Rush hour metro crowd governed by people’s eagerness to go home (Vol. 49 No.4)

New model examines the relative role of random interactions between individuals in a crowd compared to interactions stemming from their eagerness to be on their way.

Ever found yourself crushed in a metro station at rush hour? The authors have developed a new model to study the movement of crowds exiting a metro station. In a recent study they have for the first time employed models typically used to study gases consisting of a large number of molecules that collide at random (known as thermostatted kinetic theory) to study the consequences of the different interactions occurring among pedestrians in a crowd while exiting a metro station. The authors assume that what motivates pedestrians to leave a metro station can be modelled as an external force that explains the conditions under which they leave due to the crowd pressure. Their model combines aspects representing the interactions between pedestrians and governed by thermostatted kinetic theory with the cooperation between pedestrians as intelligent and self-organised decision-makers, which is governed by game theory. Numerical simulations on the magnitude of the external force explain how internal interactions between pedestrians can be affected by an external force driving them to leave the station. What matters most is that all of the pedestrians are individually in the same hurry to exit the station and get away from the crowd.

C. Bianca, and C. Mogno, A thermostatted kinetic theory model for event-driven pedestrian dynamics, Eur. Phys. J. Plus, 133, 213 (2018)
[Abstract]

Insect-like vibrating winged NAV (Vol. 49 No.4)

This work presents an original concept using the combination of two resonant vibration modes of the flexible wings of a Nano-Air Vehicle to reproduce insect wings kinematics and generate lift. Since insects use for flying a kinematics which combines flapping and twisting motions with a specific phase shift, the main goal of this study is to design the artificial wings such that they feature two vibrations modes which are producing flapping and twisting deformations and to combine them with the appropriate phase shift. For this purpose, a polymeric prototype was micromachined with a wingspan of 3 cm, flexible wings and a single electromagnetic actuator as illustrated in the figure.

An optimal wings configuration was determined with a modelling and validated through experimental analyses to observe the vibrating behaviour of the prototype. A dedicated lift force measurement bench was then used to demonstrate a lift force equivalent to the prototype weight. Finally, at the maximum lift frequency, high-speed camera measurements confirmed a kinematics of the flexible wings with flapping and twisting motions combined in the expected phase shift.

D. Faux, O. Thomas, E. Cattan and S. Grondel, Two modes resonant combined motion for insect wings kinematics reproduction and lift generation, EPL 121, 66001 (2018)
[Abstract]

Nanoscale light trapping in graphene/h-BN by sound waves (Vol. 49 No.4)

Graphene plasmonics is a rapidly emerging field exploiting the extreme light confinement provided by the 2D nature of graphene that results in enhanced light-matter interaction. However, a large momentum mismatch needs to be overcome by a photon to couple with a plasmon into a plasmon polariton. In this work, the authors have demonstrated a unique way for exciting polaritons in graphene/h-BN systems on a piezoelectric material by using a surface acoustic wave (SAW) generated by an integrated transducer. The SAW acts as a virtual diffraction grating providing the extra momentum needed to generate propagating polaritons using a simple laser. h-BN is widely used as an ideal substrate for graphene providing it with very high mobility. Moreover, h-BN is a natural hyperbolic insulator (i.e. the dielectric function has opposite sign in the two reststrahlen bands), where graphene plasmons couple strongly to both surface and hyperbolic (waveguided) phonons in h-BN, leading to hybridized surface and hyperbolic plasmon-phonon polaritons. These results pave the way for engineering SAW-driven graphene/h-BN plasmonic devices and metamaterials covering the mid-IR to THz range.

R. Fandan, J. Pedrós, J. Schiefele, A. Boscá, J. Martínez and F. Calle, Acoustically-driven surface and hyperbolic plasmon-phonon polaritons in graphene/h-BN heterostructures on piezoelectric substrates, J. Phys. D: Appl. Phys. 51, 204004 (2018)
[Abstract]

The Soreq Applied Research Accelerator Facility (SARAF) (Vol. 49 No.4)

