, the condition of the system is first evaluated. If intervention is necessary, the modules are then checked to make individual decisions based on their states, and so on. Replacement or repair will be carried out as appropriate. An optimization problem is formulated as a function of the length of the inspection period and the intervention cost incurred over the useful life of the system. Our method shows the advantages, providing compact and implementable expressions. The model is illustrated on a submarine Electrical Control Unit (ECU)., Comment: 43 pages, 6 figures

Given a prime number $q$ and a squarefree integer $C_1$, we develop a method to explicitly determine the tuples $(y, n, \alpha)$ for which the difference $y^n-q^\alpha$ has squarefree part equal to $C_1$. Our techniques include the combination of the local information provided by Galois representations of Frey-Hellegouarch curves with the effective resolution of Thue-Mahler equations, as well as the use of improved lower bounds for $q-$adic and complex logarithms. As an application of this methodology, we will completely resolve the case when $1 \le C_1 \le 20$ and $ 2 \le q < 25$., Comment: 40 pages

We offer a Reply to Document arXiv:2303.12174 by M. Boninsegni et al., commenting on arXiv:2301.02854 .

Outflows are believed to be ubiquitous in all AGNs, although their presence in low luminosity AGNs, in particular, for LINERs, has only started to be explored. Their properties (geometry, mass and energetics) are still far from being properly characterised. We use integral field spectroscopic data from the MEGARA instrument, at GTC, to analyse a small sample of nine LINERs, candidates of hosting ionised gas outflows. We aim to study the main emission lines in the optical to identify their properties and physical origin. We obtained data cubes at the lowest (R$\sim$6000) and highest (R$\sim$20000) spectral resolution of MEGARA. We modelled and subtracted the stellar continuum to obtain the ionised gas contribution, and then fitted the emission lines to extract their kinematics (velocity and velocity dispersion). We identified outflows as a secondary component in the emission lines. The primary component of the emission lines was typically associated to gas in the galactic disc. For some objects, there is an enhanced-$\sigma$ region co-spatial with the secondary component. We associated it to turbulent gas produced due to the interaction with the outflows. We find signatures of outflows in six LINERs, with mass outflow rates ranging from 0.004 to 0.4 M$_{sun}$yr$^{-1}$ and energy rates from $\sim$10$^{38}$ to $\sim$10$^{40}$ erg s$^{-1}$. Their mean electronic density is 600cm$^{-3}$, extending to distances of $\sim$400 pc at an (absolute) velocity of $\sim$340 km s$^{-1}$ (on average). They tend to be compact and unresolved, although for some sources they are extended with a bubble-like morphology. Our results confirm the existence of outflows in the best LINER candidates identified using previous long-slit spectroscopic and imaging data. These outflows do not follow the scaling relations obtained for more luminous AGNs. For some objects we discuss jets as the main drivers of the outflows, Comment: 31 pages, 15 figures. Accepted for publication in Astronomy & Astrophysics

As new wireless standards are developed, the use of higher operation frequencies comes in hand with new use cases and propagation effects that differ from the well-established state of the art. Numerous stochastic fading models have recently emerged under the umbrella of generalized fading conditions to provide a fine-grain characterization of propagation channels in the mmWave and sub-THz bands. For the first time in literature, this work carries out an experimental validation of a class of such ray-based models in a wide range of propagation conditions (anechoic, reverberation and indoor scenarios) at mmWave bands. These models allow to characterize the communication channel with a reduced number of physically interpretable parameters. In specific, we show that the independent fluctuating two-ray (IFTR) model has good capabilities to recreate rather dissimilar environments with high accuracy and only four parameters. We also put forth that the key limitations of the IFTR model arise in the presence of reduced diffuse propagation, and also due to a limited phase variability for the dominant specular components., Comment: 14 pages, 13 figures, in IEEE Transactions on Wireless Communications, 2024

Safety-critical systems such as those in automotive, avionics and space, require appropriate safety measures to avoid silent data corruption upon random hardware errors such as those caused by radiation and other types of electromagnetic interference. Those safety measures must be able to prevent faults from causing the so-called common cause failures (CCFs), which occur when a fault produces identical errors in redundant elements so that comparison fails to detect the errors and a failure arises. The usual solution to avoid CCFs in CPU cores is using lockstep cores, so that two cores execute the same flow of instructions, but with some time staggering so that their state is never identical and faults can only lead to different errors, which are then detectable by means of comparison. This paper extends Gaisler's RISC-V NOEL-V core with lockstep; and presents future prospects for its use and distribution., Comment: Abstract presented at the RISC-V Summit, June 2023, Barcelona (Spain)

