ON NEUTRON STARS'CRUST BREAKING AND GRAVITATIONAL WAVES EMISSION

Many different astrophysical events related to pulsars are taught to be due to starquakes, that could be caused by various possible loadings acting on the crust. However, at the present time, there is still a lack of theoretical well based modelling for most of these loadings and, therefore, we have only a very rough knowledge of the physics of neutron stars\u2019 crust response. This PhD work wants to be a first development of a quite realistic calculation of the effects of chosen loadings, being that the forces due to uniform rotation, differential rotation or pinning, on the crust of pulsars. A Newtonian model, already used in Geophysics, has been adapted to the very different physical conditions of neutron stars\u2019 physics and used to describe self-gravitating neutron stars, both in the incompressible and compressible scenario, subjected to different kinds of loadings. In particular, the deformations due to uniform rotation, differential rotation and slack pinning are studied. It is found that the response of the star is very sensitive to the adiabatic index value, while it is weakly influenced by the stellar mass. In all the cases, the strain developed between two glitches is found to be insufficient to break the crust, a result that challenges the standard picture of pulsar glitches based on crustquakes. Finally, attention is focused on accreting neutron stars in low-mass X-ray binaries and millisecond pulsars. The scenario is the following: the star spins up due to the accretion of matter thus building up stress; the mass quadrupole moment associated with crustal failures leads to the emission of gravitational waves which, in turn, spins down the star until equilibrium. The equilibrium frequency calculated is found compatible with observations. It is also argued that these gravitational waves could be potentially detected by the LIGO-Virgo interferometers in the near future

Incompressible analytical models for spinning-down pulsars

We study a class of Newtonian models for the deformations of non-magnetized neutron stars duringtheir spin-down. The models have all an analytical solution, and thus allow to understand easily thedependence of the strain on the star\u2019s main physical quantities, such as radius, mass and crust thickness.In the first \u201chistorical\u201d model the star is assumed to be comprised of a fluid core and an elastic crustwith the same density. We compare the response of stars with different masses and equations of stateto a decreasing centrifugal force, finding smaller deformations for heavier stars: the strain angle ispeaked at the equator and turns out to be a decreasing function of the mass.We introduce a second,more refined, model in which the core and the crust have different densities and the gravitationalpotential of the deformed body is self-consistently accounted for. Also in this case the strain angle isa decreasing function of the stellar mass, but its maximum value is at the poles and is always largerthan the corresponding one in the one-density model by a factor of two. Finally, within the presentanalytic approach, it is possible to estimate easily the impact of the Cowling approximation: neglectingthe perturbations of the gravitational potential, the strain angle is 40% of the one obtained with thecomplete model

Experimental and theoretical study on bond behavior of GFRP bars in steel fiber reinforced self compacting concrete

To estimate the cracking and the deformational behavior of steel fiber reinforced selfcompacting concrete (SFRSCC) beams reinforced with glass fiber reinforced polymer (GFRP) bars, it is fundamental to understand the interfacial bond behavior of embedded bars. Hence, the evaluation of the bond behavior between GFRP and (SFRSCC) was investigated in this study. A closed-form formulation was derived, adopting a new local bond stress-slip relationship. Furthermore, an experimental program composed of pullout bending tests was carried out in order to assess the influence of the following parameters on the bond behavior: bar diameter, bar surface treatment, embedment length and SFRSCC cover thickness. Finally, a numerical simulation was performed with a FEM-based computer program in order to simulate the bond behavior between GFRP bar and SFRSCC by means of a non-linear bond-slip relationship assigned to the interface finite element. The predictive performance of the theoretical models was appraised by comparing experimental and numerical results

Mid-Infrared Plasmonic Platform Based on n-Doped Ge-on-Si: Molecular Sensing with Germanium Nano-Antennas on Si

CMOS-compatible, heavily-doped semiconductor films are very promising for applications in mid-infrared plasmonic devices because the real part of their dielectric function is negative and broadly tunable in this wavelength range. In this work we investigate n-type doped germanium epilayers grown on Si substrates. We design and realize Ge nanoantennas on Si substrates demonstrating the presence of localized plasmon resonances, and exploit them for molecular sensing in the mid-infrared

Thermoplasmonic Effect of Surface-Enhanced Infrared Absorption in Vertical Nanoantenna Arrays

Thermoplasmonics is a method for increasing temperature remotely using focused visible or infrared laser beams interacting with plasmonic nanopartides. Here, local heating induced by mid-infrared quantum cascade laser illumination of vertical gold-coated nanoanterma arrays embedded into polymer layers is investigated by infrared nanospectroscopy and electromagnetic/thermal simulations. Nanoscale thermal hotspot images are obtained by a phototherrnal scanning probe microscopy technique with laser illumination wavelength tuned at the different plasmonic resonances of the arrays. Spectral analysis indicates that both Joule heating by the metal antennas and surface-enhanced-infrared absorption (SEIRA) by the polymer molecules located in the apical hotspots of the antennas are responsible for thermoplasmonic resonances, that is, for strong local temperature increase. At odds with more conventional planar nanoantennas, the vertical antenna structure enables thermal decoupling of the hotspot at the antenna apex from the heat sink constituted by the solid substrate. The temperature increase was evaluated by quantitative comparision of data obtained with the photothermal expansion technique to the results of electromagnetic/thermal simulations. In the case of strong SEIRA by the C=O bond of poly-methylmethacrylate at 1730 cm(-1), for focused mid-infrared laser power of about 20 mW, the evaluated order of magnitude of the nanoscale temperature increase is of 10 K. This result indicates that temperature increases of order of hundreds of K may he attainable with full mid-infrared laser power tuned at specific molecule vibrational fingerprints