The Soreq Applied Research Accelerator Facility (SARAF) is under construction in the Soreq Nuclear Research Center at Yavne, Israel. Phase I of SARAF (SARAF-I) is already in operation, generating scientific results in several fields of interest, especially the astrophysical s-process. When completed at the beginning of the next decade, SARAF-II will be a user facility for basic and applied nuclear physics, based on a 40 MeV, 5 mA CW proton/deuteron superconducting linear accelerator. This review presents first a technical overview of SARAF-I and II, including a description of the accelerator and its irradiation targets, and provides a survey of existing research programs at SARAF-I. It then describes in some detail the research potential at the completed facility. SARAF-II’s cutting-edge specifications, with its unique liquid lithium target technology, will enable world-competitive research plans in several disciplines: precision studies of beyond-Standard-Model effects by trapping light exotic radioisotopes (including meaningful studies already at SARAF-I); extended nuclear astrophysics research with higher-energy neutrons, including generation and studies of exotic neutron-rich isotopes relevant to the astrophysical r-process; nuclear structure of exotic isotopes; high-energy neutron cross sections for basic nuclear physics and material science research, including neutron-induced radiation damage; neutron-based imaging with an imaging plane flux similar to that of a 5 MW research reactor; accelerator-based neutron therapy; and, last but not least, novel radiopharmaceuticals development and production.

I. Mardor and 28 co-authors, The Soreq Applied Research Accelerator Facility (SARAF): Overview, research programs and future plans, Eur. Phys. J. A 54, 91 (2018)
[Abstract]

Evidence of Long-range Correlations in Shallow Earthquakes (Vol. 49 No.4)

Earthquakes are one of the most devastating natural disasters by the number of casualties and the negative economic impact. Seismic phenomena have been studied from the viewpoint of complex systems, where complex patterns arise from nonlinear interactions between their elements. One of such ways is using networks of geographical sites; we introduce a new methodology to construct networks of epicenters and applied it to global catalogs of shallow earthquakes. It involves essentially the introduction of a time window, which works as a temporal filter for vertices connections. The resulting network constructed has small-world properties and presents scale-free properties in its connectivity distribution, which we proved to be invariant with respect to the value of the time window adopted. Vertices with larger connectivity in the network correspond to areas with very intense seismic activity in the period considered. These new results constitute evidences of possible spatial and temporal long-range correlations between earthquakes.

D. Ferreira, J. Ribeiro, A. Papa and R. Menezes, Towards evidence of long-range correlations in shallow seismic activities, EPL 121, 58003 (2018)
[Abstract]

Temperature-driven ballistic magnon transport (Vol. 49 No.4)

Application of a temperature gradient to a magnetic medium leads to the generation of a spin current referred to as the longitudinal spin Seebeck effect (LSSE). In a magnetic insulator such a current is created by a flux of thermal magnons. Using spin-dependent electron scattering processes in the adjacent normal metal this current can be converted to an electric voltage. The voltage evolution is determined by the development of the temperature gradient ∇T(x,t) and by the characteristics of the magnon’s motion.
By analysis of the time-dependent LSSE voltages in platinum-coated Yttrium Iron Garnet (YIG) ferrimagnetic films, the authors assumed that thermally-driven magnons with energies above 20 K move through the YIG layer ballistically due to their almost linear quasi-acoustic dispersion law. Consequently, the interaction processes within the ‘acoustic’ magnon mode do not change the magnon propagation velocity, while the number of magnons decays exponentially within an effective propagation length of 425nm. This length was found to be mostly independent on film thickness that proves the ballistic magnon transport scenario.

T. B. Noack and eleven co-authors, Spin Seebeck effect and ballistic transport of quasi-acoustic magnons in room-temperature yttrium iron garnet films, J. Phys. D: Appl. Phys. 51, 234003 (2018)
[Abstract]

Liquid Metal Energy Harvester by Acoustic Wave (Vol. 49 No.4)

Application of a temperature gradient to a magnetic medium leads to the generation of a spin current referred to as the longitudinal spin Seebeck effect (LSSE). In a magnetic insulator such a current is created by a flux of thermal magnons. Using spin-dependent electron scattering processes in the adjacent normal metal this current can be converted to an electric voltage. The voltage evolution is determined by the development of the temperature gradient ∇T(x,t) and by the characteristics of the magnon’s motion.
By analysis of the time-dependent LSSE voltages in platinum-coated Yttrium Iron Garnet (YIG) ferrimagnetic films, the authors assumed that thermally-driven magnons with energies above 20 K move through the YIG layer ballistically due to their almost linear quasi-acoustic dispersion law. Consequently, the interaction processes within the ‘acoustic’ magnon mode do not change the magnon propagation velocity, while the number of magnons decays exponentially within an effective propagation length of 425nm. This length was found to be mostly independent on film thickness that proves the ballistic magnon transport scenario.

J. Jeon, S. K. Chung, J.-B. Lee, S. Joo Doo and D. Kim, Acoustic wave-driven oxidized liquid metal-based energy harvester, Eur. Phys. J. Appl. Phys. 81, 20902 (2018)
[Abstract]