Gap opening in graphene by shear strain

We exploit the concept of strain-induced band structure engineering in graphene through the calculation of its electronic properties under uniaxial, shear, and combined uniaxial-shear deformations. We show that by combining shear deformations to uniaxial strains it is possible modulate the graphene energy gap value from zero up to $0.9$ eV. Interestingly enough, the use of a shear component allows for a gap opening at moderate absolute deformation, safely smaller than the graphene failure strain.Comment: to appear on PRB - Rapid Communicatio

The effect of the hydrogen coverage on the Young modulus of graphene

We blend together continuum elasticity and first principles calculations to measure by a computer experiment the Young modulus of hydrogenated graphene. We provide evidence that hydrogenation generally leads to a much smaller longitudinal extension upon loading than in pristine graphene. Furthermore, the Young modulus is found to depend upon the loading direction for some specific conformers, characterized by an anisotropic linear elastic behavior

The Optical Counterpart to the Accreting Millisecond X-ray Pulsar SAX J1748.9-2021 in the Globular Cluster NGC 6440

We used a combination of deep optical and Halpha images of the Galactic globular cluster NGC 6440, acquired with the Hubble Space Telescope, to identify the optical counterpart to the accreting millisecond X-ray pulsar SAX J1748.9-2021during quiescence. A strong Halpha emission has been detected from a main sequence star (hereafter COM-SAX J1748.9-2021) located at only 0.15" from the nominal position of the X-ray source. The position of the star also agrees with the optical counterpart found by Verbunt et al. (2000) during an outburst. We propose this star as the most likely optical counterpart to the binary system. By direct comparison with isochrones, we estimated that COM-SAX J1748.9-2021 has a mass of 0.70 Msun - 0.83 Msun, a radius of 0.88 pm 0.02 Rsun and a superficial temperature of 5250pm80 K. These parameters combined with the orbital characteristics of the binary suggest that the system is observed at a very low inclination angle (~8 deg -14 deg) and that the star is filling or even overflowing its Roche Lobe. This, together with the equivalent width of the Halpha emission (~20 Ang), suggest possible on-going mass transfer. The possibile presence of such a on-going mass transfer during a quiescence state also suggests that the radio pulsar is not active yet and thus this system, despite its similarity with the class of redback millisecond pulsars, is not a transitional millisecond pulsar.Comment: 8 pages, 6 figures. Accepted for publication in Ap

Folds and Buckles at the Nanoscale: Experimental and Theoretical Investigation of the Bending Properties of Graphene Membranes

The elastic properties of graphene crystals have been extensively investigated, revealing unique properties in the linear and nonlinear regimes, when the membranes are under either stretching or bending loading conditions. Nevertheless less knowledge has been developed so far on folded graphene membranes and ribbons. It has been recently suggested that fold-induced curvatures, without in-plane strain, can affect the local chemical reactivity, the mechanical properties, and the electron transfer in graphene membranes. This intriguing perspective envisages a materials-by-design approach through the engineering of folding and bending to develop enhanced nano-resonators or nano-electro-mechanical devices. Here we present a novel methodology to investigate the mechanical properties of folded and wrinkled graphene crystals, combining transmission electron microscopy mapping of 3D curvatures and theoretical modeling based on continuum elasticity theory and tight-binding atomistic simulations

Using long-term millisecond pulsar timing to obtain physical characteristics of the bulge globular cluster Terzan 5

Over the past decade the discovery of three unique stellar populations and a large number of confirmed pulsars within the globular cluster Terzan 5 has raised questions over its classification. Using the long-term radio pulsar timing of 36 millisecond pulsars in the cluster core, we provide new measurements of key physical properties of the system. As Terzan 5 is located within the galactic bulge, stellar crowding and reddening make optical and near infrared observations difficult. Pulsar accelerations, however, allow us to study the intrinsic characteristics of the cluster independent of reddening and stellar crowding and probe the mass density profile without needing to quantify the mass to light ratio. Relating the spin and orbital periods of each pulsar to the acceleration predicted by a King model, we find a core density of $1.58\times$10$^6$ M$_\odot$ pc$^{-3}$, a core radius of 0.16 pc, a pulsar density profile $n\propto r^{-3.14}$, and a total mass of M$_{\rm T}$($R_\perp<$1.0 pc)$\simeq3.0\times$10$^5$ M$_\odot$ assuming a cluster distance of 5.9 kpc. Using this information we argue against Terzan 5 being a disrupted dwarf galaxy and discuss the possibility of Terzan 5 being a fragment of the Milky Way's proto-bulge. We also discuss whether low-mass pulsars were formed via electron capture supernovae or exist in a core full of heavy white dwarfs and hard binaries. Finally we provide an upper limit for the mass of a possible black hole at the core of the cluster of 3.0$\times$10$^4$ M$_\odot$.Comment: 27 pages, 20 figures, 5 tables, thesis research, accepte

Graphene under strain. A combined continuum-atomistic approach

By combining continuum elasticity theory and atomistic simulations, we provide a picture of the elastic behavior of graphene, which was addressed as a two-dimensional crystal membrane. Thus, the constitutive nonlinear stress-strain relations for graphene, as well as its hydrogenated conformers, have been derived in the framework of the two-dimensional elastic theory, and all the corresponding linear and nonlinear elastic moduli have been computed by atomistic simulations. Moreover, we discuss the effects of an applied stretching on graphene lattice to its electronic band structure, in particular regards the concept of strain-induced band gap engineering. Finally, we focus on the emergence of a stretching field induced on a graphene nanoribbon by bending, providing that such an in-plane strain field can be decomposed in a first contribution due to the actual bending of the sheet and a second one due to the edge effects induced by the finite size of the nanoribbon.------------------------------------------------------ABSTRACT ITA-------Combinando la teoria dell‘elasticità del continuo con calcoli eseguiti attraverso simulazioni atomistiche, si è affrontato lo studio del comportamento elastico del grafene, ovvero di una struttura cristallina bidimensionale a base carbonio. In tal modo, nell‘ambito della teoria elastica bidimensionale, sono state derivate le equazioni costitutive non lineari per il grafene e per il suo composto con l‘idrogeno, detto grafane; conseguentemente sono stati determinati per mezzo di simulazioni atomistiche tutti i relativi moduli elastici lineari e non lineari. Inoltre, abbiamo discusso gli effetti dovuti a deformazioni omogenee applicate al reticolo di grafene sulle sue bande elettroniche, con particolare attenzione al concetto di ingegnerizzazione della gap elettronica indotta da deformazione. Infine, discutiamo l‘insorgenza di un campo di deformazione su un campione di grafene finito sottoposto a piegamento, evidenziando come tale campo possa essere decomposto in un contributo causato della flessione reale subita e in un secondo dovuto ai soli effetti di bordo.