Electronic response and bandstructure modulation of carbon nanotubes in a transverse electrical field

The electronic properties of carbon nanotubes in a uniform transverse field are investigated within a single orbital tight-binding model. For doped nanotubes, the dielectric function is found to depend not only on symmetry of the tube, but also on radius and Fermi level position. Bandgap opening/closing is predicted for zigzag tubes, while it is found that armchair tubes always remain metallic, which is explained by the symmetry in their configuration. The bandstructures for both types are considerably modified when the field strength is large enough to mix neighboring subbands.Comment: Accepted for publication in Nanoletters, 8 pages, 3 figure

Metal-Semiconductor Transition in Armchair Carbon Nanotubes by Symmetry Breaking

The electronic band structure of armchair carbon nanotubes may be considerably modified by potentials with angular dependence. Different angular modes V_q ~ cos(q*theta) have been studied within a tight-binding scheme. Using symmetry arguments, we demonstrate a bandgap opening in these metallic nanotubes when certain selection rules are satisfied for both potential and nanotube structure. We estimate the bandgap opening as a function of both the external potential strength and the nanotube radius and suggest an effective mechanism of metal-semiconductor transition by combination of different forms of perturbations.Comment: 3 pages, 3 figures, published on AP

A Backscattering Model Incorporating the Effective Carrier Temperature in Nano MOSFET

In this work we propose a channel backscattering model in which increased carrier temperature at the top of the potential energy barrier in the channel is taken into account. This model represents an extension of a previous model by the same authors which highlighted the importance of considering the partially ballistic transport between the source contact and the top of the potential energy barrier in the channel. The increase of carrier temperature is precisely due to energy dissipation between the source contact and the top of the barrier caused by the high saturation current. To support our discussion, accurate 2D full band Monte Carlo device simulations with quantum correction have been performed in double gate nMOSFETs for different geometries (gate length down to 10 nm), biases and lattice temperatures. Including the effective carrier temperature is especially important to properly treat the high inversion regime, where previous backscattering models usually fail

Study of Warm Electron Injection in Double Gate SONOS by Full Band Monte Carlo Simulation

In this paper we investigate warm electron injection in a double gate SONOS memory by means of 2D full-band Monte Carlo simulations of the Boltzmann Transport Equation (BTE). Electrons are accelerated in the channel by a drain-to-source voltage VDS smaller than 3 V, so that programming occurs via electrons tunneling through a potential barrier whose height has been effectively reduced by the accumulated kinetic energy. Particle energy distribution at the semiconductor/oxide interface is studied for different bias conditions and different positions along the channel. The gate current is calculated with a continuum-based post-processing method as a function of the particle distribution obtained from Monte Carlo. Simulation results show that the gate current increases by several orders of magnitude with increasing drain bias and warm electron injection can be an interesting option for programming when short channel effects prohibit the application of larger drain bias

Epistemological activators and students' epistemologies in learning modern STEM topics

This dissertation is a collection of studies developed during my Ph.D. program within the Physics Education Research group of the University of Bologna. The entire work is driven by the role of epistemology in science as a means to orient learning and identity construction. Specifically, the study aims (i) to characterize epistemologically the design of teaching modules for High School on two main modern STEM topics: Artificial Intelligence (AI) and Quantum Physics (QP), and (ii) to investigate the so-called ‘students’ epistemologies’ in the context of learning QP. In the first part, the use that I do of epistemology involves the individuation of transversal themes, activities, and ideas – that I define ‘epistemological activators’ - that can structure students’ knowledge on a meta-level and foster them to reflect on the nature of disciplines and knowledge in general; this results in the proposal of teaching paths and insights for High School both in the contexts of QP and AI. In the second part, I conduct a qualitative study on students’ epistemologies in learning QP. Previous analysis showed evidence of three specific requirements that students show in learning QP, which I referred to as epistemic needs: the needs of visualization, comparability and ‘reification’. Along with these results, I decided to conduct a study on the nature of the factors that trigger students’ stances towards and acceptance of QP, building on the research literature on personal epistemologies. To this extent, I collected extensive written and recorded data of High School students participating in an introductory course on QP. The analysis mainly highlighted (i) evidence of expectations about the role of ‘visual modeling’ and ‘math’ as two personally reliable means to bridge classical and quantum domains., and (ii) evidence of entanglement between specific students’ epistemologies and their meta-affective stances towards challenges in learning QP

Metal-Semiconductor Transition and Fermi Velocity Renormalization in Metallic Carbon Nanotubes

Angular perturbations modify the band structure of armchair (and other metallic) carbon nanotubes by breaking the tube symmetry and may induce a metal-semiconductor transition when certain selection rules are satisfied. The symmetry requirements apply for both the nanotube and the perturbation potential, as studied within a nonorthogonal $\pi$-orbital tight-binding method. Perturbations of two categories are considered: an on-site electrostatic potential and a lattice deformation which changes the off-site hopping integrals. Armchair nanotubes are proved to be robust against the metal-semiconductor transition in second-order perturbation theory due to their high symmetry, but can develop a nonzero gap by extending the perturbation series to higher orders or by combining potentials of different types. An assumption of orthogonality between $\pi$ orbitals is shown to lead to an accidental electron-hole symmetry and extra selection rules that are weakly broken in the nonorthogonal theory. These results are further generalized to metallic nanotubes of arbitrary chirality.Comment: Submitted to Phys. Rev. B, 23 pages, 4 figure

A Universal Framework for Generalized Run Time Assurance with JAX Automatic Differentiation

With the rise of increasingly complex autonomous systems powered by black box AI models, there is a growing need for Run Time Assurance (RTA) systems that provide online safety filtering to untrusted primary controller output. Currently, research in RTA tends to be ad hoc and inflexible, diminishing collaboration and the pace of innovation. The Safe Autonomy Run Time Assurance Framework presented in this paper provides a standardized interface for RTA modules and a set of universal implementations of constraint-based RTA capable of providing safety assurance given arbitrary dynamical systems and constraints. Built around JAX, this framework leverages automatic differentiation to populate advanced optimization based RTA methods minimizing user effort and error. To validate the feasibility of this framework, a simulation of a multi-agent spacecraft inspection problem is shown with safety constraints on position and velocity