1,659 research outputs found
Non-local transport and the Hall viscosity of 2D hydrodynamic electron liquids
In a fluid subject to a magnetic field the viscous stress tensor has a
dissipationless antisymmetric component controlled by the so-called Hall
viscosity. We here propose an all-electrical scheme that allows a determination
of the Hall viscosity of a two-dimensional electron liquid in a solid-state
device.Comment: 12 pages, 4 figure
Helicons in Weyl semimetals
Helicons are transverse electromagnetic waves propagating in
three-dimensional (3D) electron systems subject to a static magnetic field. We
present a theory of helicons propagating through a 3D Weyl semimetal. Our
approach relies on the evaluation of the optical conductivity tensor from
semiclassical Boltzmann transport theory, with the inclusion of certain Berry
curvature corrections that have been neglected in the earlier literature (such
as the one due to the orbital magnetic moment). We demonstrate that the axion
term characterizing the electromagnetic response of Weyl semimetals
dramatically alters the helicon dispersion with respect to that in
nontopological metals. We also discuss axion-related anomalies that appear in
the plasmon dispersion relation.Comment: 5 pages, 1 figur
Spin-resolved optical conductivity of two-dimensional group-VIB transition-metal dichalcogenides
We present an ab-initio study of the spin-resolved optical conductivity of
two-dimensional (2D) group-VIB transition-metal dichalcogenides (TMDs). We
carry out fully-relativistic density-functional-theory calculations combined
with maximally localized Wannier functions to obtain band manifolds at
extremely high resolutions and focus on the photo-response of 2D TMDs to
circularly-polarized light in a wide frequency range. We present extensive
numerical results for monolayer TMDs involving molybdenum and tungsten combined
with sulphur and selenium. Our numerical approach allows us to locate with a
high degree of accuracy the positions of the points in the Brillouin zone that
are responsible for van Hove singularities in the optical response.
Surprisingly, some of the saddle points do not occur exactly along
high-symmetry directions in the Brillouin zone, although they happen to be in
their close proximity.Comment: 9 pages, 5 figure
Magnetic hallmarks of viscous electron flow in graphene
We propose a protocol to identify spatial hallmarks of viscous electron flow
in graphene and other two-dimensional viscous electron fluids. We predict that
the profile of the magnetic field generated by hydrodynamic electron currents
flowing in confined geometries displays unambiguous features linked to
whirlpools and backflow near current injectors. We also show that the same
profile sheds light on the nature of the boundary conditions describing
friction exerted on the electron fluid by the edges of the sample. Our
predictions are within reach of vector magnetometry based on nitrogen-vacancy
centers embedded in a diamond slab mounted onto a graphene layer.Comment: 5 pages, 6 figure
Design Optimization of a Natural Gas Substation With Intensification of the Energy Cycle
Abstract
Design Optimization of a Natural Gas Substation with Intensification of the Energy Cycle
(Arcangelo Pellegrino and Francesco Villecco)
Natural gas is currently the natural substitute of petroleum as an energy source, since the foreseen
ending up of this latter in the next decades. As a matter of fact, natural gas is easier to handle,
less dangerous to be transported, somehow environmentally more friendly. The gas ducts operate
with large flow rates over very long distances at high pressures, which are usually lowered in
proximity of the final substations by lamination valves which, in fact, dissipate energy. However,
a careful management of the pressure reduction may allow an energy recovery while using the gas
expansion to operate a turbine. In this case, gas must be preheated to compensate for the energy
required by the expansion. A proper control of all the parameters involved becomes crucial to an
intelligent use of these resources. In this paper, the possibility of using a pre-heating system has
been examined as a way to intensify the energy cycle in an expansion substation of the city gas
network. Fuzzy logic has been used to optimize the natural gas expansion in a turbine to produce
electrical energy. A fuzzy system has been designed and realized to control the whole process of
gas expansion, from the gas pre-heating to the pressure reduction. The system operates over the
whole year, accounting for the pressure, temperature, and gas flow rate variations experienced in
the gas line. The exit values of the latter and the inlet value of the gas pressure are selected as input
variables, being the output variable the temperature of the pre-heating water at the heat exchanger
inlet
Modulated phases of graphene quantum Hall polariton fluids
There is growing experimental interest in coupling cavity photons to the
cyclotron resonance excitations of electron liquids in high-mobility
semiconductor quantum wells or graphene sheets. These media offer unique
platforms to carry out fundamental studies of exciton-polariton condensation
and cavity quantum electrodynamics in a regime in which electron-electron
interactions are expected to play a pivotal role. Focusing on graphene, we
present a theoretical study of the impact of electron-electron interactions on
a quantum Hall polariton fluid, that is a fluid of magneto-excitons resonantly
coupled to cavity photons. We show that electron-electron interactions are
responsible for an instability of graphene integer quantum Hall polariton
fluids towards a modulated phase. We demonstrate that this phase can be
detected by measuring the collective excitation spectra, which soften at a
characteristic wave vector of the order of the inverse magnetic length.Comment: 26+17 pages, 5+3 figure
Electron hydrodynamics dilemma: whirlpools or no whirlpools
In highly viscous electron systems such as, for example, high quality
graphene above liquid nitrogen temperature, a linear response to applied
electric current becomes essentially nonlocal, which can give rise to a number
of new and counterintuitive phenomena including negative nonlocal resistance
and current whirlpools. It has also been shown that, although both effects
originate from high electron viscosity, a negative voltage drop does not
principally require current backflow. In this work, we study the role of
geometry on viscous flow and show that confinement effects and relative
positions of injector and collector contacts play a pivotal role in the
occurrence of whirlpools. Certain geometries may exhibit backflow at
arbitrarily small values of the electron viscosity, whereas others require a
specific threshold value for whirlpools to emerge
Numerical Methods for the Nonlocal Wave Equation of the Peridynamics
In this paper we will consider the peridynamic equation of motion which is
described by a second order in time partial integro-differential equation. This
equation has recently received great attention in several fields of Engineering
because seems to provide an effective approach to modeling mechanical systems
avoiding spatial discontinuous derivatives and body singularities. In
particular, we will consider the linear model of peridynamics in a
one-dimensional spatial domain. Here we will review some numerical techniques
to solve this equation and propose some new computational methods of higher
order in space; moreover we will see how to apply the methods studied for the
linear model to the nonlinear one. Also a spectral method for the spatial
discretization of the linear problem will be discussed. Several numerical tests
will be given in order to validate our results
Automatic design of Synchronous Reluctance motors focusing on barrier shape optimization
The automated design of Synchronous Reluctance motors based on Multi-Objective, Genetic Optimization and Finite Element Analysis is considered in this paper. Three types of barrier shapes are considered, all described by an effective, limited set of input variables. The three solutions are investigated to establish which of the geometries can give the best torque output and also which one represents the best compromise between output performance and computational time. The analysis presented in this paper shows that Synchronous Reluctance motors designed automatically can give a good performance, can be designed in a reasonable time and it is also shown that not all design degrees of freedom are useful in terms of motor performance. Two prototypes of automatically designed machines have been fabricated and experimentally compared to a third prototype designed according to state-of-the-art design principle
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