Electronic and vibrational properties of TiSe2\textup{TiSe}_2 in the charge-density wave phase from first principles

We study the charge-density wave phase in $\textup{TiSe}_2$ by using first principle calculations. We show that, regardless of the local functional used and as long as the cell parameters are in agreement with the experiment, density-functional calculations are able to reproduce not only the structural instability of $\textup{TiSe}_2$, but also the effective distortion observed in the experiments. We study the electronic structure evolution of the system under the charge-density wave deformation. In particular, we show that the energy bands for the distorted superstructure, unfolded into the original Brillouin zone, are in reasonable agreement with angle-resolved photoemission spectroscopy (ARPES) data taken at low temperature. On the contrary, the energy bands for the undistorted structure are not in good agreement with ARPES at high temperature. Motivated by these results, we investigate the effect of the correlation on the electrons of the localized Ti-$d$ orbitals by using the LDA+$U$ method. We show that within this approximation the electronic bands for both the undistorted and distorted structure are in very good agreement with ARPES. On the other hand, the $U$ eliminates the phonon instability of the system. Some possible explanations for this counter intuitive result are proposed. Particularly, the possibility of taking into account the dependence of the parameter $U$ from the atomic positions is suggested.Comment: 21 pages, 16 figures, 3 pages of Supplementary materia

Coastal resilience potential as a coefficient of the coastal erosion risk assessment, and the management of risk areas via nature-based solutions

Climate change has now become a global problem since the level of awareness of civil society has grown. Even non-professionals have understood that this phenomenon is strongly influenced by human activities and by local or specific dynamics, which exacerbate its effects, especially in the case of coastal environment conservation. Coastal erosion is a natural process that is exacerbated by climate change and is considered a natural hazard since it threatens the safety of humans and their properties. As with other natural hazards, such as fire and hydraulic, the risk of coastal erosion is mainly driven by urban spreading and inadequate land management. There are numerous proposals in the scientific literature based on good practices, guidelines and studies on the assessment and management of coastal erosion to solve the problem. However, coasts around the world are still experiencing significant imbalances, and future forecasts on this issue are even more pessimistic. On a technical level, numerous solutions have been tested in recent decades. Unfortunately, these alternatives have been fundamentally oriented towards the construction of hard coastal engineering structures, which instead of solving the problem, on many occasions have created new phenomena of instability such as the generation of coastal narrowing and the translation of erosive phenomena along the coast. Solutions were proposed and tested considerably, during the last century; almost everywhere they comprised hard structures of coastal engineering that instead of solving the problem, created new instabilities, such as coastal squeezing and erosive shifting. These consequences, and the trends observable on the world's shores, have required the conversion of coastal engineering into a more sustainable discipline that strongly supports natural resilience. More generally, resilience represents the ability of natural systems, such as a coast, a community or an individual, to cope and respond to a traumatic event by drawing on their own resources. As for the loss of the beaches, this intrinsic character must allow the system to use the sedimentary stock, to rebalance the dynamics and feedbacks coming from each of its physical and biological components. This would allow the beach to "jump back" and reach the morpho-dynamic equilibrium it had in the phase preceding the erosive trauma. This work proposes an integrated method for calculating the resilience potential that can address both the assessment and management phases of coastal erosion risk. The proposed evaluation methodology comprises the use of innovative technologies, such as geographic information systems (GIS) for the mapping and spatial analysis of morphological trends, integrated in the analysis of economic and social dynamics. Such matrices differ greatly due to their different nature but must be considered as the product of vulnerability and exposure in risk assessment. Adopting an approach oriented to the use of multi-parametric indices, the resilience potential was calculated and integrated into the vulnerability assessment. This is essential as from a regulatory point of view areas exposed to natural hazards must be transformed into low risk levels to improve their natural stability before their use. In this regard, the coastal strip of the Municipality of San Vincenzo (Livorno, Italy) has been mapped and its potential use and regulation have been evaluated independently of purely economic or political approaches. However, this still represents a great challenge as the economy plays a strong role in valuation formulas, as well as management plans, which for these reasons are rarely decisive. The study was tested within the Interreg MAREGOT Project, of which the Department of Earth Sciences of the University of Florence is a partner. Initially a morphodynamic evaluation of the studied site was carried out, followed by the drafting of a morpho-sedimentological map. These activities were carried out at the Laboratory of Applied Geomorphology of the Center for Geotechnology (CGT) of the University of Siena. Subsequently, the morphological trends and the economic parameters examined were converted into diagnostic indicators of territorial change, at the Department of Civil Engineering of the Polytechnic University of Cartagena (Spain). The results highlight that an assessment of the resilience potential is not only necessary to address the effects of climate change, but it is mandatory to plan corrective actions that quantify the real capacity of coastal areas to cope with extreme events. Furthermore, in anthropized coastal environments the high density of concessions for recreational activities, the high price ranges of services, and the construction of buildings on the coastal area imply significant limitations that can be related to both social and morphological risks. The most important ones concern free access, the exercise of the right to swim, as well as the provision of a right of possession of the built works to the concessionaires, and subsequently the duty to protect them (by the administrations) through rigid works in emergency conditions. climatic. As already mentioned, these works are generally to be considered as the last alternative, as they can give rise to phenomena of contraction of the coastal strip at a local level, and sometimes cause the loss of some habitats. The European Commission's EU market regulation rules have sometimes highlighted the existence of shortcomings in the management of state-owned concessions in the maritime field. Indeed, in Italy they are generally automatically renewed to concessionaires for indefinite periods, without considering its implications on sedimentary dynamics of the coast. As already mentioned, this has influenced coastal engineering and urban planning on the territory of the entire Italian state. Hence, the Nature Based Solution tested in this study consists of a method that includes the potential for resilience in the management of areas subject to coastal erosion. This phenomenon is analyzed in terms of socio-morphological vulnerability on a local scale (beach), since it represents the administrative dimension within which this procedure is most applicable, and in which the concept of risk is linked to the loss of sedimentary stock. The approach adopted is oriented to the use of multi-parametric spatial indices through GIS tools, and shows the applicability of the method that allows to identify areas with different potential for relative resilience. Furthermore, it allows to generate territorial management strategies on a local scale consistent with the existence of morphological and social vulnerabilities.Escuela Internacional de Doctorado de la Universidad Politécnica de CartagenaUniversidad Politécnica de CartagenaPrograma de Doctorado en Tecnología y Modelización en Ingeniería Civil, Minera y Ambienta