HPC (High Performance Computing) devices increasingly become the only alternative to deliver the performance needed in safety-critical autonomous systems (e.g., autonomous cars, unmanned planes) due to deploying large and powerful multicores along with accelerators such as GPUs. However, the support that those HPC devices offer to realize safety-critical systems on top is heterogeneous. Safety islands have been devised to be coupled to HPC devices and complement them to meet the safety requirements of an increased set of applications, yet the variety of concepts and realizations is large. This paper presents our own concept of a safety island with two goals in mind: (1) offering a wide set of features to enable the broadest set of safety applications for each HPC device, and (2) being realized with open source components based on RISC-V ISA to ease its use and adoption. In particular, we present our safety island concept, the key features we foresee it should include, and its potential application beyond safety., Comment: White paper

Due to its high energy efficiency and environmental friendliness, solid-state cooling based on the barocaloric (BC) effect represents a promising alternative to traditional refrigeration technologies relying on greenhouse gases. Plastic crystals displaying orientational order-disorder solid-solid phase transitions have emerged among the most gifted materials on which to realize the full potential of BC solid-state cooling. However, a comprehensive understanding of the atomistic mechanisms on which order-disorder BC effects are sustained is still missing, and rigorous and systematic methods for quantitatively evaluating and anticipating them have not been yet established. Here, we present a computational approach for the assessment and prediction of BC effects in orientationally disordered materials that relies on atomistic molecular dynamics simulations and emulates quasi-direct calorimetric BC measurements. Remarkably, the proposed computational approach allows for a precise determination of the partial contributions to the total entropy stemming from the vibrational and molecular orientational degrees of freedom. Our BC simulation method is applied on the technologically relevant material CH$_{3}$NH$_{3}$PbI$_{3}$ (MAPI), finding giant BC isothermal entropy changes ($|\Delta S_{\rm BC}| \sim 10$ J K$^{-1}$ kg$^{-1}$) under moderate pressure shifts of $\sim 0.1$ GPa. Intriguingly, our computational analysis of MAPI reveals that changes in the vibrational degrees of freedom of the molecular cations, not their reorientational motion, have a major influence on the entropy change that accompanies the order-disorder solid-solid phase transition., Comment: 13 pages, 8 figures

This special session paper introduces the Horizon Europe NEUROPULS project, which targets the development of secure and energy-efficient RISC-V interfaced neuromorphic accelerators using augmented silicon photonics technology. Our approach aims to develop an augmented silicon photonics platform, an FPGA-powered RISC-V-connected computing platform, and a complete simulation platform to demonstrate the neuromorphic accelerator capabilities. In particular, their main advantages and limitations will be addressed concerning the underpinning technology for each platform. Then, we will discuss three targeted use cases for edge-computing applications: Global National Satellite System (GNSS) anti-jamming, autonomous driving, and anomaly detection in edge devices. Finally, we will address the reliability and security aspects of the stand-alone accelerator implementation and the project use cases., Comment: 10 pages, 2 figures, conference

The accuracy of camera-based object detection (CBOD) built upon deep learning is often evaluated against the real objects in frames only. However, such simplistic evaluation ignores the fact that many unimportant objects are small, distant, or background, and hence, their misdetections have less impact than those for closer, larger, and foreground objects in domains such as autonomous driving. Moreover, sporadic misdetections are irrelevant since confidence on detections is typically averaged across consecutive frames, and detection devices (e.g. cameras, LiDARs) are often redundant, thus providing fault tolerance. This paper exploits such intrinsic fault tolerance of the CBOD process, and assesses in an automotive case study to what extent CBOD can tolerate approximation coming from multiple sources such as lower precision arithmetic, approximate arithmetic units, and even random faults due to, for instance, low voltage operation. We show that the accuracy impact of those sources of approximation is within 1% of the baseline even when considering the three approximate domains simultaneously, and hence, multiple sources of approximation can be exploited to build highly efficient accelerators for CBOD in cars.

