Unphysical features in the application of the Boltzmann collision operator in the time dependent modelling of quantum transport

In this work, the use of the Boltzmann collision operator for dissipative quantum transport is analyzed. Its mathematical role on the description of the time-evolution of the density matrix during a collision can be understood as processes of adding and subtracting states. We show that unphysical results can be present in quantum simulations when the old states (that built the density matrix associated to an open system before the collision) are different from the additional states generated by the Boltzmann collision operator. As a consequence of the Fermi Golden rule, the new generated sates are usually eigenstates of the momentum or kinetic energy. Then, the different time-evolutions of old and new states involved in a collision process can originate negative values of the charge density, even longer after the collision. This unphysical feature disappears when the Boltzmann collision operator generates states that were already present in the density matrix of the quantum system before the collision. Following these ideas, in this paper, we introduce an algorithm that models phonon-electron interactions through the Boltzmann collision operator without negative values of the charge density. The model only requires the exact knowledge, at all times, of the states that build the density matrix of the open system.Comment: 14 pages, 4 figure

Time-dependent simulation of particle and displacement currents in THz graphene transistors

Although time-independent models provide very useful dynamical information with a reduced computational burden, going beyond the quasi-static approximation provides enriched information when dealing with TeraHertz (THz) frequencies. In this work, the THz noise of dual-gate graphene transistors with DC polarization is analyzed from a careful simulation of the time-dependent particle and displacement currents. From such currents, the power spectral density (PSD) of the total current fluctuations are computed at the source, drain and gate contacts. The role of the lateral dimensions of the transistors, the Klein tunneling and the positive-negative energy injection on the PSD are analyzed carefully. Through the comparison of the PSD with and without Band-to-Band tunneling and graphene injection, it is shown that the unavoidable Klein tunneling and positive-negative energy injection in graphene structures imply an increment of noise without similar increment on the current, degrading the (either low or high frequency) signal-to-noise ratio. Finally, it is shown that the shorter the vertical height (in comparison with the length of the active region in the transport direction), the larger the maximum frequency of the PSD. As a byproduct of this result, an alternative strategy (without length scaling) to optimize the intrinsic cut-off frequency of graphene transistors is envisioned.Comment: 22 pages, 9 figures, proceeding of UPoN201

From cyber-security deception to manipulation and gratification through gamification

Over the last two decades the field of cyber-security has experienced numerous changes associated with the evolution of other fields, such as networking, mobile communications, and recently the Internet of Things (IoT) [3]. Changes in mindsets have also been witnessed, a couple of years ago the cyber-security industry only blamed users for their mistakes often depicted as the number one reason behind security breaches. Nowadays, companies are empowering users, modifying their perception of being the weak link, into being the center-piece of the network design [4]. Users are by definition "in control" and therefore a cyber-security asset. Researchers have focused on the gamification of cyber- security elements, helping users to learn and understand the concepts of attacks and threats, allowing them to become the first line of defense to report anoma- lies [5]. However, over the past years numerous infrastructures have suffered from malicious intent, data breaches, and crypto-ransomeware, clearly showing the technical "know-how" of hackers and their ability to bypass any security in place, demonstrating that no infrastructure, software or device can be consid- ered secure. Researchers concentrated on the gamification, learning and teaching theory of cyber-security to end-users in numerous fields through various techniques and scenarios to raise cyber-situational awareness [2][1]. However, they overlooked the users’ ability to gather information on these attacks. In this paper, we argue that there is an endemic issue in the the understanding of hacking practices leading to vulnerable devices, software and architectures. We therefore propose a transparent gamification platform for hackers. The platform is designed with hacker user-interaction and deception in mind enabling researchers to gather data on the techniques and practices of hackers. To this end, we developed a fully extendable gamification architecture allowing researchers to deploy virtualised hosts on the internet. Each virtualised hosts contains a specific vulnerability (i.e. web application, software, etc). Each vulnerability is connected to a game engine, an interaction engine and a scoring engine