High-pressure phase diagram of hydrogen and deuterium sulfides from first principles: structural and vibrational properties including quantum and anharmonic effects

We study the structural and vibrational properties of the high-temperature superconducting sulfur trihydride and trideuteride in the high-pressure $Im\bar{3}m$ and $R3m$ phases by first-principles density-functional-theory calculations. On lowering pressure, the rhombohedral transition $Im\bar{3}m \rightarrow R3m$ is expected, with hydrogen bond desymmetrization and occurrence of trigonal lattice distortion. In hydrostatic conditions we find that, contrary to what suggested in some recent experiments, if the rhombohedral distortion exists it affects mainly the hydrogen-bonds, whereas the resulting cell distortion is minimal. We estimate that the occurrence of a stress anisotropy of approximately $10\%$ could explain this discrepancy. Assuming hydrostatic conditions, we calculate the critical pressure at which the rhombohedral transition occurs. Quantum and anharmonic effects, which are relevant in this system, are included at nonperturbative level with the stochastic self-consistent harmonic approximation (SSCHA). Within this approach, we determine the transition pressure by calculating the free energy Hessian. We find that quantum anharmonic effects are responsible for a strong reduction of the critical pressure with respect to the one obtained with the classical harmonic approach. Moreover, we observe a prominent isotope effect, as we estimate higher pressure transition for D${}_3$S than for H${}_3$S. Finally, within SSCHA we calculate the anharmonic phonon spectral functions in the $Im\bar{3}m$ phase. The strong anharmonicity of the system is confirmed by the occurrence of very large anharmonic broadenings leading to complex non-Lorentzian line shapes. However, for the vibrational spectra at zone center, accessible e.g. by infrared spectroscopy, the broadenings are very small (linewidth at most around 2~meV) and anharmonic phonon quasiparticles are well defined

Boost symmetry in the Quantum Gravity sector

We perform a canonical quantization of gravity in a second-order formulation, taking as configuration variables those describing a 4-bein, not adapted to the space-time splitting. We outline how, neither if we fix the Lorentz frame before quantizing, nor if we perform no gauge fixing at all, is invariance under boost transformations affected by the quantization.Comment: 8 pages, to appear in the proceedings of'' 4th Italian-Sino Workshop on Relativistic Astrophysics'', AIP Conference Serie

The luminosity evolution over the EQuiTemporal Surfaces in the prompt emission of Gamma-Ray Bursts