Solid-state cooling applications based on the electrocaloric (EC) effect are particularly promising from a technological point of view due to their downsize scalability and natural implementation in circuitry. However, EC effects typically occur far from room temperature, involve materials that contain toxic substances and require relatively large electric fields ($\sim 100$-$1000$ kV cm$^{-1}$) that cause fateful leakage current and dielectric loss problems. Here, we propose a possible solution to these practical issues that consists in concertedly applying hydrostatic pressure and electric fields on lead-free multiferroic materials. We theoretically demonstrate this strategy by performing first-principles simulations on supertetragonal BiFe$_{1-x}$Co$_{x}$O$_{3}$ solid solutions (BFCO). It is shown that hydrostatic pressure, besides adjusting the occurrence of EC effects to near room temperature, can reduce enormously the intensity of the driving electric fields. For pressurized BFCO, we estimate a colossal room-temperature EC strength, defined like the ratio of the adiabatic EC temperature change by the applied electric field, of $\sim 1$ K cm kV$^{-1}$, a value that is several orders of magnitude larger than those routinely measured in uncompressed ferroelectrics., Comment: 10 pages, 5 figures

Solid-state methods for cooling and heating promise a more sustainable alternative to current compression cycles of greenhouse gases and inefficient fuel-burning heaters. Barocaloric effects (BCE) driven by hydrostatic pressure ($p$) are especially encouraging in terms of large adiabatic temperature changes ($|\Delta T| \sim 10$ K) and colossal isothermal entropy changes ($|\Delta S| \sim 100$ JK$^{-1}$kg$^{-1}$). However, BCE typically require large pressure shifts due to irreversibility issues, and sizeable $|\Delta T|$ and $|\Delta S|$ seldom are realized in a same material. Here, we demonstrate the existence of colossal and reversible BCE in LiCB$_{11}$H$_{12}$, a well-known solid electrolyte, near its order-disorder phase transition at $\approx 380$ K. Specifically, for $\Delta p \approx 0.23$ $(0.10)$ GPa we measured $|\Delta S_{\rm rev}| = 280$ $(200)$ JK$^{-1}$kg$^{-1}$ and $|\Delta T_{\rm rev}| = 32$ $(10)$ K, which individually rival with state-of-the-art barocaloric shifts obtained under similar pressure conditions. Furthermore, over a wide temperature range, pressure shifts of the order of $0.1$ GPa yield huge reversible barocaloric strengths of $\approx 2$ JK$^{-1}$kg$^{-1}$MPa$^{-1}$. Molecular dynamics simulations were carried out to quantify the role of lattice vibrations, molecular reorientations and ion diffusion on the disclosed colossal BCE. Interestingly, lattice vibrations were found to contribute the most to $|\Delta S|$ while the diffusion of lithium ions, despite adding up only slightly to the accompanying entropy change, was crucial in enabling the molecular order-disorder phase transition. Our work expands the knowledge on plastic crystals and should motivate the investigation of BCE in a variety of solid electrolytes displaying ion diffusion and concomitant molecular orientational disorder., Comment: 13 pages, 7 figures

Helium, the second most abundant element in the universe, exhibits an extremely large electronic band gap of about $20$ eV at low pressures ($\le 0.1$ GPa). While the metallization pressure of hcp helium has been accurately predicted, thus far little attention has been paid to the specific mechanisms driving the band-gap closure and electronic properties of this quantum crystal in the terapascal regime (1 TPa $= 1,000$ GPa). Here, we employ state-of-the-art density functional theory and many-body perturbation theory calculations to fill up this knowledge gap. It is found that prior to reaching metallicity bulk solid helium becomes an excitonic insulator (EI), an exotic state of matter typically observed in low-dimensional systems in which electrostatically bound electron-hole pairs form spontaneously. Furthermore, it is shown that electron-phonon coupling (EPC) is significantly enhanced across the EI to metal phase transition as signaled by prominent phonon softening and giant EPC strength values ($\lambda \sim 10-100$) estimated at specific reciprocal space points. Accordingly, we predict metallic helium to be a superconductor with a critical temperature of $\approx 30$ K at $20$ TPa and of $\approx 100$ K at $100$ TPa. These unforeseen phenomena have important consequences on the elastic, thermodynamic and transport properties of metallic helium hence may be critical for improving our fundamental understanding and modelling of celestial bodies., Comment: 25 pages, 18 figures