Limiting mechanisms for photon recycling in thin-film GaAs solar cells

Photon recycling mechanisms in single junction thin-film GaAs solar cells are evaluated in this study. Modelling supported by experimentally obtained results is used in order to correlate the reflectance of the cell's rear layers, the photon recycling probability, and the solar cell performance. Solar cells with different top and bottom metallization configurations are produced, and their performance is analyzed from the optical and electrical point of view. It is shown that the photon recycling probability increases with the rear mirror reflectance and solar cell thickness, which results in the increase of the devices open circuit voltage. However, the front grid coverage, usually disregarded in rear mirror focused studies, strongly reduces the photon recycling probability. Furthermore, perimeter and interface recombination hinder the internal radiative efficiency of the solar cells, preventing further increase of the devices' open circuit voltage as a result of improvements of the rear mirror reflectivity. In order to exploit the significant benefit of increased photon recycling probability to the solar cell performance, these limiting mechanisms need to be properly addressed

A guided inquiry based teaching and learning sequence on oscillations based on experiments and data-logging techniques

We present here a teaching and learning sequence on oscillations entirely based on experiments and data logging techniques. The sequence has been proposed to three different groups of students during curricular and extracurricular lessons. The purpose of this paper is to discuss a way to introduce upper secondary school students to complicated topics, such as those of coupled oscillations, avoiding the use of too much mathematics and calculus, but with an intense use of data logging techniques

Infrared nanospectroscopy of individual extracellular microvesicles

Extracellular vesicles are membrane-delimited structures, involved in several inter-cellular communication processes, both physiological and pathological, since they deliver complex biological cargo. Extracellular vesicles have been identified as possible biomarkers of several pathological diseases; thus, their characterization is fundamental in order to gain a deep understanding of their function and of the related processes. Traditional approaches for the characterization of the molecular content of the vesicles require a large quantity of sample, thereby providing an average molecular profile, while their heterogeneity is typically probed by non-optical microscopies that, however, lack the chemical sensitivity to provide information of the molecular cargo. Here, we perform a study of individual microvesicles, a subclass of extracellular vesicles generated by the outward budding of the plasma membrane, released by two cultures of glial cells under different stimuli, by applying a state-of-the-art infrared nanospectroscopy technique based on the coupling of an atomic force microscope and a pulsed laser, which combines the label-free chemical sensitivity of infrared spectroscopy with the nanometric resolution of atomic force microscopy. By correlating topographic, mechanical and spectroscopic information of individual microvesicles, we identified two main populations in both families of vesicles released by the two cell cultures. Subtle differences in terms of nucleic acid content among the two families of vesicles have been found by performing a fitting procedure of the main nucleic acid vibrational peaks in the 1000–1250 cm-1 frequency range

"Good Vibrations" : A workshop on oscillations and normal modes

We describe some theatrical strategies adopted in a two hour workshop in order to show some meaningful experiments and the underlying useful ideas to describe a secondary school path on oscillations, that develops from harmonic motion to normal modes of oscillations, and makes extensive use of video analysis, data logging, slow motions and applet simulations. Theatre is an extremely useful tool to stimulate motivation starting from positive emotions. That is the reason why the theatrical approach to the presentation of physical themes has been explored by the group "Lo spettacolo della Fisica" (http://spettacolo.fisica.unimi.it) of the Physics Department of University of Milano for the last ten years (Carpineti et al., JCOM, 10 (2011) 1; Nuovo Cimento B, 121 (2006) 901) and has been inserted also in the European FP7 Project TEMI (Teaching Enquiry with Mysteries Incorporated, see http://teachingmysteries.eu/en) which involves 13 different partners coming from 11 European countries, among which the Italian (Milan) group. According to the TEMI guidelines, this workshop has a written script based on emotionally engaging activities of presenting mysteries to be solved while participants have been involved in nice experiments following the developed path

Detecting anharmonicity at a glance

Harmonic motion is generally presented in such a way that most of the students believe that the small oscillations of a body are all harmonic. Since the situation is not actually so simple, and since the comprehension of harmonic motion is essential in many physical contexts, we present here some suggestions, addressed to undergraduate students and pre-service teachers, that allow one to find out at a glance the anharmonicity of a motion. Starting from a didactically motivated definition of harmonic motion, and stressing the importance of the interplay between mathematics and experiments, we give a four-point criterion for anharmonicity together with some emblematic examples. The role of linear damping is also analysed in relation to the gradual changing of harmonicity into anharmonicity when the ratio between the damping coefficient and the zero-friction angular frequency increases