Due to the ultrarelativistic velocity of the expanding "fireshell" (Lorentz gamma factor \gamma \sim 10^2 - 10^3), photons emitted at the same time from the fireshell surface do not reach the observer at the same arrival time. In interpreting Gamma-Ray Bursts (GRBs) it is crucial to determine the properties of the EQuiTemporal Surfaces (EQTSs): the locus of points which are source of radiation reaching the observer at the same arrival time. In the current literature this analysis is performed only in the latest phases of the afterglow. Here we study the distribution of the GRB bolometric luminosity over the EQTSs, with special attention to the prompt emission phase. We analyze as well the temporal evolution of the EQTS apparent size in the sky. We use the analytic solutions of the equations of motion of the fireshell and the corresponding analytic expressions of the EQTSs which have been presented in recent works and which are valid for both the fully radiative and the adiabatic dynamics. We find the novel result that at the beginning of the prompt emission the most luminous regions of the EQTSs are the ones closest to the line of sight. On the contrary, in the late prompt emission and in the early afterglow phases the most luminous EQTS regions are the ones closest to the boundary of the visible region. This transition in the emitting region may lead to specific observational signatures, i.e. an anomalous spectral evolution, in the rising part or at the peak of the prompt emission. We find as well an expression for the apparent radius of the EQTS in the sky, valid in both the fully radiative and the adiabatic regimes. Such considerations are essential for the theoretical interpretation of the prompt emission phase of GRBs.Comment: 11 pages, 5 figures, in the Proceedings of the 1st Galileo-Xu GuangQi Meeting, October 26-30, 2009, Shangha

Second order structural phase transitions, free energy curvature, and temperature-dependent anharmonic phonons in the self-consistent harmonic approximation: theory and stochastic implementation

The self-consistent harmonic approximation is an effective harmonic theory to calculate the free energy of systems with strongly anharmonic atomic vibrations, and its stochastic implementation has proved to be an efficient method to study, from first-principles, the anharmonic properties of solids. The free energy as a function of average atomic positions (centroids) can be used to study quantum or thermal lattice instability. In particular the centroids are order parameters in second-order structural phase transitions such as, e.g., charge-density-waves or ferroelectric instabilities. According to Landau's theory, the knowledge of the second derivative of the free energy (i.e. the curvature) with respect to the centroids in a high-symmetry configuration allows the identification of the phase-transition and of the instability modes. In this work we derive the exact analytic formula for the second derivative of the free energy in the self-consistent harmonic approximation for a generic atomic configuration. The analytic derivative is expressed in terms of the atomic displacements and forces in a form that can be evaluated by a stochastic technique using importance sampling. Our approach is particularly suitable for applications based on first-principles density-functional-theory calculations, where the forces on atoms can be obtained with a negligible computational effort compared to total energy determination. Finally we propose a dynamical extension of the theory to calculate spectral properties of strongly anharmonic phonons, as probed by inelastic scattering processes. We illustrate our method with a numerical application on a toy model that mimics the ferroelectric transition in rock-salt crystals such as SnTe or GeTe

Causality and momentum conservation from relative locality

Theories with a curved momentum space, which became recently of interest in the quantum-gravity literature, can in general violate many apparently robust aspects of our current description of the laws of physics, including relativistic invariance, locality, causality and global momentum conservation. We here explore some aspects of the particularly severe pathologies arising in generic theories with curved momentum space for what concerns causality and momentum conservation. However, we also report results suggesting that when momentum space is maximally symmetric, and the theory is formulated (DSR-)relativistically, with the associated relativity of spacetime locality, momentum is globally conserved and there is no violation of causality.Comment: 20 pages, 6 figures, latex (V2: minor editing

Multiservice UAVs for Emergency Tasks in Post-disaster Scenarios

UAVs are increasingly being employed to carry out surveillance, parcel delivery, communication-support and other specific tasks. Their equipment and mission plan are carefully selected to minimize the carried load an overall resource consumption. Typically, several single task UAVs are dispatched to perform different missions. In certain cases, (part of) the geographical area of operation may be common to these single task missions (such as those supporting post-disaster recovery) and it may be more efficient to have multiple tasks carried out as part of a single UAV mission using common or even additional specialized equipment. In this paper, we propose and investigate a joint planning of multitask missions leveraging a fleet of UAVs equipped with a standard set of accessories enabling heterogeneous tasks. To this end, an optimization problem is formulated yielding the optimal joint planning and deriving the resulting quality of the delivered tasks. In addition, a heuristic solution is developed for large-scale environments to cope with the increased complexity of the optimization framework. The developed joint planning of multitask missions is applied to a specific post-disaster recovery scenario of a flooding in the San Francisco area. The results show the effectiveness of the proposed solutions and the potential savings in the number of UAVs needed to carry out all the tasks with the required level of quality