152 research outputs found
Plasmon losses due to electron-phonon scattering: the case of graphene encapsulated in hexagonal Boron Nitride
Graphene sheets encapsulated between hexagonal Boron Nitride (hBN) slabs
display superb electronic properties due to very limited scattering from
extrinsic disorder sources such as Coulomb impurities and corrugations. Such
samples are therefore expected to be ideal platforms for highly-tunable
low-loss plasmonics in a wide spectral range. In this Article we present a
theory of collective electron density oscillations in a graphene sheet
encapsulated between two hBN semi-infinite slabs (hBN/G/hBN). Graphene plasmons
hybridize with hBN optical phonons forming hybrid plasmon-phonon (HPP) modes.
We focus on scattering of these modes against graphene's acoustic phonons and
hBN optical phonons, two sources of scattering that are expected to play a key
role in hBN/G/hBN stacks. We find that at room temperature the scattering
against graphene's acoustic phonons is the dominant limiting factor for
hBN/G/hBN stacks, yielding theoretical inverse damping ratios of hybrid
plasmon-phonon modes of the order of -, with a weak dependence on
carrier density and a strong dependence on illumination frequency. We confirm
that the plasmon lifetime is not directly correlated with the mobility: in
fact, it can be anti-correlated.Comment: 14 pages, 4 figure
Programmable data gathering for detecting stegomalware
The 'arm race' against malware developers requires to collect a wide variety of performance measurements, for instance to face threats leveraging information hiding and steganography. Unfortunately, this process could be time-consuming, lack of scalability and cause performance degradations within computing and network nodes. Moreover, since the detection of steganographic threats is poorly generalizable, being able to collect attack-independent indicators is of prime importance. To this aim, the paper proposes to take advantage of the extended Berkeley Packet Filter to gather data for detecting stegomalware. To prove the effectiveness of the approach, it also reports some preliminary experimental results obtained as the joint outcome of two H2020 Projects, namely ASTRID and SIMARGL
Environmental induced renormalization effects in quantum Hall edge states
We propose a general mechanism for renormalization of the tunneling exponents
in edge states of the fractional quantum Hall effect. Mutual effects of the
coupling with out-of-equilibrium 1/f noise and dissipation are considered both
for the Laughlin sequence and for composite co- and counter-propagating edge
states with Abelian or non-Abelian statistics. For states with
counter-propagating modes we demonstrate the robustness of the proposed
mechanism in the so called disorder-dominated phase. Prototypes of these
states, such as \nu=2/3 and \nu=5/2, are discussed in detail and the rich
phenomenology induced by the presence of a noisy environment is presented. The
proposed mechanism justifies the strong renormalizations reported in many
experimental observations carried out at low temperatures. We show how
environmental effects could affect the relevance of the tunneling excitations,
leading to important implications in particular for the \nu=5/2 case.Comment: 14 pages, 4 figure
Hybrid quantum thermal machines with dynamical couplings
Quantum thermal machines can perform useful tasks, such as delivering power, cooling, or heating. In this work, we consider hybrid thermal machines, that can execute more than one task simultaneously. We characterize and find optimal working conditions for a three-terminal quantum thermal machine, where the working medium is a quantum harmonic oscillator, coupled to three heat baths, with two of the couplings driven periodically in time. We show that it is possible to operate the thermal machine efficiently, in both pure and hybrid modes, and to switch between different operational modes simply by changing the driving frequency. Moreover, the proposed setup can also be used as a high-performance transistor, in terms of output–to–input signal and differential gain. Owing to its versatility and tunability, our model may be of interest for engineering thermodynamic tasks and for thermal management in quantum technologies
Plasmons and Coulomb drag in Dirac/Schroedinger hybrid electron systems
We show that the plasmon spectrum of an ordinary two-dimensional electron gas
(2DEG) hosted in a GaAs heterostructure is significantly modified when a
graphene sheet is placed on the surface of the semiconductor in close proximity
to the 2DEG. Long-range Coulomb interactions between massive electrons and
massless Dirac fermions lead to a new set of optical and acoustic intra-subband
plasmons. Here we compute the dispersion of these coupled modes within the
Random Phase Approximation, providing analytical expressions in the
long-wavelength limit that shed light on their dependence on the Dirac velocity
and Dirac-fermion density. We also evaluate the resistivity in a Coulomb-drag
transport setup. These Dirac/Schroedinger hybrid electron systems are
experimentally feasible and open new research opportunities for fundamental
studies of electron-electron interaction effects in two spatial dimensions.Comment: 7 pages, 4 figure
Highly confined low-loss plasmons in graphene-boron nitride heterostructures
Graphene plasmons were predicted to possess ultra-strong field confinement
and very low damping at the same time, enabling new classes of devices for deep
subwavelength metamaterials, single-photon nonlinearities, extraordinarily
strong light-matter interactions and nano-optoelectronic switches. While all of
these great prospects require low damping, thus far strong plasmon damping was
observed, with both impurity scattering and many-body effects in graphene
proposed as possible explanations. With the advent of van der Waals
heterostructures, new methods have been developed to integrate graphene with
other atomically flat materials. In this letter we exploit near-field
microscopy to image propagating plasmons in high quality graphene encapsulated
between two films of hexagonal boron nitride (h-BN). We determine dispersion
and particularly plasmon damping in real space. We find unprecedented low
plasmon damping combined with strong field confinement, and identify the main
damping channels as intrinsic thermal phonons in the graphene and dielectric
losses in the h-BN. The observation and in-depth understanding of low plasmon
damping is the key for the development of graphene nano-photonic and
nano-optoelectronic devices
Plasmons in layered structures including graphene
We investigate the optical properties of layered structures with graphene at
the interface for arbitrary linear polarization at finite temperature including
full retardation by working in the Weyl gauge. As a special case, we obtain the
full response and the related dielectric function of a layered structure with
two interfaces. We apply our results to discuss the longitudinal plasmon
spectrum of several single and double layer devices such as systems with finite
and zero electronic densities. We further show that a nonhomogeneous dielectric
background can shift the relative weight of the in-phase and out-of-phase mode
and discuss how the plasmonic mode of the upper layer can be tuned into an
acoustic mode with specific sound velocity.Comment: 18 pages, 6 figure
Type-Decomposition of a Pseudo-Effect Algebra
The theory of direct decomposition of a centrally orthocomplete effect
algebra into direct summands of various types utilizes the notion of a
type-determining (TD) set. A pseudo-effect algebra (PEA) is a (possibly)
noncommutative version of an effect algebra. In this article we develop the
basic theory of centrally orthocomplete PEAs, generalize the notion of a TD set
to PEAs, and show that TD sets induce decompositions of centrally orthocomplete
PEAs into direct summands.Comment: 18 page
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